Radioactive waste

Nuclear waste locations in USA

Radioactive waste is waste type containing radioactive chemical elements that does not have a practical purpose. It is sometimes the product of a nuclear process, such as nuclear fission. The majority of radioactive waste is "low-level waste", meaning it has low levels of radioactivity per mass or volume. This type of waste often consists of items such as used protective clothing, which is only slightly contaminated but still dangerous in case of radioactive contamination of a human body through ingestion, inhalation, absorption, or injection.

Basic overview

Physics

The radioactivity of all nuclear waste diminishes with time. All radioisotopes contained in the waste have a half-life - the time it takes for any radionuclide to lose half of its radioactivity and eventually all radioactive waste decays into non-radioactive elements. Certain radioactive elements (such as plutonium-239) in “spent” fuel will remain hazardous to humans and other living beings for hundreds of thousands of years. Other radioisotopes will remain hazardous for millions of years. Thus, these wastes must be shielded for centuries and isolated from the living environment for hundreds of millennia [1]. Some elements, such as ¹³¹I, have a short half-life (around 8 days in this case) and thus they will cease to be a problem much more quickly than other, longer-lived, decay products but their activity is much greater initially.

The faster a radioisotope is decaying, the more radioactive it will be. The energy and the type of the ionizing radiation emitted by a pure radioactive substance are important factors in deciding how dangerous it will be. The chemical properties of the radioactive element will determine how mobile the substance is and how likely it is to spread into the environment and contaminate human bodies. This is further complicated by the fact that many radioisotopes do not decay immediately to a stable state but rather to a radioactive decay product leading to decay chains.

Biochemistry

Depending on the decay mode and the biochemistry of an element, the threat due to exposure to a given activity of a radioisotope will differ. For instance ¹³¹I is a short-lived beta and gamma emitter but because it concentrates in the thyroid gland, it is more able to cause injury than TcO₄^- which, being water soluble, is rapidly excreted in urine. In a similar way, the alpha emitting actinides and radium are considered very harmful as they tend to have long biological half-lives and their radiation has a high linear energy transfer value. Because of such differences, the rules determining biological injury differ widely according to the radioisotope, and sometimes also the nature of the chemical compound which contains the radioisotope.

Philosophy

The main objective in managing and disposing of radioactive (or other) waste is to protect people and the environment. This means isolating or diluting the waste so that the rate or concentration of any radionuclides returned to the biosphere is harmless. To achieve this the preferred technology to date has been deep and secure burial for the more dangerous wastes; transmutation, long-term retrievable storage, and removal to space have also been suggested.

The phrase which sums up the area is ' Isolate from man and his environment ' until the waste has decayed such that it no longer poses a threat.

Fiction

In fiction, radioactive waste is often cited as the reason for gaining super-human powers and abilities. An example of this fictional scenario is the 1981 movie "Modern Problems" in which actor Chevy Chase portrays a jealous, harried air traffic controller Max Fiedler; Max Fiedler, recently dumped by his girlfriend, comes into contact with nuclear waste and is granted the power of telekinesis, which he uses to not only win her back, but to gain a little revenge. Another more widely known character affected by a bite from a radioactive spider is Spider-man. The Spider-man character was developed by Marvel Comics (see also Stan Lee) and was portrayed on the big screen by actor Tobey Macguire in two films the first in 2002 and the second in 2004.

In reality, exposure to high levels of radioactive waste may cause serious harm or death. It is interesting to note that the treatment of an adult animal with radiation or some other mutation causing effect, such as a cytotoxic anti-cancer drug, cannot cause that adult animal to become a mutant. It is more likely that a cancer will be induced in the animal. In humans it has been calculated that a 1 sievert dose has a 5% chance of causing cancer and a 1% chance of causing a mutation in a gamete (e.g. egg) or a gamete forming cell such as those in the testis which can be passed to the next generation. If a developing organism such as an unborn child is irradiated, then it is possible to induce a birth defect but it is unlikely that this defect will be in a gamete or a gamete forming cell.

Types of radioactive waste

Removal of very low-level waste

Although not significantly radioactive, uranium mill tailings are waste. They are byproduct material from the rough processing of uranium-bearing ore. They are sometimes referred to as 11(e)2 wastes, from the section of the U.S. Atomic Energy Act that defines them. Uranium mill tailings typically also contain chemically-hazardous heavy metals such as lead and arsenic. Vast mounds of uranium mill tailings are left at many old mining sites, especially in Colorado, New Mexico, and Utah.

Low Level Waste (LLW) is generated from hospitals 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. Commonly, LLW waste is designated as such as a precautionary measure if it originated from any region of an 'Active Area', which frequently includes offices with only a remote possibility of being contaminated with radioactive materials. Such LLW waste typically exhibits no higher radioactivity than one would expect from the same material disposed of in a non-active area, such as a normal office block. No LLW waste requires shielding during handling and transport and is suitable for shallow land burial. To reduce its volume, it is often compacted or incinerated before disposal. Low level waste is divided into four classes, class A, B, C and GTCC, which means "Greater Than Class C".

Intermediate Level Waste (ILW) contains higher amounts of radioactivity and in some cases requires shielding. ILW includes resins, chemical sludge and metal reactor fuel cladding, as well as contaminated materials from reactor decommissioning. It may be solidified in concrete or bitumen for disposal. As a general rule, short-lived waste (mainly non-fuel materials from reactors) is buried in shallow repositories, while long-lived waste (from fuel and fuel-reprocessing) is deposited in deep underground facilities. U.S. regulations do not define this category of waste; the term is used in Europe and elsewhere.

High Level Waste flasks are transported by train in the United Kingdom. Each flask is constructed of 3ft thick solid steel and weighs in excess of 50 tons

High Level Waste (HLW) is produced by nuclear reactors. It contains fission products and transuranic elements generated in the reactor core. It is highly radioactive and often thermally hot. HLW accounts for over 95% of the total radioactivity produced in the process of nuclear electricity generation.

Transuranic Waste (TRUW) as defined by U.S. regulations is, without regard to form or origin, waste that is contaminated with alpha-emitting transuranic radionuclides with half-lives greater than 20 years, and concentrations greater than 100nCi/g, excluding High Level Waste. Elements that have an atomic number greater than uranium are called transuranic ("beyond uranium"). Because of their long half-lives, TRUW is disposed more cautiously than either low level or intermediate level waste. In the U.S. it arises mainly from weapons production, and consists of clothing, tools, rags, residues, debris and other items contaminated with small amounts of radioactive elements (mainly plutonium).

Under U.S. law, TRUW is further categorized into "contact-handled" (CH) and "remote-handled" (RH) on the basis of radiation dose measured at the surface of the waste container. CH TRUW has a surface dose rate not greater than 200 mrem per hour, whereas RH TRUW has a surface dose rate of 200 mrem per hour or greater. CH TRUW does not have the very high radioactivity of high level waste, nor its high heat generation, but RH TRUW can be highly radioactive, with surface dose rates up to 1000 rem per hour. The United States currently permanently disposes of TRUW generated from nuclear power plants and military facilities at the Waste Isolation Pilot Plant.

Management of medium level waste

It is common for medium active wastes in the nuclear industry to be treated with ion exchange or other means to concentrate the radioactivity into a small volume. The much less radioactive bulk (after treatment) is often then discharged. For instance, it is possible to use a ferric hydroxide floc to remove radioactive metals from aqueous mixtures [2]. After the radioisotopes are absorbed onto the ferric hydroxide, the resulting sludge can be placed in a metal drum before being mixed with cement to form a solid waste form. In order to get better long-term performance (mechanical stability) from such forms, they may be made from a mixture of fly ash, or blast furnace slag, and portland cement, instead of the normal cement (made with portland cement, gravel and sand).

Management of high level waste

Storage

High-level radioactive waste is stored temporarily in spent fuel pools and in dry cask storage facilities. This allows the shorter-lived isotopes to decay before further handling.

Vitrification

A fissile material storage facility in Mayak, Russia

Long-term storage of radioactive waste requires the stabilization of the waste into a form which will not react, nor degrade, for extended periods of time. One way to do this is through vitrification. Currently at Sellafield, the high-level waste (PUREX first cycle raffinate) is mixed with sugar and then calcined. Calcination involves passing the waste through a heated, rotating tube. The purposes of calcination are to evaporate the water from the waste, and de-nitrate the fission products to assist the stability of the glass produced.

The 'Calcine' generated is fed continuously into an induction heated furnace with fragmented glass[3]. The resulting glass is a new subtance in which the waste products are bonded into the glass matrix when it solidifies. This product, as a molten fluid, is poured into stainless steel cylindrical containers ("cylinders") in a batch process. When cooled, the fluid solidifies ("vitrifies") into the glass. Such glass, after being formed, is very highly resistant to water. [4] According to the ITU, it will require about 1 million years for 10% of such glass to dissolve in water.

After filling a cylinder, a seal is welded onto the cylinder. The cylinder is then washed. After being inspected for external contamination, the steel cylinder is stored, usually in an underground repository. In this form, the waste products are expected to be immobilised for a very long period of time (many thousands of years).

The glass inside a cylinder is usually a black glossy substance. All this work (in the United Kingdom) is done using hot cell systems. The sugar is added to control the ruthenium chemistry and to stop the formation of the volatile RuO_{4 containing radioruthenium. In the west, the glass is normally a borosilicate glass (similar to Pyrex {NB Pyrex is a trade name}), while in the former Soviet bloc it is normal to use a phosphate glass. The amount of fission products in the glass must be limited because some (palladium, the other Pt group metals, and tellurium) tend to form metallic phases which separate from the glass. In Germany a vitrification plant is in use; this is treating the waste from a small demonstration reprocessing plant which has since been closed down.}

In 1997, in the 20 countries which account for most of the world's nuclear power generation, spent fuel storage capacity at the reactors was 148,000 tonnes, with 59% of this utilized. Away-from-reactor storage capacity was 78,000 tonnes, with 44% utilized. With annual additions of about 12,000 tonnes, issues for final disposal are not urgent.

In 1989 and 1992, France commissioned commercial plants to vitrify HLW left over from reprocessing oxide fuel, although there are adequate facilities elsewhere, notably in the United Kingdom and Belgium. The capacity of these western European plants is 2,500 canisters (1000 t) a year, and some have been operating for 18 years.

Synroc

The Australian Synroc (synthetic rock)[5] is a more sophisticated way to immobilize such waste, and this process may eventually come into commercial use for civil wastes (it is currently being developed for US military wastes). The synroc contains pyrochlore and cryptomelane type minerals. The original form of synroc (synroc C) was designed for the liquid high level waste (PUREX raffinate) from a light water reactor. The main minerals in this synroc are hollandite (BaAl_{2Ti6O16), zirconolite (CaZrTi2O7) and perovskite (CaTiO3). The zirconolite and perovskite are hosts for the actinides. The strontium and barium will be fixed in the perovskite. The cesium will be fixed in the hollandite.}

Synroc was invented by the late Prof Ted Ringwood (a geochemist) at the Australian National University.

Geological disposal

The process of selecting appropriate deep final repositories is now under way in several countries with the first expected to be commissioned some time after 2010. In Switzerland, the Grimsel Test Site is an international research facility investigating the open questions in radioactive waste disposal ([6]). Sweden is well advanced with plans for direct disposal of spent fuel, since its Parliament decided that this is acceptably safe, using the KBS-3 technology. In Germany, there is a political discussion about the search for an Endlager (final repository) for radioactive waste, accompanied by loud protests especially in the Gorleben village in the Wendland area, which was seen ideal for the final repository until 1990 because of its location next to the border to the former German Democratic Republic. Gorleben is presently being used to store radioactive waste non-permanently, with a decision on final disposal to be made at some future time. The U.S. has opted for a final repository at Yucca Mountain in Nevada, but this project is widely opposed and is a hotly debated topic. There is also a proposal for an international HLW repository in optimum geology, with Australia or Russia as possible locations; however, the proposal for a global repository for Australia has raised fierce domestic political objections, making such a dump unlikely.

Sea-based options for disposal of radioactive waste [7] include burial beneath a stable abyssal plain, burial in a subduction zone that would slowly carry the waste downward into the Earth's mantle, and burial beneath a remote natural or human-made island. While these approaches all have merit and would facilitate an international solution to the vexing problem of disposal of radioactive waste, they are currently not being seriously considered because of the legal barrier of the Law of the Sea and because in North America and Europe sea-based burial has become taboo from fear that such a repository could leak and cause widespread damage, though the evidence that this would happen is lacking. Dumping of radioactive waste from ships has reinforced this taboo. However, sea-based approaches might come under consideration in the future by individual countries or groups of countries that cannot find other acceptable solutions.

A more feasible approach termed Remix & Return [8] would blend high-level waste with uranium mine and mill tailings down to the level of the original radioactivity of the uranium ore, then replace it in empty uranium mines. This approach has the merits of totally eliminating the problem of high-level waste, of placing the material back where it belongs in the natural order of things, of providing jobs for miners who would double as disposal staff, and of facilitating a cradle-to-grave cycle for all radioactive materials.

Transmutation

There have been proposals for reactors that consume nuclear waste and transmute it to other, less-harmful nuclear waste. In particular, the Integral Fast Reactor was a proposed nuclear reactor with a nuclear fuel cycle that produced no transuranic waste and in fact, could consume transuranic waste. It proceeded as far as large-scale tests but was then cancelled by the US Government. Another approach, considered safer but requiring more development, is to dedicate subcritical reactors to the transmutation of the left-over transuranic elements.

There have also been theoretical studies involving the use of fusion reactors as so called "actinide burners" where a fusion reactor plasma such as in a tokamak, could be "doped" with a small amount of the "minor" transuranic atoms which would be transmuted to lighter elements upon their successive bombardment by the very high energy neutrons produced by the fusion of deuterium and tritium in the reactor. It was recently found by a study done at MIT, that only 2 or 3 fusion reactors with parameters similar to that of the International Thermonuclear Experimental Reactor (ITER) could transmute the entire annual actinide production from all of the light water reactors presently operating in the United States fleet while simultaneously generating approximately 1 gigawatt of power from each reactor.

Reuse of waste

Another option is to find applications of the isotopes in nuclear waste so as to reuse them. [9] . Already, cesium 137, strontium 90, technetium 99, and a few other isotopes are extracted for certain industrial applications such as food irradiation and RTGs.

Space disposal

Space disposal is an attractive notion because it permanently removes nuclear waste from the environment. However, it has significant disadvantages, not least of which is the potential for catastrophic failure of a launch vehicle. Furthermore, the high number of launches that would be required makes the proposal impractical. To further complicate matters, international agreements on the regulation of a such a program would need to be established.[10]

Accidents involving radioactive waste

While radioactive waste is not as sensitive to disruption as an active nuclear reactor, it is often treated as regular waste and forgotten. A number of incidents have occurred when radioactive material was disposed of improperly, simply abandoned or even stolen from a waste store.

Scavenging of abandoned radioactive material has been the cause of several other cases of radiation exposure, mostly in developing nations, which usually have less regulation of dangerous substances (and sometimes less general education about radioactivity and its hazards) and a market for scavenged goods and scrap metal. The scavengers and those who buy the material are almost always unaware that the material is radioactive and it is selected for its aesthetics or scrap value. A few are aware of the radioactivity, but are either ignorant of the risk or believe that the material's value outweighs the danger. Irresponsibility on the part of the radioactive material's owners, usually a hospital, university or military, and the absence of regulation concerning radioactive waste, or a lack of enforcement of such regulations, have been significant factors in radiation exposures. For details of radioactive scrap see the Goiânia accident.

Transportation accidents involving spent nuclear fuel from power plants are unlikely to have serious consequences due to the strength of the spent nuclear fuel shipping casks.

See also

• Global Nuclear Energy Partnership
• Hot cell
• List of nuclear accidents
• Nuclear power
• Stored Waste Examination Pilot Plant

References

Fentiman, Audeen W. and James H. Saling. Radioactive Waste Management. New York: Taylor & Francis, 2002. Second ed. An overview of waste from the nuclear fuel cycle was written by B.V. Babu and S. Karthik, Energy Education Science and Technology, 2005, 14, 93-102.

External links

• Key Radionuclides and Generation Processes (DOE)
• Alsos Digital Library - Radioactive Waste (bibliography)
• Belgian Nuclear Research Centre - Activities (documents and links)
• Belgian Nuclear Research Centre - Scientific Reports (documents)
• Critical Hour: Three Mile Island, The Nuclear Legacy, And National Security (PDF)
• Environmental Protection Agency - Yucca Mountain (documents)
• Grist.org - How to tell future generations about nuclear waste (article)
• A discussion on the secrecy surrounding plans for radioactive waste in the UK (article)
• International Atomic Energy Agency - Nuclear Fuel Cycle and Waste Technology Program (program objectives)
• International Atomic Energy Agency - Internet Directory of Nuclear Resources (links)
• Nuclear Files.org - Yucca Mountain (documents)
• Nuclear Regulatory Commission - Radioactive Waste (documents)
• Nuclear Regulatory Commission - Spent Fuel Heat Generation Calculation (guide)
• Oak Ridge National Laboratory - Coal Combustion: Nuclear Resource or Danger (document)
• Radwaste.org (links)
• Radwaste Blog (weblog)
• Surviving on Nuclear Waste (book)
• The Nuclear Energy Option - Hazards of High-Level Radioactive Waste (book)
• Uranium Information Center - Radioactive Waste (briefing papers)
• United States Geological Survey - Radioactive Elements in Coal and Fly Ash (document)
• World Nuclear Association - Radioactive Waste (briefing papers)
• Satirical look at radioactive nuclear waste disposal plans (article)