basic atomic and nuclear physics

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Engineering Fundamentals CBT

Printout of CBT Content for Reference Purposes Only

Reference CBT:

Basic Atomic and Nuclear Physics V 1.0

1019164


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ELECTRIC POWER RESEARCH INSTITUTE3420 Hillview Avenue, Palo Alto, California 94304-1338 PO Box 10412, Palo Alto, California 94303-0813 USA

800.313.3774 650.855.2121 [email protected] www.epri.com

Engineering Fundamentals CBT:

Printout of CBT Content for Reference Purposes Only

Reference CBT:

Basic Atomic and Nuclear Physics V 1.0

1019164

June 2009

EPRI Project Manager

Ken Caraway


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PRODUCT DESCRIPTION

Summary

This document provides a printout of the CBT content for use as a reference document only.Students are encouraged to use the CBT as animations, flash video, and interactive features are

intended to enhance their learning experience.

NOTE:The CBT should be used to validate information as errors may have been introduced

when converting the graphics, equations, etc.

Abstract

The Basic Atomic and Nuclear Physics Version 1.0 module of Engineering Fundamentalsprovides a basic overview of this topic applicable to all engineering disciplines beginning their

career in the nuclear power industry.

Description

The Basic Atomic and Nuclear Physics module covers basic atomic structure, fission,

radioactivity, reactor operation, and nuclear safety. This course will help new engineersunderstand how their work might impact reactor operations and nuclear safety. This module is

intended for use as orientation training for new engineering support personnel.

Software Requirements

Windows 2000 SP2, Windows XP, Windows Vista

Application, Value and Use

Allows engineering support personnel to review the content when they desire and at their own

pace

Uses interactive features and graphics to illustrate key concepts & enhance training

Keywords

Training

Fundamentals

Atomic structure

Nuclear physics

Radioactivity

Fission

Reactor operation

Reactor systems

Nuclear safety


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ACKNOWLEDGEMENTS

EPRI would like to acknowledge the following individuals for their active participationand significant contributions toward the development of this training course:

Ken Caraway EPRIJack Feimster Exelon CorporationNate Granger Wolf Creek Nuclear Operating CorporationBeth Hughes Handshaw, Inc.Don Lesnick Exelon CorporationJoe Montague DominionHenry Nicholson Duke Energy CorporationLiz Sisk EPRITerry Stuchlik Wolf Creek Nuclear Operating Corporation


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CONTENTS

1 INTRODUCTION TO BASIC ATOMIC AND NUCLEAR PHYSICS ......................................1-1

2 BASIC ATOMIC STRUCTURE .....................................ERROR! BOOKMARK NOT DEFINED.

3 THE FISSION PROCESS AND NEUTRON INTERACTIONSERROR! BOOKMARK NOT DEFINED.

4 RADIOACTIVITY ..........................................................ERROR! BOOKMARK NOT DEFINED.

5 REACTOR OPERATION ..............................................ERROR! BOOKMARK NOT DEFINED.

6 NUCLEAR SAFETY .....................................................ERROR! BOOKMARK NOT DEFINED.


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1-1

1INTRODUCTION TO BASIC ATOMIC & NUCLEAR

PHYSICSIn troduct ion

Welcome to the Basic Atomic and Nuclear Physics course. In this course, you will learnabout basic atomic structure, radioactivity, fission, reactor operation, and nuclear safety.Regardless of your discipline, you can make changes that affect reactor operations andnuclear safety in your day-to-day job. Changes that may affect operationsinclude instrumentation and control changes, electrical and/or mechanical systemalignment or availability, and introduction of foreign material (items unintended forsystem use) including coatings inside containment. You must be aware of how all

equipment and systems in the plant might be impacted by what you and others do.

After completing this lesson, you will be able to:

Describe how a nuclear power plant generates electricity

Describe the design differences between the two types of reactor systemstypically used in the United States

If you are not familiar with the navigation features used in this course, click the Abouttab to review the navigation information.


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Nuclear Steam Supply System s

First, lets take a high-level look at how a nuclear power plant works. The purpose of anuclear power plant is to produce steam that is used to generate electricity. Theunderlying concept behind this Nuclear Steam Supply System (NSSS), as it is called, is

straightforward. Nuclearfission indirectly creates heat, which is then used to generatesteam. From this point, a nuclear power plant functions very much like a fossil fuelgenerating plant. The steam flows throughturbines,rotating a shaft, which then turnsthe generator to produce electricity.

Each component and system in a nuclear power plant is designed and must beoperated to ensure that fission remains a safe, economic, reliable source of power.

Although the basic premise is simple, the NSSS is a complex system requiring afundamental understanding of certain aspects of nuclear physics.

The diagram below shows the main components of a PWR NSSS system. Roll your mouse over each component to identify itsfunction. (Note: This function will not work in this Word document.)
http://showglossary%28%27fission%27%29/http://showglossary%28%27turbine%27%29/http://showglossary%28%27turbine%27%29/http://showglossary%28%27fission%27%29/


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1-3

Fuel

Let's take a closer look at how an NSSS works. The fuel for an NSSS is typicallyenriched uranium that fissions easily with proper geometry and moderation. For U.S.nuclear plants, fuel is typically contained in small, cylindrical pellets, which measureapproximately 0.4 inch in diameter by 0.5 inch long. Pellets have small diameters so

that heat can be removed effectively. One pellet of uranium can generate as muchelectricity as 4 barrels of oil or 1.3 tons of coal. Clearly, the reactor fuel has high powerdensity (power per unit volume) compared to fossil fuels.

The pellets are stacked one on top of another to form a 12-foot long column andplaced into a sealed tube. The tube is called a fuel rod and is cladded with a wallthickness typically about 0.04 inches thick. The cladding is made of a zirconium alloythat transmits heat, absorbs few neutrons, and is non-corrosive, characteristicsimportant for maintaining the integrity of the tubes while efficiently transferring the heatand neutrons produced by fission. The cladding provides the first layer in the defense-in-depth design for keeping radioactive fission products contained. A set of fuel rods isbundled in a square lattice called a fuel assembly. Assemblies are placed side-by-sidein a nearly cylindrical array inside a large steel vessel called the reactor vessel, which ispart of the Reactor Coolant System boundary - the second barrier to radioactivity. Thetotal number of fuel assemblies depends on the type and power level of the reactor.


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Moderator and Coolant

Interactions of freeneutrons with nuclei in the fuel can result in fission. Fission occurswhen a free neutron collides with anucleus,is absorbed by the nucleus, and causes thenucleus to split into two fragments and emit additional neutrons. During fission, energyis released in the form of heat. If the newly emerged neutrons cause additional fissions,a chain reaction results. Self-sustained fission chain reactions generate a steady supplyof heat, the source of nuclear power in an NSSS.

However, neutrons born from fission typically have high kinetic energy and tend not tocause fission. Therefore, to facilitate fission of uranium, neutrons must be slowed down.This is achieved by using amoderator,which surrounds the fuel rods. Fast neutronsbounce off nuclei of similar mass in the moderator. With each collision, the neutronslose some of their kinetic energy and start slowing down.

In U.S. reactors, the moderator is water, which also serves as thecoolant.The heatfrom fission transfers from the fuel rods to the coolant, which is used for steamgeneration, but also keeps the fuel from overheating. Reactors that use "light" water (noneutron in the hydrogen atom) as moderator and coolant are called light water reactors,or LWRs. In other countries such as Canada, other media are often used for the coolant

and moderator, including deuterium (known as "heavy water"). In those cases, themoderator and coolant may be located in physically separate volumes.
http://showglossary%28%27neutron%27%29/http://showglossary%28%27nucleus%27%29/http://showglossary%28%27moderator%27%29/http://showglossary%28%27coolant%27%29/http://showglossary%28%27coolant%27%29/http://showglossary%28%27moderator%27%29/http://showglossary%28%27nucleus%27%29/http://showglossary%28%27neutron%27%29/


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Shielding

The nuclear reactor consists of the reactor vessel and everything inside it, including thecore, support structures, and other components. The core is the heart of the reactor, thepart that generates heat. It consists of the fuel assemblies, the coolant, the moderator,and the control rods. Fission in the reactor core not only creates heat, it producessignificant amounts of nuclearradiation and radioactive byproducts harmful to peopleand the environment. To safeguard workers inside the plant, a concrete biological shieldsurrounds the vessel, while other individual components that may contain radioactivematerials are shielded by the reactor building wall or other concrete structures.

In addition to the fuel cladding and reactor coolant system boundary (includes thereactor vessel) preventing the release of radioactive materials into the environment inthe event of an accident, the NSSS is completely enclosed in a steel-lined, concretecontainment barrier, which in many nuclear plants is the large dome visible on the site.Extensive additional systems and design features, which vary by reactor type, containand prevent accidental releases of radiation.
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Steam Generation

Once fission starts to heat up the coolant in the reactor, what happens next? Dependingon the type of reactor, the heated water is pumped to aheat exchanger,as in thisexample, or is allowed to boil within the reactor itself.

Click each picture below to view a larger version.

In a Boiling Water Reactor, or BWR, bulk boiling occurs in the upper part of the reactor

and nucleate boiling in the lower portion of the reactor core.

In a Pressurized Water Reactor, or PWR, high pressure keeps the water in the reactor

(called the primary coolant) from bulk boiling by raising its boiling point (localizednucleate boiling can still occur). The pressure is controlled by a pressurizer. This highlypressurized and heated primary coolant is pumped to a heat exchanger called a steamgenerator. The steam generator is a large, cylindrical steel vessel containing water at alower pressure (called the secondary coolant). The high pressure primary coolant flowsthrough tubes in the steam generator and heats up the surrounding lower-pressuresecondary coolant, causing it to boil. While remaining isolated in the primary coolantloop, after transferring heat energy to the secondary coolant, the primary coolant flowsback into the reactor to be reheated.
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Power Reactor Types

As you've already seen in this lesson, PWR systems and BWR systems generate steamin different ways.

In the table below, click each button on the left to learn about more differences between the two systems.

Design

Steam Production

Reactivity Control


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1-9

Creating Electr ici ty

From the point after the steam is created, a nuclear power plant is very similar to a fossilfuel power plant. The steam that has been created in the steam generator (PWR) or inthe reactor (BWR) will then flow to the steam turbines. The turbines then turn a shaft asthe hot steam expands through them. Large nuclear power plants typically have onehigh pressure and three low pressure turbines. The turbine shaft is coupled to anelectrical generator, which converts the mechanical energy of the turbines to electricalenergy. About one-third of the fission heat is converted to electricity in U.S. nuclear

generating stations.


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Cool ing

After passing through the turbines, the steam is exhausted into acondenser,whichconsists of hundreds of tubes designed to remove the latent heat of vaporization. Thesteam condenses and is termed condensate, as it passes around the tubes, which arefilled with cool circulating water. The condensate is pumped back to the steamgenerator in a PWR, called the secondary loop, or to the reactor in a BWR. The steam

cycle is now complete.The heat transferred from the steam to the water inside the condenser tubes must beremoved. To remove it, the heated condenser water is pumped to a cooling tower or acooling canal, where it cools by evaporation. At that point, it is either reused for cooling,as in a closed cooling cycle with a cooling tower, or it is discharged to an impoundmentor stream according to state and federal requirements on thermal discharge. In somelocations, state and federal environmental regulations allow the cooling water from thecondenser to be pumped directly to a body of water, such as a river or ocean.
http://showglossary%28%27condenser%27%29/http://showglossary%28%27condenser%27%29/http://showglossary%28%27condenser%27%29/http://showglossary%28%27condenser%27%29/


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Review Question 1

Match the nuclear power function in Column 2 with the component in Column 1.

Moderator A. Rotated by expanding steam to produce mechanical energy

Control rods B. Controls the reactor power and also provides a mechanism for rapid shutdown

of the reactor

Biological Shielding C. Slows down fast neutrons

Steam generator D. Prevents the release of radioactive materials into the environment in the eventof an accident

Turbine E. Converts the secondary coolant in a PWR into steam

The correct matching sequence is CBDEA.


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Review Question 2

In a BWR, bulk boiling occurs in the upper region of the core, not in separate steamgenerators.

True or False?

The correct answer is True. In a BWR, bulk boiling occurs in the upper region of thecore, while in a PWR, bulk boiling occurs in the steam generator.


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Conclus ion

You have now completed the Introduction to Basic Atomic and Nuclear Physics lesson,and have learned about the fundamental concepts of nuclear power generation.

Now that you have completed this lesson, you can do the following:

Describe how a nuclear power plant generates electricity

Describe the design differences between the two types of reactor systemstypically used in the United States

In the next lesson, you will learn about the basic atomic structure and the properties of anucleus.


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2BASIC ATOMIC STRUCTUREIn troduct ion

In the last lesson, you learned how a nuclear power plant creates electrical energy. Inthis lesson, you will learn about the characteristics that make an atom the key to nuclearenergy. You will learn about the structure and properties of an atom, as well as theforces that influence it.

After you have completed this lesson, you will be able to:

Define the following terms: nucleus, proton, neutron, electron, isotope, atomicmass unit (u), electron volt (eV)

Given a nuclide symbol, identify the number of neutrons, number of protons,number of nucleons, chemical element, and mass number

Define the ground and excited states of a nucleus and, in general terms, explainhow a nucleus changes energy levels

Describe the Line of Stability


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2-3

The Atom

An atom is too tiny to be seen very clearly, even with an electron microscope. For thisreason, models that explain physically observed atomic phenomena are used todescribe an atom. The simplest of these, theBohr model of an atom, describes a smallnucleus where over 99.9% of the atom's mass is concentrated, surrounded by electrons

that orbit the nucleus in shells of discrete radii. Imagine a penny in the center of abaseball stadium, and you have an idea of the nucleus's relative size compared to therest of the atom. In fact, as a whole, an atom is mostly empty space.

Below is a picture of an atom. Click each labeled part to learn more.

Nucleus: The nucleus is the center of an atom that is comprised of a tightly-packedcluster of particles called neutrons and protons. It contains 99.9% of its mass and iscomprised of neutrons and protons held together in a very small volume by nuclearforce.

Neutron: A neutron is a sub-atomic particle with no charge that has about the same

mass as a proton.

Proton: A proton is a positively-charged particle that, along with neutrons, comprise anucleus. Protons have a positive charge exactly equal in magnitude to the charge of anelectron.

Electron: An electron is a negatively-charged particle with little mass that orbits thenucleus. An atom with an equal number of electrons and protons has a neutral charge.

Electron Shell: An electron shell is the orbital area around the nucleus that correspondsto energy levels of electrons.
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Atom ic Elements

The number of protons in a nucleus determines the atomicelement.There are currently113 confirmed elements. An atom with only one proton is hydrogen (H). In fact, it is theonly element that may not have any neutrons. An atom with two protons is helium (He),with three, lithium (Li), and so on. You can use theperiodic table to look up informationabout elements.

Click each element below to see a diagram of its structure.

Isotopes are atoms of the same element (same number of protons) that may havedifferent numbers of neutrons. Every element can exist in the form of several differentisotopes. Hydrogen isotopes, for example, include an atom with just one proton in thenucleus, another with a proton and a neutron, and yet another with one proton and twoneutrons.
http://showglossary%28%27element%27%29/http://showglossary%28%27periodic%20table%27%29/http://showglossary%28%27isotopes%27%29/http://showglossary%28%27isotopes%27%29/http://showglossary%28%27periodic%20table%27%29/http://showglossary%28%27element%27%29/


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Standard Notation

The standard notation of an atom is shown below.

Click each part of the notation to learn more.

When interpreting standard notations of atoms, you should remember the following:

The mass number (A) is the sum of the atomic number (Z) and the number ofneutrons (N).

The number of neutrons (N) is usually excluded from the notation. It can becalculated by subtracting the atomic number (Z) from the mass number (A).

The atomic number (Z) may be left off of the notation because an element, as

specified by its symbol, uniquely determines the number of protons.

Don't confuse the mass of an atom with its atomic number (Z).

X is the chemical symbol for the element.A, the mass number, is the total number of protons and neutrons.Z, the atomic number or charge number, is the number of protons.N is the number of neutrons.


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Standard Notation, Continu ed

Remember, you can calculate the number of neutrons by subtracting the atomic number(Z) from the mass number (A).

Try calculating the number of neutrons in each element below. Then, click the element to show the answer.

Suppressing the atomic number and the neutron number in the notation, the aboveisotopes can also be represented as 1H, 6He, 12C and 235U. They are also sometimeswritten in X-A notation, as in H-1, He-6, C-12 or U-235.

The hydrogen atom has 0 neutrons.The helium atom has 4 neutrons.The carbon atom has 6 neutrons.

The uranium atom has 143 neutrons.


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Sub-Atomic Notat ion

Sub-atomic particles have similar representations.

Click each particle below to view its notation.

Proton

This notation shows that the mass number for a proton is 1 and the charge for a protonis +1.

Neutron

This notation shows that the mass number for a neutron is 1, and the neutron hasno charge.

Electron

The mass number of an electron is 0 and carries a-1 charge.

Nuclei frequently interact in some way with many of the other particles they encounter.Balance equations use "isotope" notation to depict the reactants and the products ofthese nuclear reactions. The total mass numbers and the charge (atomic) numbers areunchanged in nuclear reactions and must balance on both sides of the equation.


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Exci ta tion and Energy

Like an orbital electron, a nucleus can exist at various, discrete levels of energy, eachisotope having a characteristic set of levels. The lowest energy state is called theground state. Higher energy states are called excited states. A nucleus gains excitationenergy in many nuclear reactions and jumps to a higher energy level. An excited

nucleus will release excess energy as radiation to return to its ground state. Like theenergy levels, the excitation energy and the emitted radiation energy are discrete andwill equal the difference in energy between the initial and final levels.

Click the Forward button in the bottom-right corner of the graphic at right to progress through each slide in the animation.


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Electron Vol ts

The energy involved in a nuclear reaction is so small that usual energy units such asJoule and Btu are cumbersome. Instead, theelectron volt,eV, is used to describeenergy at atomic and sub-atomic levels. One eV is equal to the amount of energy

gained by an electron when accelerated through a potential difference of one volt.

Nuclear reaction energy is typically of the order of keV or MeV.
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Atom ic Mass

The mass of an atom is so small that it is usually expressed in terms of the convenientatomic mass units or u (amu). One u is defined as 1/12 the mass of a carbon-12 atomor 1.66054 x 10-24grams. The mass of H, the smallest atom, is 1.0078250 u. The mass

of a C-12 atom is 12 u. The tables below show the atomic mass of sub-atomic particlesand a comparison of the atomic mass of the nuclei of three atoms.

Click each table to learn more.

Notice the masses of nuclear particles. Which has the greater mass, a proton or aneutron? The neutron is more massive. At less than one billionth of a gram, a proton ora neutron is still about 1,840 times more massive than an electron. The mass of anatom overall is only very slightly more than the mass of its nucleus, since essentially allof the mass is concentrated in the nucleus.

Compare the mass and radii of three different nucleihydrogen, boron, and uranium.The size of a nucleus varies with the number of nucleons (protons and neutrons) it has.

A nucleus with a small number of nucleons is called a light nucleus. One with a largenumber is a heavy nucleus.


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Since like charges repel, why aren't the positively-charged protons in a nucleus sent offin every direction? The nucleus, with its compact collection of nucleons, is held intact bya specific force. Which of the following forces do you think keeps a nucleus intact?

A. Electrostatic attractionB. MagnetismC. GravityD. Nuclear force

The nuclear force, or "strong force," is the attractive force between thenucleonsthatholds them very tightly together to form the nucleus. It impacts protons and neutronsequally and is independent of charge. Although it is the strongest force known, thenuclear force acts only over very short distances and has no effect outside the nucleus.
http://showglossary%28%27nucleon%27%29/http://showglossary%28%27nucleon%27%29/http://showglossary%28%27nucleon%27%29/http://showglossary%28%27nucleon%27%29/


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Binding Energy and Mass Defect

The nucleus is not easily broken apart. The stability of the nucleus is maintained by itsbinding energy.Nuclear binding energy is the energy that binds all the nucleons in thenucleus and, therefore, is also the energy required to break the nucleus into individualnucleons. Every time a nucleus fissions and splits into two, it is releasing a small part of

its binding energy. The major component of the binding energy is the potential energyassociated with the nuclear force between neighboring nucleons. Binding energy canalso be explained by examining the mass defect.

The energy equivalent of this value can be determined by using Einstein's energy-massequivalence equation, E=mc2.

A nucleus has less mass than the total masses of its parts (mass defect), but it hasbinding energy equivalent to the difference. Since energy and mass areinterchangeable, nothing is really missing!

Binding energy can also be expressed in terms of its average value for any individual

nucleon. Binding energy per nucleon is an index of a nucleus's stability: the larger itsvalue, the more tightly bound the nucleons and the more stable the nucleus. Thegraphic at right shows an example of how binding energy is calculated for a deuterium(2H) atom.
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Line of Stabi l i ty

Every isotope has unique characteristics, including atomic mass, binding energy, atomicpercent abundance, and radioactivity data for radioactive isotopes. Information aboutisotopes can be found in the Chart of Nuclides (CN), (http://atom.kaeri.re.kr/ ). Below is acondensed graphical representation of the most commonly used properties of isotopes.

Navigate to the website for a larger view of the chart.Below is an image of the Chart of Nuclides. Click each section to learn more.

Nuclides to the right and left of the line of stability are unstable and strive to becomestable through radioactive decay. Elements with Z>83 (bismuth) have no stableisotopes. Radioactivity explains much of what happens in a nuclear reactor. You willlearn more about radioactivity in the next lesson.

Nuclides to the left of the line of stability generally have an excess of protons comparedto the stable nuclides.

Stable nuclides (blue squares) are clustered in a narrow band called the Line ofStability. They have a balanced number of neutrons and protons because at smallnumbers, the nuclear force is strong enough to counteract the Coulombic repulsion.However, with more protons in the nucleus, the Coulombic repulsion is more difficult tocounteract. Therefore, larger stable nuclei tend to have more neutrons than protons.Neutrons increase the distance between protons, acting as buffers among them.

Nuclides to the right of the line usually have an excess of neutrons and are consideredneutron-rich.
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Review Question 3

Below is a diagram of the standard notation of an atom. Match the description inColumn 2 with the letter in Column 1.

X A. The chemical symbol for the element

A B. The number of neutrons

Z C. The atomic number, or the number of protons

N D. The mass number, or total number of protons and neutrons

The correct matching sequence is ADCB.


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Review Question 4

An excited nucleus will release excess energy as radiation to return to its ground state.

True or False?

The correct answer is True. An excited nucleus will release excess energy as radiationto return to its ground state.


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Review Question 5

What is the force trying to pull a nucleus apart?

A. Nuclear forceB. Coulombic repulsion

C. Binding energyD. Kinetic energy

The correct answer is B. Coulombic repulsion is the electrostatic force that makesparticles with like charge repel each other along a straight line between their centers.


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Conclus ion

You've now completed the Basic Nuclear Structure lesson and have learned about theproperties and characteristics of an atom.

Now that you've completed this lesson, you can:

Define the following terms: nucleus, proton, neutron, electron, isotope, atomicmass unit (u), electron volt (eV)

Given a nuclide symbol, identify the number of neutrons, number of protons,number of nucleons, chemical element, and mass number

Define the ground and excited states of a nucleus and, in general terms, explainhow a nucleus changes energy levels

Describe the Line of Stability


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3THE FISSION PROCESS AND NEUTRON

INTERACTIONSIn troduct ion

Now that you've learned about the structure and properties of an atom, you'll learnabout the process that makes nuclear power possible - fission. A nuclear reactor isdesigned to sustain the fission process in a safe and controlled manner whiletransferring the heat energy and containing the radioactive fission products. This lessonwill describe the neutron interactions that initiate and sustain the process, and how theprocess is controlled.


Describe the fission process

Describe the life cycle of a neutron

Define criticality and reactivity

Describe how a chain reaction is controlled and maintained

State the difference between fuel and non-fuel absorption (poisoning) and ascattering (moderation) type of neutron interaction


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The Fiss ion Process

When a nucleus absorbs a neutron, it gains excitation energy from the neutrons massand kinetic energy. In certain heavy isotopes, this added excitation energy may put thecompound nucleus over its critical energy threshold. This may induce the nucleus to

split, or fission, into two lighter radioactive nuclei (also calledfission fragments,fissionproducts), ordaughters.

During fission, some of the total mass of the original heavy nucleus and absorbedneutron is lost in the reaction. In accordance withEinstein's equation (E = mc2), this lostmass defect is converted to about 200 MeV of energy. Fission also releases two tothree other (fission) neutrons along with heat energy and radiation.
http://showglossary%28%27fission%20fragments%27%29/http://showglossary%28%27fission%20products%27%29/http://showglossary%28%27fission%20products%27%29/http://showglossary%28%27daughter%27%29/http://showglossary%28%27mass-energy%20equivalence%27%29/http://showglossary%28%27mass-energy%20equivalence%27%29/http://showglossary%28%27daughter%27%29/http://showglossary%28%27fission%20products%27%29/http://showglossary%28%27fission%20products%27%29/http://showglossary%28%27fission%20fragments%27%29/


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Neutron Moderation

Neutrons born from fission typically have high kinetic energy (are "fast"). A very smallfraction of the high energy neutrons cause fast fissionin U-238. As neutrons slowdown, the probability of a neutron causing fission (cross section)in certain heavy nucleiincreases. This probability is greatest when neutrons slow down to energies that are in

thermal equilibrium with their surroundings (i.e., the fuel nuclei).

As you learned earlier in this course, U.S. reactors incorporate a moderator to slowdown fast neutrons and facilitate fission. The moderator, which is water in the U.S.,surrounds the fuel rods to slow neutrons down to thermal equilibrium with the U-235 fuelmaterial, in order to undergo thermal fission. When small hydrogen atoms collide with aneutron, they scatter. Much of the neutrons energy is lost, similar to when two billiardballs have an elastic collision. While not absorbed, the moderated neutron losesenergy and slows down toward thermal energy levels.

Click the Forward arrow in the bottom-right corner of the graphic at right to progress through each slide inthe scattering animation.
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Fiss ion Fuels

Because they can fission with low energy thermal neutrons, certain isotopes of Uraniumand Plutonium (i.e., U-235, Pu-239, and Pu-241) are used in U.S. reactors. Theseisotopes, displayed in red in the table at right, are referred to asfissile.Because they

readily fission, they make excellent fuels.

U-235 Enrichment

Uranium is the most widely used isotope. It occurs naturally all over the world; however,less than 1% of natural uranium is fissile U-235. In order to use its fissile quality tosustain fission reactions, the concentration of U-235 atoms in U.S. reactor fuel designsis increased to approximately 4-5% U-235 by weight through enrichment. The balanceof these fuel rods is composed almost entirely of 95-96% U-238 isotopes.

Plutonium Generation

While most of the fission energy comes from enriched U-235 fuel, in U.S. reactors, asignificant amount of energy comes from Pu fission. Over time, non-fissile isotopes,such as U-238 (called fertile isotopes, displayed in blue in the table at right), captureneutrons and decay to fissile isotopes, such as Pu-239. Since approximately 95% of thefuel is U-238 conversion to fissile Pu isotopes, orbreeding reaction,is natural.

Clickhere to see a diagram of a breeding reaction.
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3-5

Sustaining the Fission Chain Reaction

As the 2-3 neutrons created in the fission process are slowed down, they can fissionother heavy nuclei in the reactor. But, every neutron does not necessarily cause fissionin the fuel. Some neutrons escape from the reactor, and others may be absorbed byother reactor materials (such as the moderator, cladding, control rods/absorbers, orstructural materials). The ratio between one generation of neutrons (n1) and theprevious generation (n0) is denoted by k, the multiplication factor, as shown below.


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Crit ical i ty

In order for the production of neutrons and a fission reaction to be sustainable, it isimperative that k = 1 where the neutron population (or power) stays constant and the

reactor is exactlycritical.When k is greater than 1, the number of neutrons grows withtime and the reactor is supercritical. When k is less than 1, the number decreases withtime and the reactor is subcritical.

Clickhere to see the multiplication factor equation from the previous page.
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3-7

Neutron Mult ip l ic ation - Six Factor Formula

From fast, high-energy birth, there are six major factors that affect neutron multiplication(k).

Six Factor Formula:

Since n1/n0is the multiplication factor, k, the equation can be re-arranged and written inits usual form as k = f PfPt. Thus, this expression for k, which is the product of thesix factors defined above, is called the six-factor formula.

, Fast fission factor:

This factor determines how many additional fission neutrons are generated by fast (non-thermal) fissions. In addition to the generation of fissile plutonium, a very small numberof fast fissions occur in U-238, while the neutrons are still fast or at high energy.

Pf, Fast non-leakage probability:

Some fast neutrons escape through the boundaries of the reactor core before they startto slow down. Pf is the probability that fast neutrons do not escape. In other words, 1-Pf

is the probability of escape. So there are n0 Pf fast neutrons that remain inside the

reactor and try to become thermal neutrons.

, Resonance escape probability:

Probability a neutron is not absorbed as they slow down to thermal energy. As theremaining neutrons continue to slow down, some will fall into energy resonance with thesurrounding material and be captured. Those that escape resonance capture will slowdown and reach thermal equilibrium with the fuel material.

Pt, Thermal non-leakage probability:

Probability a neutron does not leak out of the core while thermal. A small fraction of thethermal neutrons will escape from the reactor. The others will get absorbed in fuel orother reactor materials (moderator and control rods).

f, Thermal utilization factor:

This is the fraction of thermal neutron absorptions that occur in the fuel as opposed to inany material in the reactor core. Since that will include absorptions in the control rods orcontrol poisons, f is primarily the factor that can be controlled by the reactor operators. fis typically in the range of 0.75 0.5 for an LWR.

, Reproduction factor:

The remaining neutrons that cause fission produce the next generation (n1) of two tothree fission neutrons for every neutron absorbed by the fuel.


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Delayed Neutrons

Most, but not all, of the n1generation neutrons come directly from fission. Fission canbe thought of as a two-stage process. In the first stage, called the prompt stage, thenucleus splits emitting fragments, neutrons, energy, and radiation. In the second, knownas the delayed stage, several seconds after fission on average, a small number of thefission products release an additionaldelayed neutron.U.S. reactor designs usedelayed neutrons to sustain criticality. This time delay using neutrons from previousgenerations provides an inherent safety control mechanism for the reaction.
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3-9

Reactivi ty

In addition to the delayed neutrons, other methods are used to control reactivity inLWRs. Change in the reactor that affects neutron multiplication is called reactivity ().Reactivity is a measure of the departure of a reactor from criticality and is described asthe percentage difference in the value of k from one, or:

Reactivity can be positive, zero, or negative. When the multiplication factor, k, is exactlyone, the reactor maintains a steady, self-sustaining chain reaction. If k is less than 1,and the reactor must be made critical, the difference, k-1, must be compensated for byusing positive reactivity. Thus, the magnitude of k, which is equal to (k-1), is ameasure of the compensation required to make a non-critical reactor become critical.

Positive (or withdrawing control rods) reactivity (e.g., creating Pu-239) increasesneutron multiplication and moves the reactor towards supercritical. Negative (orinserting control rods) reactivity (e.g., depleting U-235 fuel) takes away neutrons andmoves the reactor towards subcriticality.

Another example of reactivity change is referred to as the Reactivity Coefficient. Anexample is the Moderator Temperature Coefficient, expressed as reactivity per degreechange in moderator temperature. Temperature affects water density and changesthe number of moderating atoms in a given volume. This changes the multiplicationfactor. In BWRs, the control of water flow changes the amount of steam production anddensity of the coolant.


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Contro l l ing Fiss ion

Materials that absorb or capture neutrons are used in the reactor to control the chainreaction. Control rods are inserted or withdrawn from the reactor to control the powerlevel or shut down the reaction. The insertion of a control rod is another example ofadding negative reactivity. Conversely, removing the control rods removes negative

reactivity. Control rods are positioned in terms of resulting reactivity changes perdistance moved. This facilitates operator-generated reactivity changes through controlrod movement.

In PWRs, the neutronpoison, boron, is dissolved throughout the reactor core in themoderator/coolant. This helps control the neutron population. The boron absorbsneutrons, thus reducing the possibility of fission. This is another example of negativereactivity. However, as the boron burns out, it introduces a positive reactivity effect.

Some fuel assemblies may also have an additionalburnable poison to compensate forthe high positive reactivity of the new fuel in the core after refueling. This neutronabsorbing material (e.g., gadolinia or boron) may be part of the fuel material, coated

externally on the fuel pellets, or inserted as discrete rods in the fuel assembly.
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3-11

Neutron Life Cycle

Throughout this lesson, you've learned that in a fission chain reaction, neutrons areborn, live for a short while, may beget new neutrons, and then die. What importantevents mark the life cycle of fission neutrons inside an LWR? The graphic below showseverything that can happen to such neutrons from the time they are born from thermal

fissions in fissile fuel nuclei to when they complete their life cycle and produce the nextgeneration of fission neutrons.


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Review Question 6

A nuclear reactor is designed to do which of the following?

A. Sustain the fission process

B. Shut down the fission processC. Transfer heat energyD. Contain the radioactive fission products

The correct answers are A, C, and D.

A nuclear reactor is designed to sustain the fission process in a safe and controlledmanner, while transferring the heat energy and containing the radioactive fissionproducts.


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3-13

Review Question 7

The ratio between one generation of neutrons and the previous generation is known aswhich of the following?

A. Criticality factor

B. Multiplication factorC. Energy-mass equivalenceD. Neutron life cycle

The correct answer is B.

The ratio between one generation of neutrons and the previous generation is known asthe multiplication factor.


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Review Question 8

Which of the following are examples of adding negative reactivity in the reactor?

A. Inserting a control rodB. Removing a control rod

C. Introducing a poisonD. Burning out a poison

The correct answers are A and C.

Negative reactivity takes away neutrons and moves the reactor towards subcriticality.Examples include inserting a control rod and introducing a poison.


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3-15

Conclus ion

You have completed this lesson and learned about the neutron interactions that initiateand sustain the fission process, and how the process is controlled.

Now that you've finished this lesson, you can:

Describe the fission process

Describe the life cycle of a neutron

Define criticality and reactivity

Describe how a chain reaction is controlled and maintained

State the difference between fuel and non-fuel absorption (poisoning) and ascattering (moderation) type of neutron interaction


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4-1

4RADIOACTIVITYIn troduct ion

Nuclear reactors use and generate a large amount of radioactive material. Radioactivityis the state describing unstable isotopes, which spontaneously emit sub-atomic particlesand energy (radiation) from their nuclei. In this lesson, you will learn about radioactivityand radioactive decay, and how they affect nuclear power plant operation.


Define the following terms: radioactivity, radioactive decay, radiation,radioisotope, half-life, and decay constant

Describe the three major modes of radioactive decay: alpha, beta, and gamma-ray emission

Determine the number of atoms of a specific radioisotope and their activity after aspecified time using the radioactive decay law and half-life


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Fiss ion Products and Radioact iv i ty

When they fission, fuel nuclei do not split evenly. As can be seen in the graph to theright, the fission products come out as a wide variety of neutron-rich isotopes rangingfrom 76 to 160 in mass number. The excess neutrons give the fission fragments toomuch energy and make them unstable. Unstable nuclei naturally release energy bymeans ofradioactive decay,otherwise known asradioactivity or activity. The mostcommon forms are Alpha and Beta, which are particles similar to neutrons, andGamma, which are similar to X-rays.
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4-3

Alph a Decay

Typically, heavy radioactive nuclei such as U-238 and Pu-239 emit alpha particles.Alpha particles resemble the nucleus of a Helium atom with two protons and twoneutrons. They are relatively big with a lot of mass and energy.

Because the alpha is such a relatively large particle, a lot of energy is released when

heavy nuclei undergo alpha decay. However, because of their size, alpha particles areeasily stopped by thin layers of shielding. A sheet of paper or the outer layers of deadskin will stop them.


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Beta and Gamma Decay

The mid-sized fission fragment nuclei typically decay by beta particle emission. Thebeta particle emission is the decay of a nucleon, either an electron or an anti-electron(positron) from the nucleus. Electron emission is known as beta minus decay andpositron emission as beta plus decay. The daughter products of the beta decays arealso usually radioactive, and thus lead to a decay chain that will terminate ultimately in astable isotope. It is because of these radioactive fission products that used fuel removedfrom the reactor must be isolated and shielded in order to minimize radiation exposureto the public and environment.


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4-5

Decay Rate

Like all radioisotopes, fission products decay at different rates and by different modes.One isotope may take billions of years to decay while another decays inseconds. Radioactive decay does not make nuclei disappear; they simply change to a

more stable configuration. The decay rate of a fission fragment is determined by its half-life (t1/2), the time for half the atoms to decay. Half-life is an inherent, unchangeablecharacteristic of aradioisotope.The decay rate or activity (A) for a radioisotopedepends on and is proportional to the current population of available atoms, N(t). Thisrelationship is represented by thedecay constant() where = A/N. The initial (time =0) activity A and population N0of a radioisotope decreases exponentially over time. Thisradioactive decay law is expressed as:

or because they are proportional

There is a unique relationship between decay rates and half-life. In the radioactivityequation, if t = t1/2, the activity must decrease to half the original value or A(t)/A0= .Therefore:

This means radioactivity decreases faster for isotopes with larger decay constants,which implies shorter half-lives. The shorter the half-life, the larger the decay constantand the faster N(t) or A(t) decreases. More highly radioactive fission products decayaway more quickly. Fission products with longer half-lives typically have lower energyradioactivity. It is important to understand radioactivity and its implications, to ensure on-site work is performed with adequate protective measures.
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Decay Heat

In addition to shielding requirements, another feature of nuclear power arising fromfission product radioactivity is that, even after the reactor is shut down and the fissionchain reaction has stopped, the fuel continues to generatedecay heat.Decay heat

generation impacts spent fuel handling and storage, waste management, and reactorsafety at a nuclear plant. A typical LWR produces over 220 MWt of decay heatimmediately upon shutdown. Since much of this early decay heat is from short-livedfission products, the heat production rate decreases sharply in the first few hours. But,following a reactor shutdown, adequate cooling must be available. Otherwise, the decayheat is sufficient to boil the core dry and melt the fuel. Reactor designs have speciallyengineered safeguards to provide such cooling.

You will learn more about decay heat in the next lesson, Reactor Operation.
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4-7

Review Question 9

Which of the following determines a fission fragment's decay rate?

A. Radioactivity

B. Decay heat

C. Beta particle emissions

D. Half-life

The correct answer is D.

The decay rate of a fission fragment is determined by its half-life, the time for half theatoms to decay.


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Review Question 10

After the reactor is shut down and the fission chain reaction has stopped, the fuel is safefor handling with special precautions and safety measures (e.g., shielding and cooling).

True or False?

The correct answer is True.

Even after the reactor is shut down and the fission chain reaction has stopped, the fuelcontinues to generate decay heat, which impacts spent fuel handling and storage,waste management, and reactor safety at a nuclear plant. When the fuel is handled,it must be moved using proper shielding and tools.


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5-1

5REACTOR OPERATIONIn troduct ion

So far in this course, you've learned about atomic structure, the fission process, andradioactivity. In this lesson, you'll learn how it all works together in the reactor to createand control nuclear energy.

At the end of this lesson, you will be able to:

Describe how temperature, pressure, voids, fuel constituents, control rods, boronconcentration, and fission product poisons affect the fission process

Describe how reactor operation affects temperature, pressure, voids, fuelenrichment (burn-up or depletion), boron concentration, and fission productpoisions

Describe how criticality and/or reactivity affect startup, power operations, andshutdown

Describe how changes in various balance of plant parameters affect reactoroperations in a PWR and a BWR

Define decay heat and describe its affect on plant operations

Describe the significance of loss of coolant accidents (LOCA) in all plant

conditions


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Reactivi ty Feedback Mechanisms

The control of reactivity is an important function in reactor operations. Physicalmechanisms, inherent to the design, that are affected by and in turn affect reactivity arecalled reactivity feedback mechanisms. Feedback can be positive or negative: positive

feedback increases reactivity and negative feedback decreases reactivity. The followingmechanisms affect reactivity in reactor operation.

Click each mechanism below to learn how it affects reactivity.

Temperature feedback:Temperature feedback results from temperature changes in the water (moderator) andthe fuel. Temperature affects both the density of the moderator and the neutronabsorption cross sections of the fuel, either of which can impact reactivity.

Pressure feedback and void feedback:Pressure feedback and void feedback, of primary concern in BWRs, is due to thechange in moderator density as a result of changes in the number of steam bubbles (orvoids) in the coolant during power operation.

Fuel enrichment:Fuel enrichment is affected by U depletion and Pu production, which alters the numberof fissile atoms in the fuel. U-235, initially loaded in the reactor fuel at the beginning ofthe cycle, is depleted by neutron absorption, resulting in removal of positive reactivity.Pu-239 is produced from neutron absorption in U-238 and increases positive reactivity.

Control rods:Control rods contain materials that absorb neutrons. Inserting more control rods into thecore is an addition of negative reactivity, while withdrawal of control rods from the coreprovides a removal of negative activity.

Burnable poison rods:Burnable poison rods also capture neutrons that would otherwise be absorbed in thefuel. They are placed in the reactor at the beginning of core life to compensate for theexcess fuel necessary for the reactor to remain critical over the irradiation cycle. Thematerial used to absorb the neutrons is gradually depleted (burned), resulting in anegative reactivity removal.


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5-3

Boron:Boron, in the form of boric acid, is used as a soluble poison in PWRs. The high positivereactivity of fresh fuel early in core life is offset by the negative reactivity of highconcentrations of boric acid. As the fuel depletes over core life, the plant operatorsreduce the boron concentration.

Fission product poisons:Fission product poisons capture neutrons that would otherwise be absorbed in the fuel,and therefore lower the multiplication factor. The production of fission product poisons isthen a source of negative reactivity. Of primary concern are Xe-135 and Sm-149 due totheir high neutron absorption cross sections.


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Moderator Temperature Feedback

In LWRs, the moderator (water) and coolant are the same. Moderator temperaturechanges generally cause changes in reactor power. An increase in reactor powergenerally results in an increase in the moderator (coolant) temperature, which

decreases the moderator density. In an under-moderated condition, the furtherreduction in moderator density decreases its effectiveness in neutron moderation,resulting in decreased reactivity (a negative effect). LWRs are designed to be under-moderated such that the moderator temperature feedback is negative.

Some PWRs experience an over-moderated condition early in core life due to the highboron concentration in the core. In an over-moderated system, a reduction in moderatordensity from increased coolant temperature reduces the number of boron atoms that actas absorbers, which increases reactivity (a positive effect). Thus, a temperatureincrease will continue to add positive reactivity, which increases temperature evenmore. This undesirable effect can be overcome by other feedback mechanisms, such asfuel temperature feedback.

Reactivity increases with moderator density until neutrons become thermal. After that, reactivitydecreases with moderator density because neutron absorption increases.


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5-5

Temperature, Pressure, and Void Feedback

Fuel Temperature Feedback

Increasing the fuel temperature in an LWR increases resonanceradiative capture in U-238, which constitutes over 96% of the uranium fuel, thereby decreasing reactivity. Adecrease in fuel temperature has the opposite effect, resulting in an increase inreactivity. Opposing this effect is the possibility that the fuel temperature increase mayalso increase absorption in resonances in the fission cross section of the fuel. However,this effect is relatively small compared to resonance capture in U-238.

Pressure and Void Feedback

The effect of the presence of steam bubbles, orvoids,in BWRs is similar to themoderator temperature effect in that the moderator density is reduced. In a BWR,increasing pressure decreases the presence of voids. Conversely, decreasing pressureincreases the presence of voids. In under-moderated systems, the presence of voidseffectively removes moderation of neutrons and lowers the reactivity. In an over-moderated system, just the opposite is true. The presence of voids removes excessmoderator, lowering absorption and increasing reactivity.
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U Depeletion and Pu Product ion

Positive reactivity is supplied by the U-235 that is blended into the U-238 in the fuelrods. The term "burn-up"is used to represent the consumption of the uranium and isquoted as MWd/Tonne U (megawatt days/metric ton of uranium). As the operating cycle

progresses, theU-235 that is initially loaded in the core gets depleted by fission. In a reactor that isoperating at steady power output, a good approximation is that the U-235 concentration,hence reactivity, decreases linearly with burn-up, a reduction in positive reactivity overthe life of the core.

As the reactor operates at power, Plutonium (Pu) isotopes are produced from neutronabsorption in the U-238 contained within the fuel rods. This increases the reactivity ofthe core because some of the fissile Pu isotopes (Pu-239 and Pu-241) undergo fissionand produce energy. By the end of an irradiation cycle, about 40% of the reactor poweris estimated to be due to the fission of these Pu isotopes. The Pu isotopes build up toequilibrium values with irradiation. The net contribution to reactivity from Pu productionis positive, but is not large enough to offset the positive reactivity decrease caused byU-235 depletion. Overall, over the fuel cycle, the positive reactivity of the coredecreases.

Click the Play button in the graphic at right to view U depletion.
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5-7

Fiss ion Product Poisons

Fission products capture neutrons and lower reactivity. Many of these isotopes havefairly low absorption cross sections; however, two isotopes that occur near the secondpeak of the fission yield curve, Xe-135 and Sm-149, have very high absorption cross

sections. Their accumulation in the fuel rods can cause major changes in reactivity. Tofully understand their effects, let's look at their production and decay modes and howtheir concentration might vary with time in a reactor:

Xe-135 is produced in two ways: directly as a fission product, and also by the decay ofI-135 (half-life = 6 h), which is produced by fission. Xe-135 has an incredibly highthermal neutron absorption cross section, which is the reason for its nuisance factor. Itdecays with a half-life of 9.1 h to Cs-135, which is practically stable. In a reactoroperating at a steady power level, the Xe concentration is held at an equilibrium valueby its destruction through radioactive decay, and through neutron absorption andconversion to Xe-136, which has a much smaller absorption cross section. If reactorpower is decreased or the reactor shuts down, I-135 continues to decay to Xe-135, butXe-135 destruction by neutron absorption stops. Since I-135 has a shorter half-life thanXe-135, this results in an initial increase in the Xe level and an accompanying additionof negative reactivity. After the relatively short-lived I-135 has decayed away, theproduction of Xe-135 stops, and the Xe-135 concentration decreases exponentially byradioactive decay. Xe can cause operational problems because of the large reactivityswings it can cause over relatively short periods of time. Clickhere to see an animationof Xe buildup after reactor shutdown.

Sm-149 is the decay product of Pm-149, a daughter of the fission product Nd-149. Thehalf-lives of Nd and Pm are 2 h and 54 h respectively. Sm-149 is a stable isotope and isdestroyed only by neutron absorption. Its thermal neutron cross section is also very high

and thus, its build-up in a reactor adds significant negative reactivity. Since Sm isstable, it simply accumulates to some maximum value following power reduction orreactor shutdown. When the reactor is restarted and increases in power, the built-up Smis destroyed by neutron absorption. Clickhere to see an animation of Sm buildup afterreactor shutdown.
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Reactivi ty Control in a PWR

Positive reactivity largely comes from the fuel or from the removal of negative reactivitythrough inherent or operator-initiated processes meant to offset the depletion of thepositive reactivity in the fuel. Reactivity in a PWR is controlled in three ways.

Click the graphic at left to view a larger image of a PWR control assembly, then click each method of reactivity control below tolearn more.

Control rods:Most control rods are made of an alloy of 80% silver, 15% indium, and 5% cadmium,provide quick power control. The control rods consist of rodlets (individual rods)connected to a spider, which form a single control rod assembly. Each assembly ismoved by a separate control rod drive mechanism that drives each control rodassembly into the top of the core. About one-third of the fuel assemblies in the core willcontain a control rod assembly.

Burnable poison rods:Some fuel assemblies may also have burnable poison rods, usually made of gadolinia(Gd2O3) that are placed in unrodded fuel pin tubes. The gadolinia absorbs neutrons,thus compensating for the high positive reactivity of the core in the fuel cycle. Thatabsorption of neutrons burns out the gadolinia, thereby reducing the negative reactivityduring the cycle.

Boric acid:Soluble boric acid serves as a chemical shim (changing boron concentration) in theprimary coolant to provide longer-term control. Increasing the boron concentration viaboration or decreasing the boron concentration via dilution provides somewhat uniformcontrol of reactivity, both vertically and radially, in the core.


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5-9

Reactivi ty Control in a BWR

Reactivity in a BWR is also controlled in three ways.

Click the graphic at left to view a larger image of a BWR control assembly, then click each method of reactivity control below tolearn more.

Control rods:

Control rods, which enter the BWR core from the bottom, are used for large changes ofpower and to compensate for fuel depletion. The control rods are made of B4C (boron)powder packed inside thin rods. The rods are canned into a cruciform (cross-shaped)configuration. Each single cruciform control blade is associated with four neighboringfuel assemblies. They are operated using hydraulic controls and a locking pistonarrangement that prevents accidental removal.

Burnable poison rods:Some fuel assemblies may also have burnable poison rods, usually made of gadolinia(Gd2O3) that are placed in unrodded fuel pin tubes. The gadolinia absorbs neutrons,thus compensating for the high positive reactivity of the core in the fuel cycle. The

absorption of neutrons burns out the gadolinia, thereby reducing the negative reactivityduring the cycle.

Recirculation flow:This method provides up to 25% power changes. Increasing the flow will decrease thenumber of steam bubbles within the core (void fraction), which will increase reactivity viaincreased moderation, thereby increasing power. Decreasing the recirculation flow willincrease the number of steam bubbles in the core, displacing the water that was usedfor neutron moderation. The decreased moderation will decrease the positive reactivity,causing the power to decrease.


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Reactivi ty Coeff icients

Short term changes in reactivity, such as those due to temperature, are often quantifiedin terms of feedbackreactivity coefficients.The reactivity coefficient is the change ofreactivity per unit change in the parameter that alters the reactivity. Common reactivity

coefficients allow quantification of the reactivity effects attributable to moderatortemperature, fuel temperature, voids, and pressure. For example, a fuel temperaturecoefficient (T) is defined as:

where is the change in reactivity and T is a unit change in the fuel temperature.

If a reactor had a positive fuel temperature coefficient, an increase in fueltemperature would result in an increase in reactivity, and an associated increasein power. The power increase would then cause a further increase in fueltemperature, and so on. A positive fuel temperature coefficient would bedetrimental to reactor operations.

Conversely, if a reactor has a negative fuel temperature coefficient, then anincrease in fuel temperature would result in a reduction in reactivity, and acorresponding reduction in reactor power. The reduction in power would lowerthe fuel temperature, which would increase the reactivity and bring the reactorback to a constant power level.
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5-11

Reactor Startup

At the beginning of the startup, the control rods are fully inserted in the core, providing alarge amount of negative reactivity to overcome the large amount of positive reactivity

contained in the fuel. As the rods are withdrawn, the number of neutrons available tocause fission increases, which increases the number of fissions. The control rods arewithdrawn incrementally until the positive reactivity in the core overcomes the remainingnegative reactivity of the control rods. Removal of the control rods continues until thestartup rate desired by the plant operators is achieved.

In this condition, the reactor is slightly super-critical, and will remain super-critical untilthe fuel temperature and moderator temperature begin to increase. As described earlier,those temperature increases will insert negative reactivity, which will offset the reactivityincrease caused by the withdrawal of control rods, and will ultimately return the startuprate to zero. The reactor will then be critical at a much higher rate offlux than waspresent prior to rod withdrawal.
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Ach ieving 100% Power

To achieve 100% power from startup condition in a PWR, the boron concentration in the reactor coolantsystem is reduced, which reduces the negative reactivity in the core, allowing the neutron flux, or powerto increase. This increase will continue until the negative reactivity feedback increase from the moderatortemperature and fuel temperature overcomes the reduction in boron concentration. The operators andreactor engineers are able to calculate the combined effects of the boron, moderator temperature, andfuel temperature to determine the power level at which the reactor will be controlled.

To achieve 100% power from a startup condition in a BWR, the control rods arewithdrawn from the core, allowing the positive reactivity of the core to increase theneutron flux. At about 70% reactor power, variable speed recirculation pumps are usedto increase the flow of water through the core. This increased flow replaces the steamvoids in the core with water, which increases the moderation of the neutrons, addingpositive reactivity to reach 100% power.


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5-13

Reactor Pow er Plant Operation

A reactor is critical when its reactivity is zero. If the change in a system parameterresults in the addition of reactivity (positive or negative) then a corresponding changemust occur in some other parameter to compensate for this reactivity insertion.

The effects on reactor operations caused by changes in the balance of plant parameterscan be viewed from the perspective of the changes in moderator temperature andpressure. For example, a turbine load increase will increase the amount of thermalenergy removed from the reactor coolant system, causing the reactor coolanttemperature to decrease. With a negative moderator temperature coefficient, the drop incoolant temperature will insert positive reactivity. The positive reactivity insertion fromthe moderator temperature coefficient will cause the reactor power to increase, whichwill increase the fuel temperature. With a negative fuel temperature coefficient, this fueltemperature increase will insert an amount of negative reactivity that balances with thepositive reactivity added by the moderator temperature. The reactor will continueoperation with a reduced coolant temperature at an increased power.

In both a PWR and a BWR, this scenario can be created by increased removal ofthermal energy via any of the following processes:

Feedwater termperature decrease

Feedwater flow increase

Steam flow increase


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Void Fraction

Unlike in a PWR, the boiling water in the core of a BWR produces steam voids thataffect reactivity. The effects on reactor operation caused by changes in balance of plantparameters can then be viewed from the perspective of the changes in the fraction of

voids in the core. In a BWR, a decrease in the void fraction results in increasedmoderation of the neutrons because more water occupies the space between the fuelrods. An increase in neutron moderation causes an increase influx due to an increasein power/reactivity.

Any of the following plant parameter changes will cause a decrease in the void fraction,thereby increasing power:

Feedwater temperature decrease

Feedwater flow increase

Steam temperature decrease Steam pressure increase

Steam flow decrease
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5-15

Reactor Shutdow n

To shutdown a PWR from a 100% power condition, the operators increase the boronconcentration of the reactor coolant system. This addition of negative reactivity causesthe reactor power to start decreasing. As the power decreases, the fuel temperature

and moderator temperature also decrease, providing a feedback of positive reactivity.The amount of boron added to the coolant can be varied by the operators to reducepower to a pre-determined level or to completely shut down the reactor. Again, thecombined effects of the boron, moderator temperature, and fuel temperature must becalculated to determine the power level at which the reactor will be controlled.

The build-up of Xe-135 during the power reduction must also be considered. As thepower decreases, less Xe-135 is burned-up by the reduced neutron flux, but the I-135continues to decay to Xe-135 at a rate commensurate with production at 100% power.In short, Xe-135 starts to add negative reactivity to the core while power is beingreduced. The plant operators will have to compensate for this negative reactivityaddition by reducing the boron concentration or withdrawing control rods if the powerdecrease is to be stopped before the reactor reaches zero power.

To shutdown a BWR from 100% power, control rods are inserted to increase theaddition of negative reactivity to decrease the neutron flux to zero power. The corerecirculation flow can also be reduced, which promotes the generation of steam in thecore. The increased voiding displaces the water that was moderating the neutrons.Reducing the moderation results in a reduced number of thermal neutrons for fissions,so reactor power decreases.


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Decay Heat

As you learned in the Radioactivity lesson, one unique feature of nuclear power is thatafter the reactor is shut down and the fission chain reaction has stopped, the fuelcontinues to generate some heat because of the decay of fission products. The amount

of decay heat decreases with time since it originates from radioactive decay processes.Still, over the first several days following a reactor shutdown, adequate cooling must beavailable to remove the decay heat to reduce the potential for core damage. Reactordesigns have specially engineered safeguards to provide such cooling.

Since much of this early decay heat will be from short-lived fission products, the heatproduction rate decreases sharply in the first few hours. For example, about 16 MWt isgenerated after one day of reactor shutdown, and it drops more slowly to about 9 MWtafter the first five days. Without cooling, such thermal power is sufficient to boil the coredry and melt the fuel. Clickhereto view the graph from the Radioactivity lesson thatillustrates the decay heat generation rate as a function of time.

Decay heat generation impacts spent fuel handling and storage, waste management,and reactor safety at a nuclear plant. The production of decay heat long after the reactorhas been shutdown requires cooling systems to provide cooling water flow to the coreduring accident conditions as well as during normal shutdown conditions. Nuclear powerplants use decay heat removal systems to reject that decay heat from the core.

Click the graphic below for operating experience related to decay heat.

At Three Mile Island Unit 2 in 1979, a loss of forced cooling water flow through the core allowed the coretemperature to increase even though the reactor had been shut down for several hours. The thermalenergy generated from radioactive decay increased the temperature in the core to a value that allowedthe fuel cladding to burn and the fuel to melt. That reactor has not been re-started.
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5-17

Loss of Coolant Accidents

Loss of coolant accidents (LOCA) can also occur with severity during routine shutdownconditions. The risk for a LOCA while shutdown is greatest when the reactor coolantsystem inventory has been reduced during a refueling outage, which reduces the

volume of water available to remove decay heat. A loss of forced cooling flow throughthe core in a reduced inventory condition can result in rapid temperature increase of thecoolant and subsequent degradation of the fuel.

To reduce the risk associated with loss of coolant during reduced inventory conditions,the nuclear power plants ensure a sufficient number of pumps with redundant powersupplies are available to provide core cooling water flow. In addition, some activities areprohibited until after the core has been shutdown for a time period that allows the decayheat generation to be reduced to an acceptable level.


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Spent Fuel

Fuel assemblies that are discharged from reactors during refueling are called spent fuel.During the course of the irradiation, much of the U-235 is depleted by fission andtransmutation to U-236. A small fraction of the U-238 changes to plutonium isotopes.

Even smaller amounts of other transuranic isotopes, such as Np, Am, and Cm, are alsoproduced by neutron transmutation. Among the Pu isotopes that are produced, Pu-239and Pu-241 are fissile and contribute to energy production.

The generation of decay heat continues long after the fuel assemblies have beenremoved from the core, so the spent fuel assemblies are stored in spent fuel poolsimmersed in water to remove the decay heat. The water in the spent fuel pools iscirculated through coolers to remove the decay heat. After allowing several years for thedecay of the radioactive isotopes in the spent fuel, the spent fuel assemblies can beremoved from the pools and stored in dry casks.

Fuel pellets that developed cracks during irradiation


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5-19

Review Question 11

How does U-235 depletion affect core reactivity?

A. It decreases linearly with burn-up as a result of U-235 buildup

B. It is not affected by U-235 depletion

C. It increases linearly with burn-up as a result of U-235 depletion

D. It decreases due to U-235 depletion and increases due to Pu production

The correct answer is D.

Core reactivity decreases due to U-235 depletion and increases due to Pu production.


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Review Question 12

Match the type of reactor in Column 2 with the reactivity control method in Column 1.

Control rods A. PWRs only

Burnable poison rods B. BWRs only

Recirculation flow C. Both PWRs and BWRs

Soluble boric acid

The correct matching sequence is CCBA.


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5-21

Review Question 13

Why are changes in the isotopic composition of fuel with burn-up a concern?

A. Because Pu production contributes to nuclear energy generation

B. Because composition changes can alter the neutron economy in the reactor and

consequently affect its operation

C. Because of the large amount of radioactivity emitted and its implications in handling

and disposal of the spent fuel

D. Because of the potential of accidental release of radioactive material from spent fuel

All of these are reasons why changes in the isotopic composition of fuel with burn-upare a concern.


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Conclus ion

Great work! You've completed the Reactor Operation lesson and have learned how thefission process is used to create energy in a nuclear power plant.

Now that you've completed this lesson, you can:

Describe how temperature, pressure, voids, fuel constituents, control rods, boronconcentration, and fission product poisons affect the fission process

Describe how reactor operation affects temperature, pressure, voids, fuelenrichment (burn-up or depletion), boron concentration, and fission productpoisions

Describe how criticality and/or reactivity affect startup, power operations, andshutdown

Describe how changes in various balance of plant parameters affect reactoroperations in a PWR and a BWR

Define decay heat and describe its affect on plant operations

Describe the significance of loss of coolant accidents (LOCA) in all plantconditions


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6-1

6NUCLEAR SAFETYIn troduct ion

Because the possibility for nuclear power plant accidents exists, every measure is takenin the design and operation of nuclear plants in the U.S. to prevent accidents and toassure safety. In this lesson, you will learn about causes of nuclear power plantaccidents and the measures taken to prevent them.

At the end of this lesson, you will be able to:

Identify the types of reactivity control accidents

List the causes of fuel failures

Describe the consequences of fuel failures

Describe three methods used to ensure nuclear safety


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Nuclear Safety

Two significant accidents have occurred in commercial nuclear power plants.

Click each power plant location below to learn about the accident and why it occurred

Chernobyl:In 1986, the Chernobyl nuclear power plant in the Ukraine experienced a steamexplosion (not a nuclear explosion), which destroyed the reactor, killed 31 people, andcaused significant health and environmental consequences. The accident was the resultof major design weaknesses in the reactor, as well as human error. This type of reactoris an older design. The accident would not have happened in a U.S. licensed reactorbecause it would not have met design requirements.

Three Mile Island:

In 1979, at the Three Mile Island (TMI) nuclear power plant in Pennsylvania, a cooling

malfunction caused the majority of the core to melt in an LWR reactor. The reactor wasseverely damaged, but radiation was contained within the containment as designed,and there were no adverse health or environmental consequences. The accident wasattributed to a mechanical failure and a series of compounded human errors. Inaddition, poor valve indication design was a factor.

Every measure is taken in the design and operation of nuclear plants in the U.S. toprevent accidents and to assure safety. The underlying philosophy is to demand veryhigh standards in the design, operation, maintenance, construction, testing, andreliability of systems and components, to employ redundant safety systems, and toemphasize conservative decision-making and sound risk assessment and management.


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6-3

Defense-in-Depth

Defense-in-depth is a design and regulatory philosophy that protects the health andsafety of the public from the uncontrolled release of radioactivity. Defense-in-depth is a

hierarchical set of different, independent levels of protection, as shown in the graphicbelow. This strategy worked at Three Mile Island. At Chernobyl, however, this strategydid not work because there was a weak reactor confinement system, no containmentstructure, and an unstable reactor design.


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Safety Features

In addition to defense-in-depth, there are various features in nuclear power plants thatpromote safety.

Click each feature below to learn more.

High standards for human performance and corrective action:The first line of defense against accidents is the actual workforce at a nuclear plant. Theworkforce must have a diligent attention to reducing human errors, and must promptlywrite up and address equipment and process issues in a corrective action program.Simply put, one should never rely on the built-in barriers as the first line of defense, butthe people and leadership of the plant.

Inherent safety features:Inherent safety features incorporated into the design of nuclear reactors result frombasic physics and properties of matter and do not require operation of any piece ofequipment. An example is the negative fuel temperature coefficient.

Active and passive safety systems:

An example of an active system is Emergency Core Cooling Systems that supplywater for cooling the reactor and remove decay heat.

An example of a passive system is the gravity-driven fall of a control rod in aPWR.

Redundant and diverse systems:

A redundant system has identical back-up components that perform the samesafety task.

Diverse types of systems can act independently to provide similar service.

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basic atomic and nuclear physics

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