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.
1. Reprocessing
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, Sweden, Finland 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,
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".
Other information on natural nuclear waste storage can be found through the following link,
- FFH_FED_Final08.pdf: FusionFissionHybrids
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