Department of Nuclear Engineering, University of California, Berkeley, CA 94720-1730
NE-161 Term Project, Fall 1994
The nuclear industry is similar to other industries with respect to waste. Both industries must properly control their waste so as to protect the environment and the public. Radioactive waste differs from other industrial waste in two ways. First of all, the risk associated with radioactive waste decreases with time. Industrial waste does not have a half live, and thus is around forever. Second, the over all volume of radioactive waste is much less then other chemical and industrial wastes. As an example of this, a 1000 MWe Coal Power Plant burns approximately 1100 tons of coal every 24 hours and discharges approximately 300 tons of SO2 and 5 tons of fly ash (containing small quantities of elements such as chlorine, cadmium, arsenic, mercury, lead, and several radioactive elements) directly to the atmosphere. A 1000 MWe nuclear power plant produces ~500 m3 of waste per year, having an average density of 160 to 240 kg/m3, none of which is released into the atmosphere.
This means that in a once through fuel cycle, the spent nuclear fuel is considered to be HLW. Other waste produced by Nuclear Power Plants such as liquid wastes resulting from the operation of the first cycle solvent extraction system, and the concentrated wastes from subsequent extraction cycles, are also considered to be HLW.
Title II of the act deals with the research and development needed for the successful completion of the goals set by the NWPA
Title III establishes a Office of Civilian Radioactive Waste Management (OCRWM), which is set up to implement the provisions state in the NWPA. Soon after the establishment of the OCRWM, DOE developed a contract guaranteeing that the federal Government will take the spent fuel from the utilities and dispose of it properly for a fee. Meanwhile, the utilities are responsible for the storage of the spent fuel prior to disposal. The Nuclear Waste Fund is made up from a tax of 1 mill/kWh of nuclear electricity generated after April 7, 1983, and is paid directly to DOE. Based on the nuclear electricity generation in 1989, DOE collects about $300 million per year from this tax.
Preceding as instructed by the NWPA, the DOE identified nine possible sites for a repository in February of 1983. These sites included one in Nevada, two in Texas, one in Washington, two in Utah, two in Mississippi, and one in Louisiana. By December of 1984, environmental assessments prepared on these sites narrowed the selection down to three sites for recommendation to the president. These where the sites in Yucca Mountain, Nevada, Deaf Smith county Texas, and in Hanford Washington. The final site selection became a political argument. None of the three states wanted to host the first nuclear waste repository. In December of 1987, Congress amended the NWPA and instructed the DOE to proceed with the characterization of Yucca Mountain in Nevada as the location for the first repository and to stop all activity on the other two locations within 90 days. Congress also recommended a study be done on the use of dry casks for the temporary storage of spent fuel at the sites of nuclear power plants.
By 1990, the DOE stopped all activities at the other two sites and strictly concentrated on the Yucca Mountain site. However, according to the NWPA, DOE had to get permits from the state of Nevada for all the work to be done at the site. Contrary to federal law, the Nevada legislature passed a law that forbids the disposal of any radioactive materials at Yucca Mountain and returned all of the DOE permit applications without processing them. The State of Nevada filed a lawsuit against the DOE in December 1989, asking the courts to order DOE to terminate all activities at the Yucca Mountain site. Since Congress had not overridden the state law disapproving the Yucca Mountain site, Nevada claimed the DOE's permit applications were meaningless. In January 1990, the DOE filed a suit of their own against the state of Nevada asking the courts to "stop the state from impending the scientific study called for by Congress to determine whether Yucca Mountain would be a suitable site for . . . a repository." The state of Nevada finally backed down when the federal government threatened to withhold the funding the state was getting by hosting the repository. Because of delays like this and many others, it is hard to estimate the date at which the repository will be opened. The DOE has already pushed the completion date back from 1998 to 2003 and now to 2010.
To get an idea of the volume of waste that is produced, the projected inventory of radioactive waste through the year 2000 is:
40200 tons of Heavy Metal of Spent fuel,
3.3x105 m3 of HLW,
and 6.3x106 m3 of LLW.
Starting from the beginning of the fuel cycle, the waste produced from mining is mostly debris. The radioactivity in the debris which is left on the surface from the mining operation, is mainly due to natural Uranium. This is a minor problem and can be safely regulated by monitoring the radiation levels from old mines, and by back filling the debris from new mines into old mines.
From the milling process, waste tailing are produced that consist of slurries of sand and clay like particles called slime. The slurries are pumped into a impoundment pond. One tonne of ore contains 0.2% of U3O8 and has about 7.3 mCi of natural uranium reactivity. Because the surrounding soil can be contaminated by wind blown tailing, and rain water can leach the tailing ponds, the DOE has authorized the Uranium Mill Tailing Remedial Action Project (UMTRA). This project is meant to stabilize the tailing ponds for inactive uranium processing sites.
The process of converting the ore to UF6, produces LLW associated with the fluoridation process. The LLW is solidified and put into drums and boxes to be disposed of at a defense LLW shallow land burial site.
Depleted uranium produced from the isotope separation process. For every tonne of low enriched uranium (3.3 % U-235), five to six tonnes of depleted uranium are generated, from which the radioactivity is minimal. The Atomic Energy Act (AEA) classified depleted uranium as "source" material, not waste, and hence DOE has no plan for disposing of the depleted uranium yet. However, there have been proposals to use the depleted uranium as liner material for waste containers which will eventually hold HLW for disposal in deep geologic repository.
The LLW produced from the conversion and fabrication process is mostly contaminated equipment. It is packaged in drums of solidified LLW for shallow land burial.
Next, looking at the governmental site of the fuel cycle, the HLW from production reactors and Navy submarines is mainly spent fuel. Submarine reactors are compact and therefore use a much higher enriched uranium level then do land based reactors. Hence the spent fuel from submarine reactors has a high uranium content. Currently the spent fuel is being stored in "swimming pools", which are basically large, deep pools of borated water in which the spent fuel is stored in, before being reprocessed. Prolonged storage in the pools could worsen the fuel degradation and cause corrosion. If the corrosion caused the fuel to fail, this would result in high radioactivity in the pool as well as in the facility atmosphere, thus effecting the safety and health of the workers. The remedy to this problem has been immediate action to adjust the water chemistry so as to slow down the corrosion process of the fuel cladding.
On the commercial site of the fuel cycle, spent fuel is also produced from the commercial Light Water Reactors (LWR) operating in this country. Here too, the spent fuel is stored in "swimming pools" at the reactor sites. At a few site, mainly in the eastern states, the spent fuel is also stored in dry casks at the reactor site. As discussed previously, the utilities pay a 1 mill/kWh to DOE to eventually take charge of the spent fuel. Because the opening of a repository has been pushed back to 2010, the utilities are running out of room to store the spent fuel. This is why spent fuel is being stored in dry casks at some reactor sites.
The defense reprocessing facilities deal mostly with liquid wastes from nuclear fission, and salt cakes contaminated with LLW. The liquid wastes are stored in single or double walled tanks, while calcined wastes are stored in bins at the Idaho National Engineering Laboratory. The waste volumes in tank form are large. Previous tanks leakages have resulted in soil contamination. However, the Defense Waste Processing Facility is now ready to vitrify the liquid waste in borosilicate glass. Similar vitrification processes are planned for the calcined wastes. The problem with disposing of the salt cake in not the radioactivity level, but the volume of the waste. Currently LLW salt cake is being disposed of on-site.
The spent fuel from the reprocessing at West Valley will be vitrified as HLW glass. The vitrified HLW glass logs are stored at West valley, pending for final disposal in a deep geological repository. The TRU waste are radioactinities that range from Pu to Am-241. The TRU waste is stored at defense complexes and will eventually be disposed of at the WIPP (Waste Isolation Pilot Plant) in New Mexico.
Finally, the waste from the manufacturing and dismantlement of weapons produces a wide range of waste products. Weapons Grade Plutonium and High Enriched Uranium (HEU) are not concerdered to be waste, but must also be disposed of carefully. The storage of these materials posses more of a security problem than a safety problem. Also, the large scale use of Pu caused non-proliferation concerns. Recent NAS recommendations have been to: One, Vitrify the Pu into glass, thus rendering it unusable. Two, use the Pu and HEU to blend into Mixed Oxide Fuels and Low Enriched Uranium fuels respectively. Mixed LLW are also produced in the form of chemical hazardous wastes. There is no disposal method available for this type of chemical and radioactive waste and is currently being stored at defense facilities. Methods to separate the mixed LLW into chemical and radioactive components are needed before the waste can be disposed of.
Table 1 The Most Important Isotopes Encountered in Radioactive Wastes
More than 350 nuclides have been identified as fission products, many having very sort half-lives. Out of these 350 nuclides, three general groups of radioisotopes are formed when fuel is irradiated in a reactor: (1) fission products, (2) actinides, and (3) activation products. The exact quantities of radioisotopes depends on the irradiation history of the fuel and the time after discharge. In principle, the concentration of any isotope can be computed using differential equations. Computer codes are used to calculate the total activity in spent fuel as a function of time. Two well-known codes that perform this task are ORIGEN and CINDER.
Because a significant amount of heat is generated the decay in spent fuel, it is important to know what the decay heat from spent fuel is, and how it decreases with time. This knowledge is important to be able to adequately cool the core after a shutdown, handle the fuel during storage and transportation, and to safely dispose of the fuel in a repository. The decay power at time t after shutdown is given by,
Eqn 1. P(t,T) = (Po/Q)*[F(t, infinity) - F(t + T, infinity)]
where the reactor is operated for a finite time T at constant power Po. Q is the recoverable fission energy in MeV, and can be found in tables for many common isotopes. The value of the function F(t, infinity) and F(t + T, infinity) for fissionable isotopes can also be found in tables.
The original fuel storage racks in the spent fuel pools where constructed out of stainless steel and had 6 in. or more of water between fuel assemblies. As spent fuel storage space became scarce, the old racks where replaced with new "high-density racks". The new racks were constructed out of a neutron absorbing material, Boral. (Boral is the trade name of a composite material consisting of boron carbide evenly dispersed within an aluminum matrix.) Boron also has the unique ability to absorb thermal neutrons without producing any significant secondary radiation. The new racks made out of Boral permitted the close spacing of fuel assemblies without criticality problems.
Several cask designs have been submitted to the NRC for licensing, however, as of the end on 1989, the NRC had granted licenses for metal casks and horizontal concrete modules. A typical metal cask is a cylindrical unit with a diameter of 8 feet and is 16 feet long. The cask is capable of storing 21 to 33 Pressurized Water Reactor (PWR) assemblies, which is the equivalent of about 9 to 14 tonnes of Heavy Metal (HM), or 45 to 70 Boiling Water Reactor (BWR) assemblies (8 to 12 t HM). Fully loaded the cask would weigh between 100 and 120 t. The cask walls are made out of iron or iron plus lead, and are thick enough to provide adequate shielding against gamma rays. A neutron absorbing material such as polyethylene or resin, also surrounds the circular surface of the cask. Finally, the external surface of the cask may have fins to enhance cooling. A metal cask is estimated to cost about $55 per kg of HM. The casks will require a storage pad area of ~25 ft2/tonne HM.
A variation of the metal cask is the duel purpose cask. This is a cask that can be used for storage as well as transportation. The duel purpose casks are similar is shape and size to the metal casks, but cost $7 /kg HM more than the single purpose cask. Figure 2 shows what a typical MPC looks like. A scenario showing the transportation of the MPC from reactor site to the repository is shown in figure 3.
Figure 2 Large MPC and Transportation Overpack
Figure 3 Schematic Diagram of Spent Fuel movement using MPC
Concrete storage casks are very similar to metal casks, except that the body of the cask is now made out of heavily reinforced concrete. The concrete wall is thick enough to provide adequate gamma ray shielding. A steel liner covers the inner surface of the body of the cask. There are two designs of concrete casks, ventilated and unventilated. Figure 4 shows an unventilated cask. Unventilated concrete casks are about 8.5 ft in diameter, 18 ft long, and weigh close to 90 t when loaded. Concrete casks rang in cost from $45 to $110 per kg HM for on site storage.
Figure 4Design of an unventilated concrete storage cask.
The majority of TRU waste is generated by DOE activities related to defense programs, specifically plutonium separation. Examples of TRU waste are contaminated glove boxes, filters, tools, and chemical sludges. Before 1970, all TRU waste was buried in shallow burial grounds at government owned and commercial sites. In 1970 the government decided that TRU waste required better disposal methods and started storing the TRU waste instead of burying it. Most TRU waste can be handled with the shielding from the waste package, and is called "contact handled." However, about 1% of TRU waste contains some isotopes that emit enough energetic gamma rays and neutrons from spontaneous fission to require special handling, in which case the waste package is called "remote handled."
Tranuranics are especially toxic because they are heavy chemical elements and emit alpha particles. When ingested, heavy metals tend to be deposited at the bone. Alpha emitting isotopes are not harmful outside the body because the heavy alpha particles have a short mean free path and do not penetrate the skin. They are very harmful if inhaled though, because when lodged in an organ of the body they deliver a high dose to the surrounding tissue. A relatively small fraction of the elements found in TRU are absorbed through the digestive system if ingested. Thus, TRU metals are much more hazardous if inhaled than if they are ingested.
The management of spent fuel is much different in other counties. France for example reprocesses all of its spent fuel from LWRs. They also have a breeder reactor program that is supported by the reprocessing of LWR spent fuel. Because of this, France has developed safe and reliable methods for the treatment of HLW. The liquid HLW is stored in refrigerated double walled stainless steel tanks for the first 1 to 5 years. From there, the waste is vitrified. The first pilot vitrification facility was started at Marcoule in 1969, and used a batch process that produced 12 tonne of glass. Excellent results were obtained from this facility, and a industrial size facility was built. 400 Kg of glass are poured into 5 mm thick stainless steel containers that are 1.3 m high and 0.43 m in diameter. Figure 5 shows the French vitrification method. The containers are closed and sent to an intermediate storage facility where the glass is allowed to cool before being sent to a geologic repository. Studies are still being done in France to determine the best geological medium.
Figure 5 Schematic of the French vitrification method.
Looking at other countries around the world, the UK has no plans for the permanent disposal of HLW. The present policy in England is to store the waste at ground level for 50 years, allowing it to cool. By the end of 50 years, the government hopes to have enough information to propose a permanent method of disposal. Japan started to construct vitrification facilities in 1988 for the vitrification of HLW. Japan also has definite plans to reprocessing and breeder reactors.
Over the years, several methods of disposal have been considered. These methods range from burial in the ground to firing the waste into space. Because of high risk or high cost, the only method that is currently being considered is deep geological burial. The deep geologic burial method is a multiple barrier approach to disposing of waste. Once placed deep underground, there are three barriers: the waste package, the disposal container, and the geologic medium. Figure 6 shows a conceptual design of a geologic repository.
When looking for a repository, four geologic media were considered: salt, tuff, basalt, and granite. Each medium has its advantages and disadvantages. Salt exhibits high plasticity, meaning that it behaves similarly to a fluid and any cracks that may form tend to close up on their own. However, salt is highly soluble in water, but generally where large deposits of salt are found, water is generally absent. Tuff is volcanic ash that has been compressed by its own weight. Because of this, the density of Tuff can very greatly. Basalt is also a volcanic material which has flowed up through fissures in the earth's crust and spread out over large areas in sheet like formations. Basalt is very hard, strong, and dense, and contains negligible amounts of moisture. Finally, Granite is a abundant material that formed from molten material that has cooled. Granite is very strong and contains little water. Other advantages of granite are that it contains minerals that tend to reduce corrosion of metals and retard the movement of waste.
Once built, a geologic repository will be at a depth of between 300 to 1200 m. The HLW will be placed in holes drilled in the rock of excavated tunnels. The waste will be able to be retrieved for a period of 100 years. If after that time, the repository has been confirmed to perform as designed, the tunnels will be back filled with the excavated rock. The backfilling will provide several benefits. First, it will act as an extra barrier between the waste and the environment. Second, it enhances the heat transfer between the waste and the surrounding rock. Finally, it reinforces the structural integrity of the mined geologic medium. In terms of risk analysis to the general population, the EPA has set an individual lifetime risk of 1 in 1,000,000 and an individual maximum lifetime risk of 1 in 10,000. This translates to mean that no person can receive a dose greater than 10 mrem/year. For comparison, natural background radiation is on the order of 100 mrem/year.
As mentioned before, DOE has chosen Yucca Mountain in Nevada to be the location of the first U.S. geologic repository. The geologic medium at Yucca Mountain is Tuff. Once complete, the repository will hold 70,000 tonne of spent fuel, 10% of this is designated for government waste, leaving 63,000 tonne of commercial spent fuel to be stored in the repository. Figure 7 shows what a typical geologic repository might look like.
Figure 7 Conceptual diagram of a geological repository..
Despite not having built the repository yet, there exists some experimental evidence to support deep geologic repositories. Strong evidence has been found that a natural fission reactor started up in the earth on its own approximately 1.8 billion years ago in a place called Oklo, which is located in Gabon West Africa. The natural reactor operated for thousands of years before it shut itself down, probably due to fuel depletion. This natural reactor was possible because at that time, the abundance of U-235 in natural uranium was about 3%. The combination of uranium ore and ground water was similar to the make up of fuel and moderator in modern day reactors. About 10 tonne of HLW was produced form this natural reactor. Through research it has been determined that most of the solid fission products and all of the TRU elements remained in the same location for 1.8 billion years, locked in the ore where they decayed. The geologic medium at Oklo is clay.
Some of the experiments planned are listed below.
1. Test the durability of the drums carrying the wastes.
2. Study the effectiveness of backfilling.
3. Evaluate rock mechanics codes using data obtained directly from the disposal medium.
4. Thermal structural experiments.
During the Pilot phase of the program, all the deposited waste will be able to be retrieved. If the facility performs as planned after the pilot phase, the operation may be expanded to permit the permanent disposal of TRU waste and HLW.
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