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