If it has been decided to reprocess spent fuel with the objective of recovering valuable uranium and plutonium, the fuel must first be transported to a repro­cessing plant using the flasks described in the previous section. The stages that the fuel then goes through in the separation process are illustrated schemati­cally in Figure 7.10. First, the flask is taken off the vehicle, the spent fuel is re­moved under water, and the flask is decontaminated and returned to the power station for further use. The fuel is loaded into a storage rack under water until it is ready to be fed into the reprocessing plant.

In a modern reprocessing plant like THORP (Thermal Oxide Reprocessing Rant) operated by British Nuclear Fuels at Sellafield, the actual separation process is undertaken after at least 5 years’ storage of the spent fuel in the ponds. The fuel element is first stripped of as much of its extraneous metal structure (grids, support plates, etc.) as possible. These remnants are stored separately and treated as intermediate-level waste (see Chapter 8). The fuel pins themselves are sheared into small lengths between 1 and 4 in.; these sheared fuel pieces fall down a chute into a perforated basket (see Figure 7.10). This basket is then transferred to the dissolver. The shear needs to be of modular construction to allow replacement of the blade and for maintenance.

In the dissolver the fuel is dissolved in hot (90°C) 7 M nitric acid. Dissolution of the fuel takes place quickly and can be controlled by the rate of shearing. The cladding pieces, or “hulls,” are withdrawn in the basket and again sent for disposal as intermediate-level radioactive waste. Various types of dissolver, both batch and continuous, have been developed. As the fuel dissolves, fission gases are released: the inert gases krypton and xenon and other volatiles such as io­dine and carbon dioxide as well as oxides of nitrogen and steam. The dissolver off-gas systems must be able to cope with this mixture. The system recovers as much of the nitrogen oxides as possible as nitric acid.

The fuel solution itself still contains some undissolved particulates, both from the cladding and from fission products. The solution is therefore clarified using a centrifuge. The clarified nitric acid solution containing the fission products, the uranium, and the plutonium is next passed through the chemical separation plant. This involves a solvent extraction system.

Solvent extraction is a process that allows separation of dissolved materials. Suppose we have two liquids that do not mix, such as oil and water. If we have a solution of two substances, A and B, in one of the liquids, and component B is soluble in the other liquid but component A is not, then we may solvent-ex­tract component B from the original mixed solution of A and B by essentially shaking up (“contacting”) the solution with an immiscible liquid in which only B is soluble. By then removing component B from the-‘ resultant solution. w,.-

have achieved a separation of A and B. Various types of equipment are used in chemical engineering for this process, and it is beyond the scope of this book to go into them in detail. Probably the most commonly used devices in repro­cessing plants use mechanical stirrers to mix the two liquids, followed by set­tling tanks that allow their separation, with each of the liquids containing the respective components. These are called mixer settlers. Alternatively, vertical pipes containing perforated metal plates may be used, with one fluid flowing up the pipe and the other flowing down it. To promote mixing of the fluids, such columns are subjected to pulses, and they are often referred to as pulsed columns. A typical pulsed column is shown in Figure 7.11. The first objective of solvent extraction in the reprocessing plant is the separation of the valuable uranium-plutonium mixture from the nitric acid solution, which also contains the fission products. This is done by contacting the nitric acid fuel solution with an organic solvent, typically tributyl phosphate (TBP) diluted with odorless kerosene (OK). In a typical extraction plant, all but about 0.1% of the uranium and plutonium in the fuel solution is removed into the TBP phase.

Separation of the uranium from the plutonium is also achieved by solvent extraction. The first step is to redissolve the mixture in a clean acid stream and then add a substance to the stream to change the condition of the plutonium and render it insoluble in TBP. Thus, when the new acid stream is contacted again with the TBP, the plutonium remains in the acid stream while the uranium passes into the TBP. The success of the extraction process is largely dependent on the efficiency of the transfer from the aqueous phase and vice versa. In gen­eral, the uranium-plutonium will dissolve preferentially in the TBP when the aqueous phase has a high nitric acid content and will dissolve preferentially in the aqueous phase when it has a low nitric acid content. Thus, the final stage of the extraction is to take the uranium from the TBP stream by contacting the stream again with an aqueous phase having a low concentration of nitric acid.

The output of the separation stages in the reprocessing plant consists of streams of uranium, plutonium, and fission products dissolved in nitric acid. Each of these streams may be concentrated by evaporation and subsequently purified, if necessaiy, by additional solvent extraction stages. The uranium and plutonium are precipitated as uranium and plutonium nitrates, which are then heated to convert them into oxides, which may then be reused in the prepara­tion of nuclear fuel. The fission product stream is usually concentrated by evap­oration and passed to storage tanks for long-term storage and ultimate conversion into a solid form; we shall discuss this process in Chapter 8.

Once the uranium and plutonium have been extracted, the decay heat gen­eration is almost totally associated with the fission product stream in the repro­cessing plant. Any heat transferred to the solvent phase, together with the intense radiation, tends to degrade the solvent and cause difficulties in the op­eration of the plant.

The thermal and radiation problems in reprocessing plants are obviously fewer the longer the fuel has been stored in the cooling ponds prior to repro­cessing. It is for this reason that for thermal reactor systems the storage period is 5 years or more. However, this is not possible for fast reactors, where the eco­nomics of the fuel cycle dictates a fast turnaround in reprocessing. Much more fissile material is contained in fast reactor fuel than in thermal reactor fuel, and failure to utilize this valuable capital resource results in a considerable eco­nomic penalty. Furthermore, the rate at which fast reactors can be built is lim­ited because of the very much larger total inventory of valuable fissile material associated with each reactor.

Therefore, fast reactors present greater difficulties for reprocessing than do thermal reactors. They already have a higher specific heat generation rate, as seen in Figure 7.7, and their spent fuel must be reprocessed on a much shorter time scale, typically 6-9 months after removal from the reactor. The very high concentration of fissile materials in the streams presents a further difficulty. In the design of a reprocessing plant for both thermal and fast reactor fuel, one must take into account the possibility of developing a nuclear reaction (critical­ity) within the plant. This can be prevented in many cases by designing the plant so that the geometry of the pipes containing the solutions of fissile mate­rial is so unfavorable to the nuclear reaction that the plant can be regarded as “ever-safe.” This is particularly important in the reprocessing of fast reactor fuels where the concentrations of fissile material are high and the throughputs are small. Such plants are successful when proper attention is given to the design details; an example is the U. K. Atomic Energy Authority’s fast reactor fuel re­processing plant at Dounreay in Scotland, which is illustrated schematically in Figure 7.12.

REFERENCES

Jenkins, G. E.C., Lee, M. D., and N. Wall 0995). “Improved Refueling of Advanced Gas- Cooled Reactors.” Nuclear Europe Worldscan 15 (March-April): 44.

Nuclear Fuel Reprocessing Technology 0985), published by British Nuclear Fuels pic, Information Services, Risley, Warrington, U. K.

Fi^we 7.12: Reprocessing plant for the U. K. prototype fast reactor :it Doirnreav. Scot!.;’

Fuel Reprocessing Services 0986), published by British Nuclear Fuels pic, Information Services, Risley, Warrington, U. K.

Fuel Handling and Site Ion Exchange Effluent Plant (1985), published by British Nu­clear Fuels pic, Information Services, Risley, Warrington, U. K.