The Challenges of Nuclear Power
Nuclear Power plants generate large quantities of highly radioactive material. This is due to the left over isoptopes (atoms) from the splitting of the atom and the creation of heavier atoms, like plutonium, which the Nuclear Power plant does not utilise. It is called nuclear waste. The actual quantity of waste output is some 100,000 times less than a Fossil Fuel plant but it is much more radioactive.
Humans are exposed to low level radioactivity constantly from naturally occuring radioactive isotopes and cosmic rays from outer space. However, in large doses, radiation has many harmful effects. Therefore it necessary for Nuclear Power plants to in-build many safety mechanisms in order to keep the population safe. This includes the workers as well as humans living around the nuclear power plant. It is also necessary for independant parties to monitor Nuclear Power plants. This ensures that plants adhere to world safery standards and to make sure none of the waste plutonium is diverted for use in nuclear weapons. The International Atomic Energy Agency (IAEA) have developed programs to detect such activity. Nuclear Power Plants in France, Sweden, Canada and Finland have shown that it is possible for the generation of electricty through nuclear power to be extremely safe. Although other nuclear power plants such as Three Mile Island and Chernobyl have had disastrous accidents, it is important to put them into context. The Three Mile Island accident, which destroyed the economic value of the plant, was caused by design flaws and poor operator training. Nevertheless most of the radioactivity was contained at the site. The Chernobyl accident was caused by numerous inherent design flaws, poor operator training and a total disregard for safety.
Another challange for nucear power is dealing with the left over, highly radioactive and long lived nuclear waste. It is necessary to isolate the waste from humans and evironment for about 100,000 years before it decays to safe levels. The consensus amongst the Nuclear Power industry is that radioactive waste should be isolated by multiple barriers and placed deep underground. However other strategies involving waste transmutation are being investigated.
Accidents at Nuclear Power Plants
Like any large scale industrial activity, there have been numerous accidents and mistakes at Nuclear Power plants and reprocessing facilities. A partial list of these is available from this wikipedia link
However two large accidents have had the greatest impact on global consciousness regarding Nuclear Power. These are:
Here are additional links that describe the after effects of the Chernobyl accident.
The Chernobyl accident resulted from a lack of "safety culture" at the plant, design flaws in the RBMK reactor and a violation of procedure. The lessons learned from Chernobyl and the phasing out of the older reactor design means that an accident of this type is most unlikely to occur again.
Since the contained meltdown at Three Mile Island reactor, many improvements have been made to Nuclear Reactor design. The third generation reactors currently proposed are designed so that a failure leading to a contained core melt-down, (which would destroy the commercial value of the reactor) should occur at the rate of 1 in 2-million reactor-years
. If the cost of a new reactor is 2 billion dollars and operates for 60 years, this risk would amount to an extra $80,000 insurance.
After the events on September 11th, 2001, many people were reasonably concerned about the safety of nuclear power plants from terrorist activities. However the containment facilities of nuclear reactors
and the structures holding spent fuel rods
are very strong and make hard terrorist targets.
The biological effects of heavy particle ionising radiation are measured with a quantity called Absorbed Dose. However, while two differing particles may have the same amount of energy, they may have very different responses from our living, biological cells, so a factor called Relative Biological Effectiveness (RBE)
is also used. RBE is a measure of the amount of cell damage a given type of particle causes as compared to a dose of x-rays with the same energy. The combination of RBE with Absorbed dose is measured in units called Sieverts.
| Estimated dose (Micro-Sieverts) || Activity |
| 5 || sleeping next to your spouse for one year |
| 10 || a year of watching TV at an average rate |
| 10 || a year of wearing a luminous dial watch |
| 10 || a year of living in the USA from nuclear fuel and power plants |
| 10 || a day from background radiation (average, varys a lot throughout the world) |
| 20 || having a chest x-ray |
| 65 || flying from Melbourne to London, via Singapore |
| 300 || Yearly dose due to body's potassium-40 |
| 460 || maximum possible offsite dose from Three Mile Island Accident |
| 400 - 1000 || Average annual dose from Medical sources |
| 7,000 || having a PET scan |
| 8,000 || having a chest CT (CAT) scan |
| 50,000 || off-site dose from accident at Chernobyl Nuclear Power Plant (estimates vary widely) |
| 2,000,000 || Typical single dose to Cancer region from Radiation Therapy |
| 700,000 - 13,000,000 || staff and firefighters at the Chernobyl Nuclear Power Plant during and immediately after the accident |
| 65,000,000 || Typical total dose to Cancer region from Radiation Therapy |
Background, naturally occurring environmental sources of radiation include cosmic radiation (which occurs when subatomic particles from outside the solar system interact with the Earth's atmosphere and produce a shower of gamma rays, neutrons and leptons), terrestrial radiation (due to naturally occurring radionuclides such as uranium, radium and thorium which are present in rocks, soil and water), internal radionuclides (most common is potassium-40 which emits beta and gamma particles as it decays, and may be found as a fraction of the total amount of potassium in the body). The largest source of our dose from background radiation is from radon-220 and radon-222 gases, which are the airborne products of the decay of terrestrial uranium. Inhalation of radon gas contributes approximately two-thirds of the dose that is received from natural, background sources.
Biological Effects of Radiation.
Thus we can see that during our normal daily activities we are continually
exposed to ionising radiation both from natural and human-made sources.
The passage of ionising radiation through the body may produce adverse
biological effects. The effects of concern are primarily, although not
solely, due to damage to the genetic material inside the cell; the DNA
or `double-helix' molecule that carries genetic information. The current
scientific data suggests that there is no safe minimum, or threshold, for adverse
radiation effects on the DNA of biological systems and that even small
doses can produce consequences for the organism. The low-dose response
of biological systems it believed to be linear - that is, smaller
doses of radiation produce a proportionately smaller risk of adverse
effects like a straight-line graph.
The biological effects fall into the following categories;
Cell Death or Apoptosis
- biological systems are composed
of many individual tiny cells; each with its own DNA (only
red blood cells don't have any DNA - they lose it during development).
If there is catastrophic damage to vital cell function by radiation
energy absorption the cell may cease to function and `die'.
- in this case the DNA is damaged or altered
but the alteration is not lethal to the cell. If the normal regulator
genes which control the rate at which cells divide and die are rendered
malfunctioned the cell may become `immortal' and multiply at an
abnormal rate producing an out-of-control growth of a line of abnormal
cells. This is a cancer. Most tissues can produce cancers with enough
radiation damage but rapidly dividing tissue lines, such as blood-forming
`haemopoietic' lines which may produce leukemias, are particularly vulnerable. The
risk for cancer production in the general population is estimated
to be approximately 0.06 cases per million Micro-Sieverts of absorbed dose.
Genetic Damage to Future Generations
- this can arise because
mutations, or changes in the pattern of bases in the DNA, can occur
in the DNA that ends up in sperm or eggs and becomes a permanent
feature of any resulting babies. Most often, radiation induced damage
to such egg or sperm DNA is incompatible with the life of the fetus
in utero but there is a finite chance of a live baby being born with
defects. This is of particular concern because the damage to the genetic
material can then be passed on to all future generations and become a
permanent feature of the gene-pool; damaging many individuals. Of course,
such mutations are also a natural part of life and the evolution of
biological systems. The concern is that we do not want to increase the
mutation rate above the natural background rate. The estimated risk
of permanent damage to a second generation (grandchild) individual
is 0.02 cases per million Micro-Sievert of exposure.
Dose-Response Tissue Reactions
- or `radiation burns'. These
are the sorts of effects seen immediately after the bombing of
Hiroshima; but lesser effects can occur with smaller doses. However, there
does seem to be a `threshold' for this kind of effect with very small
doses encountered in normal life (apart from sun-burn!) not producing
More information on the biological effects of radiation can be found
at the International Commission on Radiological Protection
A Study on Childhood Cancers in Great Britain
A study on the incidence of childhood cancer around nuclear power plants in Great Britain by the Health Protection Agency for the committee on Medical Aspects of Radiation in the environment concluded that there have been no increase in childhood cancers for children living less than 25km from a nuclear power plant. The report was published in 2005.
Download the report here
Waste from Nuclear Power
What is Nuclear Reactor Waste?
The generation of electricity in a Nuclear Power Plant is made by splitting uranium atoms. The uranium used as fuel in a nuclear plant is formed into ceramic pellets about the size of the tip of your little finger. The uranium atoms in these pellets are bombarded by atomic particles, they split (or fission) to release particles of their own. These particles, called neutrons, strike other uranium atoms, splitting them. When the atoms split, they also release heat. This heat is known as nuclear energy and is essentially responsible for creating electricity.
Other reactions also take place in the nuclear reactor such as neutron capture. Neutron capture is a term used for the scenario where a neutron comes close to a nucleus (in this case uranium) and the nucleus captures it and becomes a different nucleus. In this case when uranium-238 captures a neutron it becomes uranium-239. After uranium-239 emits a beta particle (electron) it becomes neptunium-239. Then neptunium-239 also emits a beta particle and becomes plutonium-239. The plutonium can also be used as nuclear fuel.
Certain changes take place in the ceramic fuel pellets during their time in the reactor of the nuclear power plant. The particles left over after the atom has split are radioactive. During the life of the fuel, these radioactive particles collect within the fuel pellets. The fuel remains in the reactor until trapped fission fragments begin to reduce the efficiency of the chain reaction. Some of the fission products are various isotopes of barium, strontium, cesium, and iodine. The spent fuel also contains plutonium and uranium that was not used up. The fission products and the left over plutonium and uranium remain within the spent fuel when it is removed from the reactor and are called high level waste as they are extremely hot and very radioactive.
A Short Review of Radioactive Isotopes and Half-lives
In this subsection a review on radioactivity is provided so that the reader will have a better understanding of the nuclear waste issue.
An atom that is radioactive will decrease its radioactivity in time and eventually decay. How fast the radioactivity decreases depends on the half-life. The half-life is defined as the time it takes for half of the radioactivity to decay. Hence an isotope with a short half-life will decay quickly. The half-life is also inversely proportional to the intensity of radioactivity. Therefore the higher the intensity of radioactivity the shorter the half-life.
The approximate half-lives of some of the isotopes in the spent nuclear fuel are listed below:
| Isotope || Half-life |
| Strontium-90 || 28 years |
| Caesium-137 || 30 years |
| Plutonium-239 || 24,000 years |
| Caesium-135 || 2.3 million years |
| Iodine-129 || 15.7 million years |
It is clear from the table above that the left over nuclear isotopes from the generation of electricity through nuclear power are extremely long lived and therefore must be shielded from humans and the environment for a long time.
Storage and Disposal of High Level Nuclear Reactor Waste
Since the spent nuclear reactor (SNF) fuel is highly radioactive initially it is too dangerous to handle and thus it is very important to shield the radioactivity from humans and the environment. The radioactive material in the SNF generally falls into three categories: (1) un-reacted fuel, usually uranium, (2) fission products, and (3) activation products, most notably plutonium. Because of the nature of radioactivity, on a per-atom or by-weight basis the fission products are by far the most radioactive, and have the shortest half-life. The un-reacted uranium and the plutonium have vastly longer half-lives, but are correspondingly less radioactive. Once the SNF has been removed from the nuclear reactor it is placed in interim storage at the reactor site. Usually this consists of putting the nuclear waste into large pools of water. The water cools the radioactive isotopes and shields the environment from the radiation. Nuclear waste is typically stored in these supervised pools between 20-40 years. As the SNF ages the radioactivity decreases, reaching the point where it does not need to be water cooled and can be placed in dry storage facilities. Throughout this time there is a great reduction in heat and radioactivity and this makes handling of nuclear waste safer and easier.
After this “cooling off” period the high level waste can be handled in different ways. It can be reprocessed then disposed of permanently or directly disposed permanently in a geological repository.
Recall that the spent nuclear fuel contains uranium and plutonium which are used as fuel in a nuclear reactor. It is possible to isolate much of the uranium and plutonium from the other fission products in spent nuclear fuel so that it can be recycled as fresh fuel to power the nuclear reactor. This is called reprocessing. After reprocessing the left over waste is largely liquid. It is then embedded into borosilicate glass and put into interim storage. Eventually it will be disposed of permanently deep underground.
2. Final Disposal
Eventually the nuclear waste will have to be stored indefinitely because of the long time it takes for some of the waste isotopes to decay to a safe level. The consensus of most waste management specialist for final disposal is to bury the waste deep underground. In doing so we must ensure that the radioactive waste does not move from its burial site or that it does not escape into the environment. If it does it could have dire consequences for future generations such as contamination of drinking water.
To ensure that the radioactive waste is contained the current consensus is to use a multi-barrier system to store the waste. The geological disposal system consists of firstly surrounding the conditioned and packaged solid waste by several human made barriers then placing this at a depth of several hundred meters in a stable geological environment. The geological formation is the most important of the isolation barriers. The barriers act in concert to initially completely isolate the radioactive particles so they can decay and then limit their release to the environment. The combination of man made and natural geological barriers is called a multibarrier system.
The solidification of nuclear waste (which is necessary for final disposal) usually consists of dispersal in a glass matrix. However, alternative techniques are being researched. One such technique consists of embedding the waste into a ceramic matrix such as Synroc.Synroc
is a “synthetic rock” invented in 1978 by Professor Ted Ringwood of the Australian National University. Synroc can incorporate nearly all the elements contained in high level waste.
The barriers surrounding the solid waste vary from country to country. However most countries believe that the barriers should be made of materials that occur naturally in the earth’s crust. In Sweden
, the barriers consist of
1. A copper canister with a cast iron insert. This barrier is closest to the waste and its function is to isolate the fuel from the environment.
2. The second layer consists of bentonite clay called a buffer. Its function is to protect the the canister against small movements in the rock and keep it in its place. The clay also acts as a filter in case any radioactive particles escape form the canister.
3. The geological rock. The rock also stops leaking of radioactive particles into the environment but its main function is to protect the canister and buffer from mechanical damage and to offer a stable environment for the isolation of the waste.
Current Programs for final disposal of nuclear waste
Currently, no country has a complete system for storing high level waste permanently but many have plans to do so in the next 10 years. There are a number of well-developed proposals from the USA
and France for the disposal of long-lived radio-active waste.
All the proposed disposal techniques employ multiple barriers, as discussed above, to isolate the waste from the biosphere for at least 100,000 years. Nevertheless every one of the proposed disposal methods faces strong opposition
from environmental groups and it is true that humans have never attempted to do anything
on this sort of timescale. However nature has plenty of examples of systems that are stable for much longer periods. The most spectacular being the trans-uranic products of the Oklo natural nuclear reactors, which are discussed below, which have not appreciably moved in over 1.7 billion years.
The World Nuclear Industry appears to have reached a consensus to pursue Geologic disposal as final phase of Nuclear waste management. The US National Academies of Science, Engineering and Medicine also conclude that deep Geologic Disposal
can provide a safe means of disposing high level waste.
There are a number of programs that have seriously mishandled the issue of waste from Nuclear Power. The British decision to reprocess spent-fuel appears to have been both an environmental and financial mistake
. The Nuclear Weapons program at the Hanford site
in Eastern Washington State, U.S.A, created an enormous environmental impact that has so far cost 5.7 billion dollars to clean up.
Currently waste from Nuclear Power plants is being held in temporary storage facilities until such time as long-term disposal is decided. This is a feasible option because of the relatively small amount of material used to generate Nuclear Power.
New Technologies for Waste Disposal
Another option for disposal of long-lived (trans-Uranic) waste is to burn it via either Accelerator Driven Systems
or within Fourth Generation
reactors. However these technologies are not yet mature. Since the waste is stored in large tanks of water for 20-40 years first, it may be that by this time these new technologies will be sufficiently developed so that waste can be destroyed using these new methods.
Read an article on transmutation from The Economist
There are also proposals to use a Fusion-Fission Hybrid
for waste-disposal. These devices use the powerful neutrons from a Deuterium-Tritium plasma to drive nuclear transmutation. In some respects this technology is similar to Accelerator driven waste transmutation and the proponents believe that a fleet of 6 Fusion-Fission hybrids, when used in conjunction with a reprocessing waste cycle, would be sufficient to destroy the remaining long-lived nuclear waste from a fleet of 100 commercial power-reactors.
Finally there are experiments with Deep-Burn
where fuels originating from reprocessed nuclear-waste would be used to power Very High Temperature Reactors
(VHTR). The result would be that a single fuel loading derived from 4 years of operation of a light-water reactor could be used to deliver all the energy needed over the 60-year life of a VHTR. This technology would not only destroy most of the long-lived waste, it would make the existing stockpiles a very valuable source of energy, since it could be used to deliver ten times the energy of the original fuel.
How much high level nuclear waste is produced in a nuclear reactor?
According to the International Atomic Energy Agency
a nuclear reactor which would supply the needs of a city the size of Amsterdam – a 1000MW(e) nuclear power station – produces approximately 30 tonnes of high level solid packed waster per year if the spent fuel is not reprocessed.
In comparison, a 1000MW(e) coal plant produces 300,000 tonnes of ash per year.
Currently, worldwide, nuclear power generation produces 10,000m3 of high-level waste per year.
Who is responsible for the nuclear waste?
In most countries it is the responsibility of the power companies to take care of the waste. They are responsible for all the cost of nuclear waste management. In fact most countries are obliged to set aside a certain amount of money each year for waste management. Furthermore there are strict guidelines imposed by governments and other bodies for safe and responsible nuclear waste management. For example the International Atomic Energy Agency runs frequent conferences on the science of nuclear waste disposal, setting and enforcing safety standards, monitoring safety and security of nuclear waste. Recent conferences can be found on the IAEA website
A natural nuclear reactor
Eventual final disposal of nuclear waste means that the radioactive isotopes will be buried (with appropriate barriers - see "Final disposal" above) in an appropriate disposal site for a very long time. One of the serious questions we should consider is are there going to be any problems far in the future associated with the burial of the waste. The system for final waste disposal is usually based on naturally occurring phenomena. One such natural analogue is naturally occurring nuclear reactors in Gabon, West Africa in the Oklo mine called the Oklo nuclear reactors.
About 1.7 billion years ago, deep underground in Africa, favorable natural conditions prompted nuclear reactions to take place. These natural conditions were sufficient amounts of Uranium-235 and the evolution of plants which subsequently caused rainwater to filter down through cracks in rocks. The water was necessary to slow down the neutrons emitted via uranium decay so that they could interact with other particles and produce nuclear chain reactions. The reactors operated for about 1 million years. The reactions stopped because the uranium depleted to amounts that were too small to keep the reactions going.
It has been shown that the Oklo reactors fissioned Uranium-238, Uranium-235 and Plutonium-239. This is exactly the elements that are fissioned in todays man-made nuclear reactors. Note also that there was no Plutonium-239 on earth when the Oklo reactors formed. This means that the reactors themselves must have produced this isotope. This is also the case for man-made nuclear reactors.
Once the natural reactors burned out they left radioactive nuclear waste. This waste is very similar to the waste generated by nuclear power stations. The nuclear waste was held in place deep underground by granite, sandstone and clays surrounding the reactors' site. The important point is that the waste has not moved much over approximately 2 billion years (see the fact sheet
from Office of Civilian Radioactive Waste Management, USA and the article by Walton and Cowan from the "Proceedings of a Symposium on the Oklo Phenomenon", 1975 listed in the bibliography at the end of this section).
The Oklo reactors give us an opportunity to observe the effects of storing waste deep underground for billions of years. By analyzing the remains of these ancient natural nuclear reactors and gaining an understanding of the conditions needed to secure and contain the nuclear waste, we can apply the same techniques to the final disposal of man-made nuclear reactor high level waste. The study of the Oklo nuclear reactors means that we can have reasonable confidence in final disposal.
Bibliography for Waste from Nuclear Power
The sources on man-made high level nuclear waste have been,
International Atomic Energy Agency Factsheet on Nuclear Waste
Nuclear Energy Agency
Swedish Nuclear Waste Management
Finland Nuclear Waste Management
French Nuclear Waste Management
USA Office of Civilian Radioactive Waste Mangagement
Technical Report series no. 43, International Atomic Energy Agency, 2003, "Scientific and Technical Basis for the Geological Disposal of Radioactive Wastes"
The sources for the Oklo natural nuclear reactors have been,
R.D Walton Jr., G.A. Cowan, "Relevance of Nuclide Migration at Oklo to the Problem of Geologic Storage of Radioactive Waste", from the International Atomic Energy Agency, Vienna, 1975, "Proceeding of a Symposium on the Oklo Phenomenon".
Curtin University Information on Oklo
Office of Civilian Radioactive Waste Management, USA - Oklo Fact Sheet
Other information on natural nuclear waste storage can be found through the following link,
Natural Analogues of Nuclear Waste Storage
Nuclear Weapons Proliferation
A connection between nuclear power and nuclear weapons exists because both require fissile materials. Some of the technology that can be used to produce or purify a fissile material for a nuclear power plant could also be applied to producing nuclear weapons. There are three main fissile materials that are used in nuclear reactions:
In addition, Plutonium-240 (240Pu) and Plutonium-241 (241Pu) are produced and consumed in Nuclear Power production but neither can be used for Nuclear Weapons.
Out of these three materials only 235U exists naturally as an isotope of natural uranium with concentrations of 0.7%. Plutonium-239 and Uranium-233 are created by neutron capture on 238U and Thorium-232 (232Th) respectively.
This is currently the most common fuel in nuclear reactors. Natural Uranium must be enriched to contain about 3-5% 235U before it can be used in most conventional reactors. The CANDU Heavy-Water reactor can use natural Uranium. To create a weapon Uranium must be enriched above 80%. Highly enriched Uranium (more than 20% enrichment) is also used for reactors in naval vessels and for research reactors.
Various techniques can be used to enrich
Uranium. Any of these techniques used to enrich Uranium for power plants could in theory be further used to produce 235U in pure enough quantities for nuclear weapons.
This is the preferred isotope for Nuclear weapon design
as it has a lower critical mass and is easier to produce in large quantities than 235U. 239Pu and 240Pu are produced in nearly all nuclear reactors by neutron capture on naturally occurring 238U, and can be easily separated from the Uranium. However, for the purposes of Nuclear Weapon's 240Pu is an unwanted component as it has a high rate of spontaneous fission which limits the a nuclear weapon from achieving critical mass for long enough to consume a large fraction of the fissile material. Weapons grade Plutonium is defined to contain no more than 7% 240Pu. That said, the USA exploded a nuclear weapon in 1962 with a 240Pu content in excess of 7%. The World Nuclear Association
estimates that the 240Pu concentration of the device was 10%. Other people
estimate that the concentration of 240Pu in the test may have been much higher and that weapons with "reactor grade" plutonium are possible.
This has very important consequences for Nuclear Weapons proliferation. If a batch of Plutonium has more than 7% 240Pu, it is unlikely to used for a Nuclear Weapon. Firstly because of the difficulty is creating a useful weapon with this material and because the mass difference between 240Pu and 239Pu is too small to allow normal isotopic enrichment procedures to work satisfactorily so that the plutonium cannot be enriched in 239Pu.
Nuclear reactors must be operated in a special and easily detectable way in order to create 239Pu with a sufficiently low abundance of 240Pu to be used in a Nuclear Weapon. What happens is this: Starting from a fuel that consists solely of 238U and 235U, the 238U will capture a neutron and convert to 239U. Shortly there-after the 239U will decay to Neptuium-239 (239Nu). The 239Nu then
decays to 239Pu. Now 3 out of 4, neutron reactions on 239Pu initiates fission which destroys the 239Pu. However one in four neutron reactions are neutron captures which instead convert 239Pu into 240Pu. 240Pu does not undergo fission and so the relative abundance of 240Pu compared to 239Pu increases with the time the nuclear fuel spends in the reactor.
See the figure below.
Consequently in order to create weapons-grade plutonium, the fuel of the reactor must be removed before the concentration of 240Pu has a chance to exceed the 7% threshold. As is shown in the figure this means the fuel must be removed before it has spent 4 months in the reactor.
For this reason, Light Water reactors are what are called "proliferation resistant". In order to remove the fuel, the reactor must be shutdown. Shutting down a 1 GigaWatt reactor is very easy detect and is clearly not in the best interests of power production. When such events occur it is easy for the International Atomic Energy Agency (IAEA)
to inspect the fuel rods and verify that no weapons grade Plutonium has been created or diverted. A standard light-water reactor that has been operated continuously for more than 4 months is not capable of producing weapons grade plutonium.
However other types of reactors such as Heavy Water reactors and some types of Breeder reactors do not have this built-in safe-guard. In these reactors fuel can be removed without a power-down. They require much more serious monitoring to ensure that weapons-grade Plutonium is not being created.
Uranium-233 is produced by neutron capture of Thorium-232 in much the same way as Plutonium-239 is produced. There is very little information on the use of Uranium-233 for constructing nuclear weapons. There are also no commercial Thorium breeder reactors.
Nuclear Non-Proliferation Treaty
The Nuclear Non-Proliferation Treaty
was designed to limit the spread of Nuclear Weapons. In exchange for full access to the most advanced Uranium fuels and nuclear technology, countries must agree to detailed inspections by the IAEA. In the past the IAEA has not always successfully detected non-compliance. For example in Iraq before the 1991 war. The IAEA has been recently successful detecting suspicious behavior, such in North Korea and Iran. The IAEA has also been forthright in declaring the absence of such behavior such as in Iraq in shortly before the second Iraq war. Director-General of the IAEA Dr. ElBaradei and the IAEA itself were awarded the Nobel Peace prize
The Australian Inquiry into developing Australia's non-fossil fuel energy industry
has received submissions critical of the IAEA
and its ability to adequately implement the NPT.
Many of these were refuted
by a subsequent submission by the Australian Safeguards and Non-Proliferation Office (ASNO).
Safety of Nuclear Power Plants
Safety is taken very seriously by those working in nuclear power plants. The main safety concern is the emission of uncontrolled radiation into the environment which could cause harm to humans both at the reactor site and off-site. A summary by the nuclear world association on environmental, health and safety issues can be found at the Nuclear World Association website
The Union of Concerned Scientists has an extensive website devoted to the detailed safety issues
faced by American Nuclear Power Industry. These provide an interesting perspective on the importance both of a vigilent safety culture and a pro-active regulatory oversight.
Safety Mechanisms of a Nuclear Power Reactor
By regulation, the design of the nuclear reactor must include provisions for human (operator) error and equipment failure. Nuclear Plants in the western world use a "Defense in Depth" concept which is a system with multiple safety components, each with back-up and design to accommodate human error. The components include:
1. Control of Radioactivity
This requires being able to control the neutron flux. Recall that in a nuclear reactor when a neutron is captured by a fuel nucleus (generally uranium) the nucleus splits releasing radioactive particles (or undergoes fission). Hence if we decrease the neutron flux we decrease the radioactivity. The most common way to reduce the neutron flux is include neutron-absorbing control rods. These control rods can be partially inserted into the reactor core to reduce the reactions. The control rods are very important because the reaction could run out of control if fission events are extremely frequent. In modern nuclear power plants, the insertion of all the control rods into the reactor core occurs in a few seconds
, thus halting the nuclear reaction as rapidly as possible.
In addition, most reactors are designed so that beyond optimal level, as the temperature increases the efficiency of reactions decreases, hence fewer neutrons are able to cause fission and the reactor slows down automatically.
2. Maintenance of Core Cooling
In any nuclear reactor some sort of cooling is necessary. Generally nuclear reactors use water as a coolant. However some reactors which cannot use water use sodium or sodium salts.
3. Maintenance of barriers that prevent the release of radiation
There is a series of physical barriers between the radioactive core and the environment. For instance at the Darling Nuclear Generation Station
in Canada the reactors are enclosed in heavily reinforced concrete which is 1.8m thick. Workers are shielded from radiation via interior concrete walls. A vacuum building is connected to the reactor buildings by a pressure relief duct. The vacuum building is a 71m high concrete structure and is kept at negative atmospheric pressure. This means that if any radiation were to leak from the reactor it would be sucked into the vacuum building and therefore prevented from being released into the environment.
The design of the reactor also includes multiple back-up components, independent systems (two or more systems performing the same function in parallel), monitoring of instrumentation and the prevention of a failure of one type of equipment affecting any other.
Further, regulation requires that a core-meltdown incident must be confined only to the plant itself without the need to evacuate nearby residence.
Safety is also important for the workers of nuclear power plants. Radiation doses are controlled via the following procedures,
- The handling of equipment via remote in the core of the reactor
- Physical shielding
- Limit on the time a worker spends in areas with significant radiation levels
- Monitoring of individual doses and of the work environment
See the following websites for more detailed information and further references,
World Nuclear Association Summary on Nuclear Reactor Safety
Wikipedia information on Nuclear Reactors
Safety Mechanisms at a Canadian Nuclear Power Station
Final report of the US Nuclear Regulatory Commission on the Advanced Westinghouse AP1000 Nuclear Reactor design
In addition readers should be aware that the safety of any large scale industrial activity is an on-going concern and must always be taken seriously. Nuclear Power is certainly no exception and unfortunately the US Nuclear Industry did not give it the full attention it needed before the Three Mile Island accident. An excellent description of the safety issues surrounding early nuclear power plants is provided by Robert Pool. It can be downloaded from here
. The article describes how complex interacting technologies offer substantial challenges to safety. Consequently a major goal of 3rd generation Power plants
is to provide inherent safety and simpler design.