The Science ofNuclear Reactors and

the Regulatory Framework

A Presentation to Wake Forest UniversitySchool of Physics

September 12, 2002M.T. Cash

Historical Perspective

Science and Law are often best understood in a historical perspective

The regulation of nuclear power reactors involves science, law, politics and many other influencing forces

A review of the development of science along with the regulatory framework is interesting and illuminating

Discovery of Nuclear Fission

James Chadwick “discovery” of the neutron (1932)

Hahn and Strassman “discovery” of barium atoms resulting from neutron bombardment of uranium (1937)

Frisch and Meitner using ideas from others develop the liquid drop model of fission to explain Hahn Strassman (January 1939)

Mass Defect

Splitting atoms was all very exciting and fascinating in 1939, but practical value?

The observed nuclear mass is always less than the summation of the constituent nucleon masses

This mass defect is embodied in the binding energy of the nucleus pursuant to E = m c2 (A. Einstein 1905)

Application to Uranium Fission

It can be shown that the binding energy per nucleon in Uranium is approximately 7.59 Mev/nucleon

U-235 FPa + FPb + Energy The resultant energy from fission will be found

to be approximately 200 Mev/Fission Combustion of Carbon atom results in an

energy release of approximately 4 ev

Chain Reacting System

Even with the enormous energy release the reaction still needed a supply of neutrons

In early 1939 it became apparent that a number of neutrons (2 to 3) were emitted per fission.

A nuclear chain reaction was possible.

Neutron Slowing Down Theory

Using computers of the day (??) Fermi and others developed a neutron slowing down model

Neutrons born at high energies Fission likelihood (cross section) changes in

relation to neutron energy Neutrons slow down through collisions as they

move through materials (loose energy)

Factors Affecting Chain Reacting System

Composition of Fuel Composition of Surrounding Materials Physical Arrangement of Fuel Physical Arrangement of other Materials Quantity of Fissionable Isotope in Fuel

Four Factor Formula

Simplified Algebraic View of Chain Reaction Material Potential for Self Sustaining Chain Reaction (Infinite Media)

K∞ = ή έ p f ή Average Number of Fast Neutrons έ Neutrons from high to low energy p Resonance Escape Probability f Thermal Utilization

Six Factor Formula Simple algebraic representation estimating the chain reacting

state of a physically real system Keff = K ∞ Pnlth Pnlfst

P represents the non-leakage probability for either fast or thermal neutrons

P represented by a formula largely dependent on physical geometry of system

The solution of the relative geometric size values given certain materials will yield the “critical size” of the reactor

Fermi and colleagues would undertake a famous experiment in this regard

An important point: The systems self sustaining capability is not dependent on the magnitude of the neutron population

Moving from Subcritical to Critical Conditions

The previous discussion focused on a chain reacting system in a a steady state condition

A physical reactor however must be assembled, and started up, hopefully not at the same time

The concept of neutron kinetics or reactor kinetics is useful and fundamental

Basic Kinetics and Inhour It can be shown, that the time dependent

neutron population in a reactor is: Φ(t) = Φ0 e (t/Tp)

Tp is known as the reactor period or time for reactor power to increase by a factor of e.

Reactor period can be estimated from certain changes in Keff

These relationships demonstrate that Tp will establish a stable value shortly after a change in Keff

Basic Kinetics and Inhour

These relationships will show that for a certain range of Keff the reactor will be supercritical on delayed neutrons

For certain large values of Keff the reactor would be supercritical on prompt neutrons (not controllable)

Delayed neutrons come from fission products, some of which have neutrons in the decay scheme. These “delayed neutrons” enter the chain reacting system. This time delayed contribution to the chain reaction acts a natural control mechanism.

Inhour Approximation for Changes in Neutron Population

Neutron population is important because fission rate is proportional to neutron population

Neutron population and fission rate starts low (at an artificial source level)

By changing reactor composition (control rods) the population is allowed to increase (by several orders of magnitude) to increase power

Control rods are re-adjusted to restore criticality and maintain constant power

Basic Kinetics and Inhour

Basic Kinetics and Inhour (Startup of CP 1)

Affects of Fission on Surrounding Materials Fission Products deposit kinetic energy very near the site

of fission. Potential for high localized heat production Heat must be transported from the fuel to avoid melting

the fuel material. Following reactor shutdown, subcritical state, decay of

fission products generate heat. Heat must be removed to avoid melting the fuel

Fission Products may exist in gaseous state in fuel rods and represent a potential hazard

Radiation from fission and decay products requires shielding of immediate environment (Particularly high energy neutrons and gamma)

Development of Modern Methods and Tools Current nuclear analysis for neutron transport, radiation

dose analysis and heat transfer relies on sophisticated computer models

Typical neutron transport is based on neutron group theory or a modified version of group theory.

Heat Transfer and Fluid Mechanics use various numerical methods solution techniques for Navier-Stokes differential equations.

The four factor and six factor formula are largely only of interest from a historical perspective

Reactor Technology Development Chicago Pile Number One First Man Made Chain Reacting System Chicago Squash Court Team led by Enrico Fermi Graphite blocks with Uranium Slugs in the blocks Taken Critical in December 1942 Operated a very low “neutron flux” (power levels) Crude safety features, no physical containment structure

Early Production Reactors By 1943 Large Plutonium production reactors were under

construction in the middle of the Eastern Washington Desert

Design was large 28 by 36 foot graphite cylinder on its side

Approximately 200 tons of small uranium plugs in aluminum tubes (1000)

Approximately 75000 gallons per minute of cooling water flowing over the uranium inside the tubes for cooling

Irradiate each plug for about 100 days then push it out to decay for about 60 days

Plugs would go to chemical separation plants to extract Plutonium for first atomic bomb

Developmental Reactors The Atomic Energy Act of 1946 as amended in 1954

established the Atomic Energy Commission to oversee weapons production and the development of commercial nuclear power

A number of different reactor types were investigated in this early phase of reactor development

The Nuclear Navy Under Admiral Rickover functioned under the AEC umbrella

By 1950 the Navy had focused on a Pressurized Water Reactor Design for a propulsion plant

Prototype PWR plant in 1953 Nautilus launches in 1954 The pressurized water reactors place as dominant reactor

design is set

Shippingport The first commercial nuclear power generating

station Pressurized water reactor design based on

naval prototype Ground broken in 1954 and generated

electricity to the grid in 1958 Part of President Eisenhower’s Atoms to

Peace Program Concept of Fission Product Barriers as a

Primary Safety Concept

Light Water Reactors

The United States 103 power reactors are all light water reactors

The moderator/coolant is ordinary water There are two designs, boiling water

reactor (BWR) and pressurized water reactor (PWR)

The PWR is the dominant reactor design

Typical Pressurized Water Reactor

Safety Considerations in Early Reactor Designs Defense in Depth and Fission Product Barriers

• The fuel cladding• The reactor coolant boundary• Containment structure

An early AEC analysis of severe accidents raised concern over large scale fission product release

Concerns in the public regarding atmospheric weapons testing became more pronounced

Remote siting and engineered safety features as alternatives were debated in early decisions

AEC at that time had a dual mandate, reactor technology development and reactor safety regulation

Development of Reactor Regulation In the 1940s and 1950s a form of governmental entity

was emerging, arguably as the dominant “branch of government”

The administrative agency was becoming a major policy setting, law making, law enforcing entity

Administrative Agencies typically have congressional delegations of power that allow rulemaking, adjudication and enforcement

These powers are parallel to legislative, judicial and executive powers

The emergence of administrative agencies is coincidental in time with the early growth of commercial atomic energy

The Atomic Energy Act The 1946 Act created the Atomic Energy Commission coming

out of World War II with exclusive control of nuclear weapons and nuclear reactor technology vested in the federal government. No civilian use authorized at that time.

The Atomic Energy Act of 1954 established the legal framework allowing commercial operation of nuclear reactors

The 1954 Amendment to the Act established a dual development and regulatory role for AEC regarding commercial reactors.

The dual role of development and oversight became increasingly problematic over a period of years

In 1974 the Energy Reorganization Act of 1974 abolished the AEC gave the weapons production responsibility to ERDA and created the Nuclear Regulatory Commission (NRC). The promotional role for government was largely abolished.

Nuclear Regulatory Commission

The NRC is a “typical” Commission based administrative agency

There are five commissioners, nominated by the President and approved by the Senate

There is a Chairman of the Commission There is a professional staff of non-political

appointees (approximately 2000 now) The professional staff act in response to

general policy direction of the Commission

Regulatory Framework The NRC has authority to:

• Promulgate rules• Issue Violations and Civil Penalties• Grant Licenses to Operate Reactors• Suspend or Revoke Licenses to Operate Reactors• Issue Orders • Conduct adjudicatory hearings

Federal Agency Regulations are in the Code of Federal Regulations (CFR)

NRC regulations are in Title 10 Energy (10 CFR)

NRC Regulatory Framework

NRC Staff and Commission Authority can be placed in two broad categories• Licensing Activities

• Centered in Washington DC in the Office of Nuclear Reactor Regulation.

• Maintain and Issue Revisions to Operating Licenses

• Enforcement and Inspection Activities• Centered in Four Regional Offices with Resident

Inspectors at Each Nuclear Power Reactor.

• Southeast Region (Region IV) is Headquartered in Atlanta

Key Regulations as Relate to Underlying Science and External Forces General Design Criteria

• 10 CFR 50 Appendix A The GDC were developed in the early phases of licensing initial

power reactors after the 1954 Act. Give objective criteria which licensees were to include in the

design of facilities Criterion 10 through 19 (Under the General Heading Protection

by Multiple Fission Product Barriers) examples• 14 Reactor Coolant Pressure Boundary integrity standards• 16 Containment Design

These fission product barrier “regulations" reflect the scientific principles associated with the cladding, coolant boundary and containment vessel as engineered features

Siting Reactors 10 CFR 100.11 As noted earlier scientific analysis had calculated potential

consequences of hypothetical accidents In addition, public concern arising mainly from weapons fallout

emphasized the focus on public safety concerns As a means of addressing the issue a regulation and review

process surrounding reactor siting was put into place• Meant to weigh and consider, actual site relative to population• Engineered Safety Features

Current Regulation is 10 CFR 100.11 “Factors to be Considered When Evaluating Site”• Sets radiation dose limits following certain postulated accidents• Exclusion Area Two Hour Dose Limits• Low Population Zone with same Limits for the entire period of the

accident

Three Mile Island and Hydrogen Generation (10 CFR 50.46) Approximately 4:00 am on March 28, 1979 an unexpected but non-

emergency automatic reactor shutdown (trip or scram) occurred at TMI Do to a number of human errors and design problem a relatively minor

operational event cascaded into a major core damage event Late into the sequence of events concern arose regarding an

unanticipated buildup of hydrogen gas in containment Experts debated at that time whether the hydrogen gas could reach

“explosive levels” Combustible levels did exist however the explosive concentrations and

conditions likely did not exist (4% by volume in air) The hydrogen was generated from a high temperature water zirconium

reaction. (Fuel clad is made of Zirconium) Normal operating temperatures of the clad during operation are

approximately 700 to 800 F0

In the accident conditions at TMI the cladding exceeded the melting point of Zirconium and the temperature for rapid hydrogen generation

Three Mile Island and Hydrogen Generation (10 CFR 50.46)

The Accident at Three Mile Island was followed by extensive changes in regulations and regulatory oversight• Including the Promulgation of 10 CFR 50.46

(Acceptance Criteria for ECCS)

• Includes limits for peak clad temperature, maximum cladding oxidation and maximum hydrogen generation

• Which might result from loss of cooling accidents

Concluding Remarks The nuclear power industry in the United States is extensively

regulated by the NRC The Three Mile Island Accident had no documented health

effects but resulted in a loss of public trust and led to significant improvements in operations, training, emergency planning and regulation

The existing regulatory framework is largely a reflection of the initial development of reactor technology and science, political forces surrounding initial reactor licensing and the Three Mile Island Accident

The technology is fundamentally sound from a public policy perspective• No greenhouse gases• Cost competitive with coal for electrical generation• Domestic Source of Energy with stable fuel cost