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Russia: Technical Uncertainties in the
Disposition Technologies
The immobilization methods for plutonium and high-level waste have not been demonstrated on the industrial scale needed for the disposition program, nor has an optimal glass or ceramic composition been chosen. However, neither problem is expected to cause unacceptable delays in the program: in fact, the relatively detailed DOE estimates in the Technical Summary Report show a 3 to 13 year advantage in completion time for the immobilization alternatives relative to the reactor alternatives.[1] Vitrification There are several large-scale high-level waste vitrification plants in the world, and a substantial body of research into the technology exists. However, this advantage is somewhat diminished in the context of plutonium disposition, since less research has been done into the vitrification of plutonium at the required concentrations. As mentioned above, Russia has a large-scale vitrification facility in operation at Mayak, (recently shut down). This plant operates with lead-phosphate glass as the immobilization medium. To be useful for plutonium disposition, it would have to be modified for use of the borosilicate glass that is better suited for the purpose. According to a DOE report describing the Mayak vitrification plant, the existing melters can process either type of glass.[2] Plutonium is present at a level of about 0.01 percent by weight in vitrified high-level waste. To immobilize military plutonium in a timely fashion, higher concentrations of about 5 to 10 percent will be required. Plutonium oxide has been vitrified alone, without high-level wastes, at concentrations as high as 14% percent by weight.[3] Outstanding technical questions for the vitrification methods include the optimal level of solubility of the plutonium in glass, the optimal glass formulation, the need to ensure that a critical mass of plutonium cannot accumulate in processing equipment, and the long-term stability of the waste form. Ceramic Immobilization The outstanding technical issues for ceramic immobilization are similar to those for vitrification. Ceramics appear to have greater stability than glass waste forms over geologic times, as is pointed to by the existence of natural mineral deposits in which uranium and similar elements have remained immobilized for millions of years. However, less is known about ceramic immobilization on an industrial scale, since most countries with nuclear power and weapons programs have chosen vitrification as their preferred method of high-level waste disposal. The key step of incorporating plutonium in ceramics alone has been demonstrated on an engineering scale, with samples containing greater than 10 percent plutonium by weight.[4] In the specific case of the can-in-canister method, existing vitrification technology would be used for encasement of the cans in radioactively laced glass. The ceramic would be used only for plutonium immobilization, rather than for simultaneous immobilization of plutonium and high-level waste. This simplifies the technical burden for the ceramic method. As a result, the problem of scaling up the ceramic can-in-canister method to the industrial levels needed for plutonium disposition is roughly comparable to the scaling problem for the glass can-in-canister method. Reactor Methods Unlike plutonium immobilization, industrial scale fabrication of MOX fuel and its use in light water reactors has been relatively well demonstrated. About 420 metric tons of MOX fuel have been fabricated, mostly in Belgium, France, and Germany, and the fuel has been used in about 17 light water reactors in France, Germany, and Switzerland.[5] Normally, MOX fuel is loaded into only 1/3 of the core of the reactor, with the remainder of the core containing LEU fuel assemblies. This loading scheme is not optimal for plutonium disposition, since either the number of reactors or the duration of the irradiation phase must be increased threefold relative to 100 percent MOX core operation. Therefore the use of MOX in the entire core (full MOX core operation) is being considered[6]. Other difficulties include the presence of a contaminant in weapons-grade stocks, and the different isotopic composition of the 30 metric tons of civil separated reactor-grade plutonium that may be included in the Russian campaign. The consequences of these changes are explored below. Use of Full MOX Cores Full MOX core operation brings about several changes in the core behavior relative to 1/3 MOX cores. Fewer 'delayed' neutrons are produced in a full MOX core, and these neutrons travel faster on the average than those in a 1/3 MOX core (or one containing ordinary LEU fuel). Absorption of delayed neutrons emitted a few seconds after fission is one of the prime means of controlling the fission reaction rate and criticality in a reactor. A reduction in the number of these neutrons makes control of the reactor core more difficult and requires an increase in the amount of absorbing material in the core. Moreover, neutrons with higher speeds are less efficiently absorbed by the fissile material in the fuel, and cause greater damage to reactor walls and encasements than lower speed neutrons. Preliminary computer simulations of VVER-1000 behavior with full MOX loading have been performed in Russia.[6] These studies confirm the need for additional reactor control mechanisms, but do not address the issue of added damage to reactor infrastructure. Presence of Gallium in Military Plutonium Gallium is added to weapon-grade plutonium as a stabilizing agent in concentrations of up to 1 percent. According to one DOE study[7], even lower gallium concentrations may adversely affect MOX fuel fabrication and use. The study's results are summarized here. Gallium causes two problems. The first problem arises in the sintering phase of the fuel fabrication process, when the plutonium and uranium oxide mixture is heated to form a ceramic pellet. The varying concentrations of gallium in different plutonium stocks may require repeated adjustment of the sintering process parameters. The second problem is that gallium may chemically degrade the zircaloy tubes which hold the MOX fuel pellets. This could cause deterioration of the casing and complicate fuel management and disposal. Gallium can be removed from plutonium with existing technologies. However, the best understood method is aqueous and generates large amounts of liquid radioactive waste. Dry processes are environmentally preferable. There are two dry processes under study at US national laboratories, processes that also serve to convert the plutonium from metal to oxide. Both may be able to reduce the amount of gallium in the oxide from 1 percent to a few hundred parts per million (ppm), corresponding to a few tens of ppms in the MOX fuel. This level of gallium concentration might be acceptable, since it is the level at which other potentially destructive contaminants are present in existing stocks of MOX fuel. However, more research must be done to determine the maximum allowable content of gallium in the fuel. Variation in Isotopic Composition As mentioned above, Russia has a 30 metric ton surplus of reactor-grade plutonium at Mayak which it also intends to use as reactor fuel. The isotopic composition of this surplus and of the weapon-grade plutonium surplus may cause difficulties in fuel fabrication and use. The Russian reactor-grade plutonium is unusual in that much of it has been stored for one to two decades. In reactor-grade plutonium, decay of Pu-241, which has a 14.4 year half life, causes the build-up of a gamma-emmitter, Am-241. The presence of Am-241 in the separated plutonium complicates fuel fabrication by substantially increasing radioactive shielding requirements in process equipment. Most MOX facilities can tolerate Am-241 contamination at levels up to about three percent. About half the Mayak surplus has an Am-241 content of about 4 to 6 percent a percentage that will increase as Pu-241 continues to decay. There are three ways to deal with the increased radiation. The first is to remove the Am-241 by chemical means. However, the cost of this process, estimated at $10 to $28 per gram of plutonium, can equal the cost of MOX fabrication itself (about $30 per gram of plutonium.)[8] The second method is to increase shielding in the initial plutonium-uranium blending stage. Blending at MOX plants is accomplished by workers using shielded glove boxes. One millimeter of steel shielding is used in a typical plant. However, for the 4 percent Am-241 concentration typical of much Russian reactor-grade plutonium, twenty millimeters of steel shielding would be required.[9] This increase in shielding requirements considerably increases the expense and difficulty of fabrication, since the thicker shielding makes glove box operations cumbersome or requires a remote-handling environment. A final possibility, suggested by one Russian study, is to blend the reactor-grade plutonium with weapon-grade plutonium prior to introduction into the MOX processing line. Since the weapon-grade isotopic composition is fixed, the quantity of weapon-grade material could be varied in the blend to obtain a fixed isotopic composition. This may be the most practical solution, although the cost and technical feasibility of the process have yet to be closely examined.[10]
Comments or questions? Contact Elena Sokova at MIIS CNS: esokovaATmiis.edu 
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