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Nuclear Waste: How big is the problem?
Nuclear power has the potential to greatly reduce our dependence on foreign oil and to significantly reduce greenhouse gas emissions. Despite these obvious advantages, public acceptance of nuclear power remains a major obstacle to the construction and operation of new commercial nuclear power plants. The issues of safety, economics, nuclear waste disposal, and the potential for nuclear weapons proliferation are some of the most important objections to commercial nuclear power. Among these issues, disposal of nuclear waste is the most commonly expressed concern. The belief that “we don’t know how to safely dispose nuclear waste” is pervasive. But is this common belief correct? Nuclear waste disposal must be carefully examined to assure that we do not prematurely eliminate an option that could help us address the serious issues of global warming, our dwindling oil reserves, and our dangerous dependence on foreign oil.
Potential consequences of exposure to nuclear waste
Nuclear radiation includes all particles emitted from the nucleus of an unstable atom. When the nucleus of an atom emits radiation, the nucleus is said to undergo radioactive decay or nuclear disintegration. Nuclear decay most commonly results in the emission of alpha or beta particles and gamma rays. The nucleus of an atom will be transformed by the emission of radiation to form a new atom. For example, uranium-234 decays by emitting an alpha particle resulting in the transformation into the atom thorium-230. Radioactive isotopes (species of atoms, such as carbon-14) are called radioisotopes.
The radioactivity of a substance is characterized by the number of nuclear disintegrations per second, and is given in units of curies, equal to 37 billion disintegrations per second. Radionuclides are also characterized by a half-life. The half-life is the time in which half the number of atoms of a particular radionuclide decays. The faster a radioisotope decays (i.e., the shorter the half-life), the more radioactive it will be. Often radioactive decay results in the formation of another unstable nucleus that will also decay. The process of radioactive decay will continue until a stable (non-radioactive) nucleus is formed; consequently, the radioactivity of all nuclear waste diminishes with time.
Potential health issues associated with nuclear waste disposal focus on the risk of accidental internalization (inhalation or ingestion) of radioactive materials. For internalized radioactive material, all three common forms of decay radiation (alpha, beta and gamma) may cause damage to internal tissue and organs. The extent of damage depends on the quantity of internalized material, the pathway for internalization (inhaled or ingested), the specific radioisotopes internalized, and the chemical and physical form of the radioactive substance. Thus, the radioactivity in curies of internalized material is not sufficient for determining adverse health effects. Potential health effects are typically correlated with radiation exposure characterized by dose expressed in units of sieverts (Sv) or rem (1 Sv equals 100 rem). Radiation dose can account for the radiation energy deposited in each body organ, the damage effectiveness of the specific form of radiation, and the sensitivity of each organ to radiation exposure. Once the radiation dose is determined, health risks can be predicted. Postulated scenarios in which humans may be exposed to radiation from nuclear waste typically fall into the category of low doses with stochastic health effects. A stochastic effect is one that is characterized by probabilities; it occurs among unexposed as well as exposed individuals. Radiation-induced cancers and genetic effects are stochastic effects; these effects are the principle concerns associated with accidental exposure to radioactive waste.
When nuclear waste includes uranium or plutonium, the possibility that they may be extracted from the waste to build nuclear weapons is also a concern. The potential for use of nuclear waste by terrorist groups has also been raised as an issue.
Sources of Nuclear Waste and Waste Categories
Nuclear waste results from the fabrication of nuclear weapons and nuclear reactors, from reactor operation, and from a variety of medical, research, and industrial uses of nuclear materials. The bulk of nuclear waste results from nuclear weapons fabrication and from commercial reactor fuel fabrication and operation. In this analysis, we are concerned with commercial reactor waste.
Nuclear waste from reactor fabrication and operation is often divided into the front end and the back end of the fuel cycle. The front end refers to all nuclear related activities prior to reactor operation. The front end includes mining uranium, extraction of uranium from the ore, chemical processing and uranium enrichment, and fabrication of the nuclear fuel. Natural uranium contains only 0.72% U-235 and 99.27% U-238. For typical water-cooled reactors, an enrichment process is used to increase the uranium-235 concentration to about 3% (94.5% U-238). Waste from the front end of the nuclear fuel cycle typically consists of alpha-emitting nuclear materials from uranium extraction and processing. These materials are primarily uranium isotopes, radium, and radium decay products. Prior to reactor operation, the typical form of commercial reactor fuel, uranium dioxide, is not very radioactive.
The back end of the nuclear fuel cycle relates to waste generated during reactor operation. Reactor power results from a neutron chain reaction in which the nuclei of uranium-235 atoms are split (nuclear fission) when the nucleus captures a neutron. The fissioning of the uranium nucleus releases energy and more neutrons, and results in two new atoms called fission products. The fission products, primarily beta emitters, are highly radioactive and produce the majority of the radioactivity associated with spent nuclear fuel. Nuclei formed by neutron capture and radioactive decay in fuel materials other than U-235, along with uranium isotopes in the original fuel, make up the actinide contribution to nuclear waste in spent fuel. Actinides are the radioactive elements between atomic number 89 and 103 in the periodic table, such as uranium-234, neptunium-237, plutonium-239, and americium-241. Many actinides are alpha particle emitters and some actinides have very long half-lives. Contaminated hardware from decommissioning of reactors will also contribute to nuclear waste for the back end of the fuel cycle.
In the United States, nuclear waste is typically categorized as either low level waste (LLW), high level waste (HLW), or transuranic waste (TRUW). Some countries also define an intermediate level waste category. Low level waste includes miscellaneous items, such as rags and clothing, slightly contaminated by mostly short-lived radioactive materials. LLW waste does not require shielding during handling and is suitable for shallow land burial. 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 is often thermally hot. HLW accounts for over 95% of the total radioactivity produced in the process of nuclear electricity generation. Elements that have an atomic number greater than uranium are called transuranic. Transuranic waste is waste that includes alpha-emitting transuranic radionuclides with half-lives greater than 20 years, and concentrations greater than 100 nanocuries per gram, excluding High Level Waste.
In this discussion, only the most challenging waste category, high level waste, will be reviewed.
The nuclear fuel in a typical water-cooled reactor consists of uranium dioxide fuel pellets stacked inside long, small diameter zircaloy tubes. The tubes are grouped together to form fuel assemblies. During operation, the uranium fuel is slowly “burned up” (fissioned), gaseous fission products build up, and the fuel pellets become damaged. After about a year or so, the fuel will need to be replaced with fresh fuel bundles, and the spent fuel is removed from the reactor. When first removed from the reactor, the spent fuel is highly radioactive and heat from radioactive decay makes the fuel thermally hot. After removal from the reactor, spent fuel is placed in water-filled storage pools to keep the fuel cool and to provide radiation protection.
In the United States, the original plan was to eventually remove the spent fuel from the storage pools and send it to a reprocessing facility where the uranium and plutonium would be separated from the fission products. The uranium and plutonium where to be used in the fabrication of new fuel rods and the fission products were to be stored and eventually disposed. Because fission products decay relatively quickly and reprocessing allowed uranium and plutonium to be salvaged, the original reprocessing plan appeared to provide both economic and waste disposal advantages. However, reprocessing fuel also increases the chance of environmental contamination and raises proliferation issues. Furthermore, the “once-through” approach (no reprocessing) appeared to show a greater economic advantage than reprocessing spent fuel. In 1977 a federal moratorium on reprocessing fuel was instituted and spent fuel has since been accumulating in reactor storage pools, the moth-balled Barnwell fuel reprocessing plant, and dry storage facilities.
A number of nuclear waste disposal options have been explored. Some of the most commonly proposed methods include:
1. Deep geological disposal
2. Above ground storage followed by geological disposal
3. Disposal in the polar ice caps
4. Seabed disposal
5. Transmutation of waste
6. Disposal in space
Disposal in space does not appear to be an attractive option because of the cost and the potential for serious accidents. The technology for economic, large-scale transmutation of waste using fusion reactors is not sufficiently developed at this time. Research into fission reactors that burn actinides, such as the Integral Fast Reactor, and reactors that do not produce actinides, such as the gas-cooled pebble bed reactor, may be attractive future options. Seabed disposal and disposal at the polar ice caps may be viable alternatives, but these options have not been explored in great depth. Another approach, called Remix and Return, has been proposed that includes the blending of high level waste with mill tailings and disposing the mix in empty uranium mines. Here, we will focus on the disposal method currently underway in the United States; i.e., deep geological disposal of spent fuel. Above ground storage followed by geological disposal and reprocessing followed by vitrification and disposal will be considered as well.
Geological disposal is the burial of nuclear waste within Earth’s crust. A number of geological disposal options have been proposed. Some of the prominent approaches include: (1) placement in very deep boreholes below the level of moving ground water; (2) using the waste heat from radioactive decay to heat and melt the surrounding rock and seal the region surrounding the buried waste; and (3) placing the nuclear waste in mined cavities hundreds of meters below the Earth’s surface. The third option has been selected by the United States and a number of other options.
A multiple barriers approach has been used for waste disposal in mined cavities. Uncertainty in the properties of natural systems is the principal reason for using the multiple-barrier approach that includes nesting engineered and natural barriers. The barriers include the waste form, a container, backfill, and natural barriers. The function of the waste form is to limit the release of radionuclides following containment failure. For reprocessed fuel, the waste form is typically a glass. For approaches that do not include reprocessing, the spent fuel is disposed directly (with possible separation from the fuel assembly). The container (or overpack) isolates the nuclear waste from groundwater for periods that may range from as short as 500 years to as long as one million years (depending on the containment strategy). The backfill functions during the containment and the controlled release periods. The host rock serves as the final barrier to the release of radioactive waste material. Potential candidates for the host rock include rock salt, granite, basalt, and tuff. Each candidate rock type has advantages and disadvantages.
The United States has chosen the Yucca Mountain in Nevada as the final repository site for high-level waste. The site will cover 1150 acres (4.7 square km), and the waste will be placed in welded tuff about 1000ft (300 m) below the surface and about 1000 ft above the water table. Spent fuel bundles will be contained in a multilayer stainless steel and nickel alloy package covered by titanium drip shields.
(Under Construction)
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http://zfacts.com/p/295.html | 01/18/12 07:22 GMT
Modified: Sun, 04 Mar 2007 20:28:26 GMT
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