Waste Disposal: Problems & Solutions
Nuclear power is characterized by a very large amount of energy available from a very small amount of fuel. Although the amount of nuclear waste (often referred to as radwaste) is relatively small, much of it is highly radioactive and must therefore be carefully managed as hazardous waste. The problems and solutions of nuclear waste disposal are becoming a major concern in the 21st Century. Many nuclear power plants, particularly in the developed countries around the world, are nearing the end of their operating lives. The end of the cold war has left us with radioactive waste from decommissioned nuclear missiles. Nuclear power is the only energy industry which takes full responsibility for all its wastes. The cost of waste disposal is included in the cost of the power produced.
Nuclear waste comprises a variety of materials requiring different types of management to protect people and the environment. One of the factors in managing nuclear wastes is the time that they are likely to remain hazardous. This depends on the kinds of radioactive isotopes in them, and particularly the half-lives characteristic of each of those isotopes. The half-life is the time it takes for a given radioactive isotope to lose half of its radioactivity. After four half-lives the level of radioactivity is 1/16th of the original and after eight half-lives 1/256th.
The various radioactive isotopes have half-lives ranging from fractions of a second to minutes, hours or days, through to billions of years. Radioactivity decreases with time as these isotopes decay into stable, non-radioactive ones.
The rate of decay of an isotope is inversely proportional to its half-life; a short half-life means that it decays rapidly. Hence, for each kind of radiation, the higher the intensity of radioactivity in a given amount of material, the shorter the half-lives involved.
Three general principles are employed in the management of radioactive wastes:
The first two are also used in the management of non-radioactive wastes. The waste is either concentrated and then isolated, or it is diluted to acceptable levels and then discharged to the environment. Delay-and-decay however is unique to radioactive waste management; it means that the waste is stored and its radioactivity is allowed to decrease naturally through decay of the radioisotopes in it.
The problem of nuclear-waste
disposal is not as much technical as one that requires efficient management.
Each country is ethically and legally responsible for its own nuclear wastes,
therefore the default position is that all nuclear wastes will be disposed of
in each of the 40 or so countries concerned.
This means that countries like Sweden or Switzerland, which have only
a few plants, still have to do the research and development and find a local
site for disposal. This makes no economic or environmental sense at all. A
better solution would be to have competitive, commercial geologic repositories
-- in stable underground sites like the one in Yucca Mountain, Nevada -- that
take waste from other countries for a fee. Plutonium, one of the most
radioactive materials, does not move significantly in ground water, and if
some did ultimately escape it would be readily detected, and measures could
then be taken to avoid contamination. A geologic repository would work
effectively for at least 100,000 years, after which the waste would be little
more radioactive than the natural uranium from which it was derived.
According to the amount and type of radioactivity in them, nuclear waste materials can generally be classified under two categories – a) low level nuclear waste and b) high level nuclear waste.
a) Low level nuclear waste: usually includes material used to handle the highly radioactive parts of nuclear reactors (i.e. cooling water pipes and radiation suits) and waste from medical procedures involving radioactive treatments or x-rays. Low-level waste is comparatively easy to dispose of. The level of radioactivity and the half-life of the radioactive isotopes in low-level waste are relatively small. Storing the waste for a period of 10 to 50 years will allow most of the radioactive isotopes in low-level waste to decay, at which point the waste can be disposed of as normal refuse.
Low-level nuclear waste is generated from hospitals, laboratories and industry, as well as the nuclear fuel cycle. It comprises paper, rags, tools, clothing, filters etc., which contain small amounts of mostly short-lived radioactivity. It is not dangerous to handle, but must be disposed of more carefully than normal garbage. Some low-level waste viz., resins, chemical sludges and reactor components, as well as contaminated materials from reactor decommissioning may require special shielding before disposal. Low-level nuclear waste is usually buried in shallow landfill sites. To reduce its volume, it is often compacted or incinerated (in a closed container) before disposal. Worldwide it comprises 97% of the volume but only 5% of the radioactivity of all nuclear waste.
b) High level nuclear waste: may be the spent fuel itself, or the
principal waste from reprocessing this.
It is generally material from the core of a nuclear reactor or nuclear
weapon. This waste includes uranium, plutonium, and other highly radioactive
elements formed during fission. Most of the radioisotopes in high level waste
emit large amounts of radiation and have extremely long half-lives (some
longer than 100,000 years) requiring long time periods before the waste will
settle to safe levels of radioactivity.
While only 3% of the volume of all radwaste,
it holds 95% of the radioactivity. It contains the highly radioactive fission
products and some heavy elements with long-lived radioactivity. It generates
a considerable amount of heat and requires cooling, as well as special
shielding during handling and transport. If the spent fuel is reprocessed,
the separated waste is vitrified by incorporating it into borosilicate
(Pyrex) glass, which is sealed inside stainless steel canisters for eventual
disposal deep underground.
As can be readily appreciated from the foregoing, disposal of the high level nuclear waste is more problematic than the low level one. This article describes some of the methods that can/are being undertaken for dealing with this problem. The methods of nuclear waste disposal include:
1. Short Term Storage
2. Long Term Storage
Short Term Storage of Nuclear Waste
Radioactive material decays in an exponential fashion. Short-term storage will reduce the radioactivity of spent nuclear fuel significantly. A ten-year storage can bring a 100 times decrease in radioactivity. A further reduction of radioactive emissions, similar to that of the first 10 years, would take another 100 years of storage. Storing the waste for at least 10 years is recommended. The reduction in radioactivity during short-term storage makes handling and shipment of the waste much easier. After short term storage the waste will be sent for transmutation or long-term storage.
2. Long Term Storage for High Level Radioactive
While there are methods of significantly reducing the amount of high-level radioactive waste, some (or all) high level radioactive waste must end its journey in long-term storage. Because "long term" refers to a period of thousands of years, security of the radioactive waste must be assured over geologic time periods. The waste must not be allowed to escape to the outside environment by any foreseeable accident, malicious action, or geological activity. This includes accidental uncovering, removal by groups intending to use the radioactive material in a harmful manner, leeching of the waste into the water supply, and exposure from earthquake or other geological activity. In addition, this security must be maintained over a period of time during which, not only will the designers of the storage area die, but also the host country will, in all likelihood, see different political regimes. Civilization has undergone tremendous changes in the last 3000 years since the Egyptian Empire, yet some high level radioactive waste will take over 20,000 years to decay to safe levels.
Posing further difficulty is the fact that some of this waste is plutonium, and other actinide elements, produced as byproducts of uranium fission. These elements are not only highly radioactive, but highly poisonous as well. The toxicity of plutonium is among the highest of any element known.
Areas currently being evaluated for long-term storage of nuclear waste are:
b) Under the sea bed, and
c) Large stable geologic formations on land.
Long-term storage on land seems to be the favorite of most countries.
a) Disposal of Nuclear Waste in Space
Outer space is the most appropriate long-term storage option for high-level nuclear waste. This would ensure it’s safe removal from humans regardless of the activities of nature or man on earth. Anybody accidentally stumbling upon this waste would be at a lesser risk as they would be using radioactive shielding for space travel. Delivery of the waste into space has a crippling drawback -- the rocket used to deliver the waste into space would need to provide enough power to escape the earth’s gravity. This is necessary for two reasons: a) to leave the waste in orbit creates space garbage that is likely to reenter the earth’s environment at some time due to collision with satellites and other orbiting waste or spacecraft; and b) the large delivery rocket would be expensive and an accident during launch could have catastrophic results. Space disposal therefore, will not be a viable option until space travel is considerably safer and less expensive.
b) Storage of Radioactive Waste in the Sea Bed
A possibility for long-term storage on the earth is burial in the seabed. The rock formations in the seabed are generally more stable than those on land reducing the risk of exposure due to seismic activity. Apart from this, there is little groundwater circulation under the seabed, reducing the possibility of radioactive material contaminating ground water available for human consumption. The greatest appeal of under sea burial is also its greatest drawback. The enormous cost and difficulty of excavating the waste would likely prevent accidental or malicious disturbing of the waste. This cost is also keeping us from burying the waste at sea.
c) Long Term Storage of Radioactive Waste on
Long term storage in tectonically stable rock formations on land is the most likely solution for high-level radioactive waste. The radioactive material may be vitrified1 and buried in caverns, created in a large rock formation. When use of the storage area is complete, it would be sealed again with stone. While still extremely expensive, and considerably unsafe, this is the most viable storage option currently available. Using methods that reduce the amount of radioactive waste could further enhance safety levels.
While most countries should not only be able to find suitably safe sites in stable geological formations, they should also demonstrate this safety so as to create public confidence. This is best achieved where there is simple geology.
The Pangea proposal
A major research program in the 1990s (by Pangea Resources) has identified Australia, southern Africa, Argentina and western China as having the appropriate geological credentials for a deep geologic repository, with Australia being favoured on economic and political grounds. It would be located where the geology has been stable for several hundred million years, so that there need not be total reliance on a robust engineered barrier system to keep the waste securely isolated for thousands of years.
It would be a commercial undertaking and would have a dedicated port and rail infrastructure. It would take spent fuel and other wastes from commercial reactors, and possibly also material from weapons disposal programs.
The following were Pangea's objectives, but they are relevant to future proposals:
Ø To site a deep geologic disposal facility in a region where the geology and biosphere conditions meet the test of simplicity coupled with robustness.
Ø to create a facility for deep geological disposal capable of accepting spent fuel, vitrified high-level waste, long-lived intermediate-level waste, and appropriately conditioned long-lived nuclear materials, such as immobilized plutonium.
Ø to provide an economic and environmentally responsible disposal option.
Ø to provide a safe and secure transportation service to the repository location.
Ø to provide the host country with the opportunity to gain substantial economic benefits and to play an important role in enhancing security and non-proliferation efforts for the benefit of all nations.
Transmutation is the transformation of one element into another. The goal of transmutation, in radioactive waste disposal, is to transmute long half-life, highly radioactive elements, into shorter half-life, less-radioactive elements. There are two methods currently proposed for the transmutation of high-level radioactive waste are:
a) fast consumer reactors, and
b) hybrid reactors.
a) Fast Consumer Reactors
Fast consumer reactors are merely variations of fast breeder reactors. These reactors take the plutonium created by nuclear reactors as byproduct, or as fuel for nuclear weapons and "consume" it. This process leaves uranium and other less dangerous radioactive waste. As an added benefit, the isotopes of the elements created as byproducts generally have shorter half-lives than the initial plutonium used as fuel.
Transmutation with a Hybrid Nuclear Reactor
Hybrid nuclear reactors
promise near complete transmutation of almost any high level radioactive
waste. The general process is to produce a sub-critical nuclear reactor (i.e.
the nuclear reactions would stop under normal conditions) and bombard the
reactor fuel with neutrons. The neutrons break apart the large radioactive
elements, releasing energy. This energy is used to power the neutron source
needed to start the fission reaction. There will be some high level
radioactive waste produced (generally parts of the neutron source) but in
comparison to the amount of radioactive waste consumed, this will be minimal.
This high level waste will need to be placed in long-term storage.
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Department of Geology
Aligarh Muslim University, Aligarh - 202 002 (India)
1 Vitrification is a method in
by which radioactive waste is mixed with silica and melted into glass beads.
It is believed that this will capture most of the radioactive elements and
prevent them from escaping into the atmosphere or ground water. The glass
beads are then put into long term storage.