Core Unit IV: Nuclear Physics
A. Natural Radioactivity
Key Concepts
Becquerel accidentally discovered that uranium compounds caused a photographic plate to
become fogged. (He was investigating the relationship between x-rays and fluorescence using
crystals of uranium potassium sulphate.)
Radioactivity is the spontaneous disintegration of an unstable atomic nucleus and the emission
of particles or electromagnetic radiation.
Pierre and Marie Curie investigated uranium ores using chemical separation. They discovered
that pitchblende and chalcocite, naturally occurring ores, were highly radioactive due to the
presence of plutonium and radium.
All naturally occurring elements with atomic numbers greater than 83, as well as some isotopes
of lighter elements, are radioactive.
Based on later work by Rutherford, Soddy, Villard, and others, three different types of radiation
were identified.
Alpha particles
are helium nuclei, containing two protons and two neutrons. They are
deflected slightly in an electric of magnetic field. Their penetrating power is very low, being
stoppable by a thin sheet of aluminum or paper.
Beta particles
are electrons capable of travelling at speeds approaching the speed of light.
Their low mass allows them to be deflected greatly in an electric or magnetic field, in the
opposite direction as the deflection of alpha particles. Their high speed gives them greater
penetrating power than alpha particles. Some beta particles can penetrate several centimetres of
aluminum. (Some texts refer to beta particles as "beta negative particles", to distinguish them
from beta positive particles -- positrons.)
Alpha particle emissions and beta particle emissions change the composition of the nucleus.
Gamma rays
are high energy electromagnetic radiation with short wavelengths. Gamma
rays, unlike alpha and beta particles, do not change the composition of the nuclide. They have
the highest penetrating power, being able to penetrate at least 30 centimetres of lead.
All radioactive nuclides have the following common characteristics:
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Their radiations affect the emulsion of photographic film, ionize surrounding air
molecules, make certain compounds fluoresce, and have certain special biological effects.
They undergo radioactive decay.
Radioactivity is found in naturally occurring sources and in artificially produced ones.
People are constantly being exposed to radiation from a variety of natural and human-created
sources. Exposure should be minimized, but it can never be reduced to zero.
Some commonly used symbols for subatomic particles are:
neutron
proton
electron (beta particle)
positron
(A positron is a particle much the same as an electron, but with a positive charge. It is an
example of "antimatter".)
(alpha particle)
gamma ray (photon)
Radioactivity can not be detected with our senses. Special detectors are needed. Because it can
not be detected by human senses it is particularly dangerous; one may unknowingly be exposed
to it for prolonged periods of time. Radiation has an effect on tissue and on genetic material.
Several devices have been developed to detect radioactivity, with the earliest being an unexposed
photographic plate placed in the vicinity of a source being detected. Other devices include a
Wilson cloud chamber, electroscopes, ionizing chambers, the Geiger-Muller tube, liquid and
electronic bubble chambers, scintillation detectors (spinthariscope), and solid state
semiconductor devices.
Dosimetry is the measurement of radiation and the study of its effects on living organisms.
There are several different units used to measure radiation.
The absorbed dose describes the amount of energy deposited per kilogram of exposure time,
measured in the gray (Gy).
1 Gy = 1 J/kg = 100 Rads
(Rads are non-SI, but in general use.)
The biological damage produced on a given organism is called the dose equivalent, measured in
sieverts (Sv).
1 Sv = 100 rem = 105 mrem
(rem -- rad equivalent man)
dose equivalent(Sv) = absorbed dose(Gy) x a quality factor(Q)
The quality factor is a number assigned to each type of radiation to describe its biological
effects.
The effect that absorbed radiation has on different types of tissues varies. Furthermore, there is
disagreement by scientists about the cumulative effects of low dosage exposure to radiation. For
these reasons, no exposure to radioactive emissions, for any period of time, should be regarded
as being "safe" to humans or other living organisms. Much research is still needed into the longterm biological effects of radiation.
The becquerel (Bq) is the activity of a source produced when one disintegration per second
occurs from a radioactive source.
1 Bq = 1 disintegration per second
kBq and MBq are often used to express the radioactivity of a source. This unit does not make
any distinctions between the effects of different types of radiation.
1 curie (Ci) = 3.7 x 1010 Bq
Core Unit IV: Nuclear Physics
B. Nuclear Fission
Key Concepts
A neutron can be captured by the nucleus of some heavy atoms. The nucleus then becomes
unstable and splits. Other neutrons are released when the nucleus splits.
Fission is the term used to describe the splitting of a heavy nucleus into two or more smaller
nuclei.
Slow moving neutrons are more easily captured by the nucleus. A moderator is a medium which
causes neutrons to travel more slowly.
Graphite, heavy water, and beryllium are all excellent moderators, capable of slowing neutrons
without absorbing them.
The neutrons liberated by fission travel very quickly unless moderated.
A very large amount of energy is released when an atom undergoes fission. ( 200 MeV)
In a typical fission reaction, the energy released is distributed as follows: 170 MeV of kinetic
energy of fission fragments, 5 MeV of kinetic energy of neutrons, 15 MeV of energy beta
particles and gamma rays, and 10 MeV as energy of antineutrinos.
An example of a typical fission is:
Mass is not conserved in a nuclear reaction. The products formed during nuclear fission have a
slightly lower mass, due to the nuclear mass defect. This nuclear mass defect can be used to
determine the nuclear binding energy which held the heavier nucleus together and was released
when fission occurred.
The energy released by a fission can be calculated by finding the difference between the mass of
the parent atom and neutron, and the masses of the daughter atoms and emitted neutrons, and
converting this mass "loss" into energy using .
Neutrons released when an atom undergoes fission are capable of causing other nuclei to
undergo
fission, if the neutrons are slowed down by a moderator.
A sustained fission reaction caused in this way is called a chain reaction.
Natural uranium ore contains about 0.7% uranium-235. To increase the likelihood of sustaining a
chain reaction for uranium, the fissionable isotope of uranium must be increased in its relative
proportion through enrichment.
A nuclear reactor produces a sustained chain reaction at a controlled rate. The heat energy
produced by the reaction is used to drive turbines, generating electricity. (Refer to section C.)
Control rods, made of materials such as cadmium which absorb neutrons, are used to control the
rate of a chain reaction in a nuclear reactor.
A critical mass of fissionable material is the minimum mass that will produce a nuclear
explosion. To produce a sustainable nuclear chain reaction requires more material than the
critical mass.
An atomic bomb explodes when two or more sub-critical masses of fissionable material are
brought together very rapidly. Chemical explosives are used to implode the sub-critical masses
together to form a mass larger than the critical mass.
An atomic bomb produces devastating destruction. Its explosive force is measured in terms of
the comparable number of megatons of conventional explosives that would be needed to produce
similar results.
Nuclear weapons produce radioactive contamination of the environment. For this and other
reasons many countries have banned atmospheric testing of these weapons.
The first atomic bombs, developed and tested by the United States during the Manhattan Project
in World War II, were dropped on the Japanese cities of Hiroshima and Nagasaki in 1945. Over
110 000 people were killed and many others suffered from the effects of the explosions for years
afterwards. Japan surren- dered shortly after the atomic bombs were dropped, bringing the war to
an end.
Leo Szilard, one of the developers of the atomic bomb, recommended that it be tested before an
inter- national audience of observers prior to being used, offering the Japanese a chance to
surrender beforehand. Whether or not the atomic bomb should have been used is an issue worthy
of debate.
Today the nuclear arsenals of the superpowers contain such vast supplies of nuclear weapons
that, according to one scenario, if a large proportion of them were deployed simultaneously, it
would render the planet virtually uninhabitable by humans. Contemporary societal reactions to
this issue are growing.
Should scientists ultimately help bring about an understanding that nuclear weapons are
immoral? Do such weapons threaten the existence of all forms of life on Earth? These are
questions worth pondering.
Should a scientifically literate society help to reduce the potential threat of nuclear war? Can
nuclear weapons be thought of as a "deterrent" if their destructive capabilities could be so severe
that it may be unreasonable to consider their use for solving international conflict?
Core Unit IV: Nuclear Physics
C. Nuclear Reactors
Key Concepts
The CANDU reactor (Canadian deuterium uranium) uses uranium, bundled in the form of
uranium oxide fuel pellets, to produce electricity. (A comparison between CANDU reactors and
other types of reactors would be an interesting optional extension topic.)
Saskatchewan has abundant deposits of uranium ore which is refined for use in nuclear reactors.
The refined uranium oxide fuel pellets are stacked into cylindrical rods. The rods are arranged
into a fuel bundle which is then ready to be placed in special pressure tubes inside the reactor.
The reactor vessel is called the calandria.
Nuclear reactors can not explode like a nuclear bomb. Even under a worst-case scenario, with a
core meltdown, a critical mass of fuel would not be present and the fuel would burn into the
ground. (This, of course, would lead to very serious consequences, including possible loss of life
and environmental damage.)
Refuelling can be done by removing fuel bundles from the pressure tubes and replacing them
with new bundles. In a CANDU reactor this can be done without having to shut the reactor
down.
Heavy water is used as the moderator in a CANDU reactor. Heavy water contains deuterium,
an isotope of hydrogen having one neutron in the nucleus. Heavy water also transfers heat from
the fuel into a heat exchanger which heats ordinary water to produce steam.
The steam produced is used to turn turbines which are connected to electric generators.
Condensers change the steam back into water so it can be cycled back to the steam generator.
Some experts believe that the design of the CANDU reactor makes them safer than other types of
nuclear reactors.
If excess heat builds up in the calandria, the heavy water can be drained out. This causes the
chain reaction to stop, because the moderator is no longer present.
Supporters of the use of nuclear energy feel that it is a safe and effective way to produce energy.
With the demand for energy increasing, and the problems associated with burning fossil fuels,
such as acid precipitation and the greenhouse effect, they regard the use of nuclear energy as
being necessary.
Nuclear energy avoids some of the problems of generating hydro-electric power. Flooding land
to build dams creates environmental and social problems.
The use of nuclear energy may avoid the need for long transmission lines. Nuclear plants can be
built in relatively close proximity to where the power is needed.
Nuclear energy produces very small amounts of waste by volume. The radioactive materials can
be concentrated for storage and monitoring in one place. Poisonous metals (such as arsenic, lead,
and mercury), toxic gases, carbon dioxide, and fly ash are not released into the atmosphere.
Critics of the use of nuclear energy cite various problems with its use. The opposition to the use
of nuclear energy has grown so strong in recent years, that some reactors have been shut down.
Other reactors scheduled for development have been delayed or were never completed because
of the social and political pressure exerted by the antinuclear lobby. The debate continues.
The Chernobyl nuclear accident lead to a justifiable scepticism about any claims of the safety of
nuclear reactors, particularly if those claims come from spokespersons of the industry, who often
cite the strict controls and regulations faced by the industry.
CANDU reactors need to be built near a large body of water. Fresh water is circulated through
the condensers. Excess heat is returned to the source. Raising the temperature reduces the oxygen
content of the water, creating an environmental stress on many kinds of living organisms. The
heated water does, however, offer some possibilities for commercial aquaculture, allowing for
warm-water species to be harvested in colder regions. The excess heat can also be used for
commercial greenhouses or other applications. Air cooling is possible.
Critics also suggest that mining safety is an issue with the use of nuclear energy. The ore is
slightly radioactive. Radon gas is often present at the mine site. The disposed tailings contain
trace amounts of uranium. Unless they can be disposed of properly, they can cause ground water
contamination and environmental damage to the land on which they are dumped.
The entire cycle, from mining the fuel to its eventual disposal after use, is called the nuclear fuel
cycle.
Used nuclear fuel is both hot and radioactive. It is stored under water in large cooling pools for
up to two years after use, until it cools. Some of the used fuel will still remain radioactive for up
to several thousand years. This concerns many people.
The storage of used fuel is a contentious issue for those concerned about the protection of the
environment. No ideal solution has yet been developed to dispose the waste. Current proposals
for waste management merely offer temporary storage solutions until better methods become
available.
Storage of waste in underground salt mines offers one possible solution. Formations in the
Canadian Shield offer other possibilities. The area being considered as a storage site must be dry
and relatively free of earthquake and volcanic activity.
Decommissioning of nuclear reactors, once they have completed their useful service, is another
issue frequently raised by opponents to the use of nuclear energy.
One of the waste materials from a nuclear reactor is plutonium. It is known to cause cancer in
extremely small quantities. It is also used to make nuclear weapons. Some argue against the use
of nuclear reactors because they provide a country with the potential to build a nuclear weapons
arsenal. (An interesting anecdote is that of the development of India's first atomic bomb, which
occurred partially as a result of their having purchased the rights to the CANDU technology from
Canada. This occurred in spite of India having been under a contractual obligation not to exploit
CANDU reactor technology for anything other than peaceful uses.)
The use of nuclear energy is controversial. Individuals need to have a solid knowledge base
before taking a stand on the issue. One needs to weigh the risks against the benefits involved.
Once an individual has taken a position regarding the use of nuclear energy, responsible action is
needed to persuade other citizens to adopt a similar position.
Arguments for or against the use of nuclear energy should be based on reason -- not emotion.
One needs to remain open-minded, listening carefully to the arguments presented by those who
hold a different position.
If one examines the uses of energy since before the Industrial Revolution, it becomes apparent
that the major source used has changed throughout time, based on economics, the development
of new technologies, and a variety of other factors. Some of these same factors are at work today,
determining which sources of energy will be most advantageous to use in the future.
A concern for the protection of the environment needs to play a prominent role whenever
decisions which might have an adverse affect on the environment are being considered.
Alternative solutions to problems need to be examined with regard to their environmental
impact.
One very important strategy is to promote conservation. Instead of demanding more and more
energy, at the expense of the environment and our resources, individuals, institutions, and
government all have to search for ways to conserve energy. If everyone strives to use energy
wisely, existing resources will last longer. Less damage to the environment will occur. Can we
reach sustainable development?