Chapter 19Nuclear Reactions

The Nucleus

• Remember that the nucleus is comprised of the two nucleons, protons and neutrons.

• The number of protons is the atomic number.• The number of protons and neutrons together

is effectively the mass of the atom.

Isotopes

• Not all atoms of the same element have the same mass due to different numbers of neutrons in those atoms.

• There are three naturally occurring isotopes of uranium:Uranium-234Uranium-235Uranium-238

Radioactivity

• It is not uncommon for some nuclides of an element to be unstable, or radioactive.

• We refer to these as radionuclides.

• There are several ways radionuclides can decay into a different nuclide.

Alpha Decay:

Loss of an -particle (a helium nucleus)

He42

U23892

U23490 He4

2+

Beta Decay:

Loss of a -particle (a high energy electron)

Where does the -particle come from?

0−1 e0

−1or

I13153 Xe131

54 + e0

−1

Positron Emission:

Loss of a positron (a particle that has the same mass as but opposite charge than an electron) Where does the positron come from?

e01

C116

B115 + e0

1

Gamma Emission:

Loss of a -ray (high-energy radiation that almost always accompanies the loss of a nuclear particle)

00

Electron Capture (K-Capture)

Addition of an electron to a proton in the nucleusAs a result, a proton is transformed into a

neutron.

p11 + e0

−1 n1

0

Neutron-Proton Ratios

• Any element with more than one proton (i.e., anything but hydrogen) will have repulsions between the protons in the nucleus.

• A strong nuclear force helps keep the nucleus from flying apart.

Neutron-Proton Ratios

• Neutrons play a key role stabilizing the nucleus.

• Therefore, the ratio of neutrons to protons is an important factor.

Neutron-Proton Ratios

For smaller nuclei (Z 20) stable nuclei have a neutron-to-proton ratio close to 1:1.

Neutron-Proton Ratios

As nuclei get larger, it takes a greater number of neutrons to stabilize the nucleus.

Stable Nuclei

The shaded region in the figure shows what nuclides would be stable, the so-called belt of stability.

Stable Nuclei

• Nuclei above this belt have too many neutrons.

• They tend to decay by emitting beta particles.

Stable Nuclei

• Nuclei below the belt have too many protons.

• They tend to become more stable by positron emission or electron capture.

Stable Nuclei

• There are no stable nuclei with an atomic number greater than 83.

• These nuclei tend to decay by alpha emission.

Radioactive Series

• Large radioactive nuclei cannot stabilize by undergoing only one nuclear transformation.

• They undergo a series of decays until they form a stable nuclide (often a nuclide of lead).

Nuclear Transformations

Nuclear transformations can be induced by accelerating a particle and colliding it with the nuclide.

Particle Accelerators

These particle accelerators are enormous, having circular tracks with radii that are miles long.

Kinetics of Radioactive Decay• The kinetics of radioactive decay obey

this equation:= kt

Nt

N0

ln

• The half-life of such a process is:

= t1/2 0.693

k• Comparing the amount of a radioactive

nuclide present at a given point in time with the amount normally present, one can find the age of an object.

Measuring Radioactivity

• One can use a device like this Geiger counter to measure the amount of activity present in a radioactive sample.

• The ionizing radiation creates ions, which conduct a current that is detected by the instrument.

Energy in Nuclear Reactions• There is a tremendous amount of energy

stored in nuclei.• Einstein’s famous equation, E = mc2, relates

directly to the calculation of this energy.• In the types of chemical reactions we have

encountered previously, the amount of mass converted to energy has been minimal.

• However, these energies are many thousands of times greater in nuclear reactions.

Energy in Nuclear Reactions

For example, the mass change for the decay of 1 mol of uranium-238 is −0.0046 g.

The change in energy, E, is then

E = (m) c2

E = (−4.6 10−6 kg)(3.00 108 m/s)2

E = −4.1 1011 J

Nuclear Fission

• How does one tap all that energy?• Nuclear fission is the type of reaction carried

out in nuclear reactors.

• Bombardment of the radioactive nuclide with a neutron starts the process.

• Neutrons released in the transmutation strike other nuclei, causing their decay and the production of more neutrons.

• This process continues in what we call a nuclear chain reaction.

Nuclear Fission

Nuclear Fission

If there are not enough radioactive nuclides in the path of the ejected neutrons, the chain reaction will die out.

Therefore, there must be a certain minimum amount of fissionable material present for the chain reaction to be sustained: Critical Mass.

Nuclear Reactors

In nuclear reactors the heat generated by the reaction is used to produce steam that turns a turbine connected to a generator.

SCRAMThe sudden shutting down of a nuclear reactor, usually by

rapid insertion of control rods, either automatically or manually by the reactor operator. May also be called a reactor trip. It is actually an acronym for "safety control rod axe man," the worker assigned to insert the emergency rod on the first reactor (the Chicago Pile) in the U.S. http://www.nrc.gov/reading-rm/basic-ref/glossary/scram.html

Nuclear Reactors

• The reaction is kept in check by the use of control rods.

• These block the paths of some neutrons, keeping the system from reaching a dangerous supercritical mass.

Fissionable Material• fissionable isotopes include U-235, Pu-

239, and Pu-240• natural uranium is less than 1% U-235

rest mostly U-238not enough U-235 to sustain chain reaction

• to produce fissionable uranium the natural uranium must be enriched in U-235to about 7% for “weapons grade”to about 3% for reactor grade

Nuclear Power Plants - Core• the fissionable material is stored in long tubes,

called fuel rods, arranged in a matrixsubcritical

• between the fuel rods are control rods made of neutron absorbing materialB and/or Cdneutrons needed to sustain the chain reaction

• the rods are placed in a material to slow down the ejected neutrons, called a moderatorallows chain reaction to occur below critical mass

Pressurized Light Water Reactor• design used in US (GE, Westinghouse)

• water is both the coolant and moderator

• water in core kept under pressure to keep it from boiling

• fuel is enriched uraniumsubcritical

• containment dome of concrete

Cooling Tower

Nuclear Power

• Nuclear reactors use fission to generate electricityAbout 20% of US electricityThe fission of U-235 produces heat

• The heat boils water, turning it to steam

• The steam turns a turbine, generating electricity

Nuclear Power Plants vs. Coal-Burning Power Plants

• Use about 50 kg of fuel to generate enough electricity for 1 million people

• No air pollution

• Use about 2 million kg of fuel to generate enough electricity for 1 million people

• Produces NO2 and SOx that add to acid rain

• Produces CO2 that adds to the greenhouse effect

Concerns About Nuclear Power

• core melt-downwater loss from core, heat melts coreChina SyndromeChernobyl

• waste disposalwaste highly radioactive reprocessing, underground storage?Federal High Level Radioactive Waste Storage

Facility at Yucca Mountain, Nevada • transporting waste• how do we deal with nuclear power plants

that are no longer safe to operate?

Fusion

+ +

21H 3

1H 42He 1

0n

deuterium + tritium helium-4 + neutron

Nuclear Fusion• Fusion is the combining of light nuclei to

make a heavier one• The sun uses the fusion of hydrogen isotopes

to make helium as a power source• Requires high input of energy to initiate the

processBecause need to overcome repulsion of positive

nuclei • Produces 10x the energy per gram as fission• No radioactive byproducts• Unfortunately, the only currently working

application is the H-bomb

Nuclear Fusion

• Fusion would be a

superior method of

generating power. The good news is that the

products of the reaction are

not radioactive. The bad news is that in order to achieve fusion, the material

must be in the plasma state at several million kelvins.

• Tokamak apparati like the one shown at the right show promise for carrying out these reactions.

• They use magnetic fields to heat the material.

Radiation Exposure

Medical Uses of Radioisotopes,Diagnosis

• radiotracerscertain organs absorb most or all of

a particular elementcan measure the amount absorbed

by using tagged isotopes of the element and a Geiger counter

use radioisotope with short half-lifeuse radioisotope low ionizing

• beta or gamma

Nuclide Half-life Organ/SystemIodine-131 8.1 days thyroidIron-59 45.1 days red blood cellsMolybdenum-99 67 hours metabolismPhosphorus-32 14.3 days eyes, liverStrontium-87 2.8 hours bonesTechnetium-99 6 hours heart, bones, liver,

lungs

Medical Uses of Radioisotopes,Diagnosis

• PET scanpositron emission tomographyC-11 in glucosebrain scan and function

Medical Uses of Radioisotopes,Treatment - Radiotherapy

• cancer treatment cancer cells more sensitive to radiation

than healthy cells1. brachytherapy

• place radioisotope directly at site of cancer

2. teletherapy• use gamma radiation from Co-60 outside to

penetrate inside

3. radiopharmaceutical therapy• use radioisotopes that concentrate in one

area of the body