Irradiated fuel

Uranium is converted by CIF, BiF, and BrP to UF. The recovery of uranium from irradiated fuels has been the subject of numerous and extensive investigations sponsored by atomic energy agencies in a number of countries (55—63). The fluorides of the nuclear fission products are nonvolatile hence the volatile UF can be removed by distiUation (see Nuclearreactors Uraniumand uranium compounds). 

The throwaway fuel cycle does not recover the energy values present ia the irradiated fuel. Instead, all of the long-Hved actinides are routed to the final waste repository along with the fission products. Whether or not this is a desirable alternative is determined largely by the scope of the evaluation study. For instance, when only the value of the recovered yellow cake and SWU equivalents are considered, the world market values for these commodities do not fully cover the cost of reprocessing (2). However, when costs attributable to the disposal of large quantities of actinides are considered, the classical fuel cycle has been the choice of virtually all countries except the United States. 

Fuel Characteristics. Historically, chemical reprocessing of irradiated fuel was developed specifically to handle the U.S. government s defense-related fuels. These were of two types. There were the very low enrichments and bum-ups, eg, 0.9% U and 2000 MW-d/t, as was used for... 

Canadian reactors (CANDU) are fueled using natural uranium. The discharged fuel contains small amounts of plutonium, but the fissile uranium content is below that of natural uranium. Therefore, the irradiated fuel is not normally considered a candidate for economic reprocessing. 

Loss of cooling to irradiated fuel in fueling machine 2.0E-3... 

Another possibility is dropping into the heat exchanger bay the 100 ton heat exchanger to cause a primary or secondary LCXZA. If irradiated fuel were still in the reactor, melting could occur. Heat exchangers have been removed with hot fuel in the reactor (at least twice since 1971). This scenario was estimated to be -- lE-4/y and procedures were changed to eliminate it... 

Many of the fission products formed in a nuclear reactor are themselves strong neutron absorbers (i.e. poisons ) and so will stop the chain reaction before all the (and Pu which has also been formed) has been consumed. If this wastage is to be avoided the irradiated fuel elements must be removed periodically and the fission products separated from the remaining uranium and the plutonijjm. Such reprocessing is of course inherent in the operation of fast-breeder reactors, but whether or not it is used for thermal reactors depends on economic and political factors. Reprocessing is currently undertaken in the UK, France and Russia but is not considered to be economic in the USA. 

Irradiated Fuel A historically important and continuing mission at the Hanford site is to chemically process irradiated reactor fuel to recover and purify weapons-grade plutonium. Over the last 40 years, or so, several processes and plants— Bismuth Phosphate, REDOX, and PUREX—have been operated to accomplish this mission. Presently, only the Hanford PUREX Plant is operational, and although it has not been operated since the fall of 1972, it is scheduled to start up in the early 1980 s to process stored and currently produced Hanford -Reactor fuel. Of nine plutonium-production reactors built at the Hanford site, only the N-Reactor is still operating. 

The reactor accident at Chernobyl in April 1986 released radionuclides into the atmosphere, mostly between April 26 and May 6. Estimates of quantities released are based on observations of deposition within 30 km of the reactor. Releases in this area were predominantly highly irradiated fuel particles. It is estimated that the discharge of 241Pu through May 6, 1986 was 5,200 TBq (140 kCi), which amounted to 3% of the reactor content of this radionuclide (Askbrant et al. 1996 Pattenden and McKay 1994). The material was released mainly in the lower troposphere. 

Dissolution procedures are described for gram samples of graphite or pyrolytic carbon, milligram samples of irradiated fuel particles, and for more readily oxidised forms of carbon, such as charcoal. The first two methods involve heating the samples with mixtures of 70% perchloric and 90% nitric acids (10 1), and must only be used for graphite or pyrolytic carbon. Other forms of carbon must not be oxidised in this way (to avoid explosions), but by a preliminary treatment with nitric acid alone and in portions. 

Fig. 12.9 Two-stage acid Thorex process for highly irradiated fuels. Numbers in the frames indicate stage number, whereas numbers on the lines indicate flow volumes relative to the feed volume (DOD and FP are dodecane and hssion products). 

Meanx hile, success in the development of the natural uranium fuelled CANDU concept had led to very low cost fuelling and effective utilization of uranium even without recovery through reprocessing. AECL therefore decided to set aside work on reprocessing and concentrate instead on the once-through fuel cycle with storage of the irradiated fuel. The evidence indicated that the zirconium clad UO fuel could be stored under water for many decades until a decision was needed regarding recycle or disposal. 

Thus the consistent Canadian philosophy has been to store irradiated fuel retrievably until a decision on its ultimate disposition is necessary or desirable. Experience with the storage of CANDU fuel now extends over 15 years and has provided confidence that this type of fuel can be stored safely and economically for the order of five decades, using proven technology. 

In the chemistry of the fuel cycle and reactor operations, one must deal with the chemical properties of the actinide elements, particularly uranium and plutonium and those of the fission products. In this section, we focus on the fission products and then chemistry. In Figures 16.2 and 16.3, we show the chemical composition and associated fission product activities in irradiated fuel. The fission products include the elements from zinc to dysprosium, with all periodic table groups being represented. 

Figure 16.2 Chemical composition of the fission products in irradiated fuel as a function of decay time after a 2-month irradiation. [From J. Prawitz and J. Rydberg, Acta. Chem. ScantL 12, 393 (1958).]...

Initially, the radioactivity levels of the irradiated fuel are very high (Fig. 16.8). Chemically, the 235U content has been reduced from its initial 3 to 1% while the... 

The original plan for this spent fuel was to store it for about 150 d and then transfer it to other facilities for disposal as waste or reprocessing. In the United States, this transfer has not occurred, and most of the fuel has remained in cooling ponds at the reactor sites, for times that have exceeded 20 y. In this case, a closer look at the properties of irradiated fuel is justified. 

Figure 16.9 Changes in the chemical composition of irradiated fuel. (From Murray, 2003.)...

An overall schematic view of the Purex process is shown in Figure 16.10. Detailed descriptions of the process can be found in Benedict and co-workers (1981) and Wymer and Vondra (1981). We shall briefly summarize the important steps. The first step is to prepare the irradiated fuel for dissolution by mechanically chopping it into small pieces ( 1—5 cm). This opening of the cladding causes the... 

Viewed in the context of the actinide lifespan, the nuclear fuel cycle involves the diversion of actinides from their natural decay sequence into an accelerated fission decay sequence. The radioactive by-products of this energy producing process will themselves ultimately decay but along quite different pathways. Coordination chemistry plays a role at various stages in this diversionary process, the most prominent being in the extraction of actinides from ore concentrate and the reprocessing of irradiated fuel. However, before considering these topics in detail it is appropriate to consider briefly the vital role played by coordination chemistry in the formation of uranium ore deposits. 

The uranium and thorium ore concentrates received by fuel fabrication plants still contain a variety of impurities, some of which may be quite effective neutron absorbers. Such impurities must be almost completely removed if they are not seriously to impair reactor performance. The thermal neutron capture cross sections of the more important contaminants, along with some typical maximum concentrations acceptable for fuel fabrication, are given in Table 9. The removal of these unwanted elements may be effected either by precipitation and fractional crystallization methods, or by solvent extraction. The former methods have been historically important but have now been superseded by solvent extraction with TBP. The thorium or uranium salts so produced are then of sufficient purity to be accepted for fuel preparation or uranium enrichment. Solvent extraction by TBP also forms the basis of the Purex process for separating uranium and plutonium, and the Thorex process for separating uranium and thorium, in irradiated fuels. These processes and the principles of solvent extraction are described in more detail in Section 65.2.4, but the chemistry of U022+ and Th4+ extraction by TBP is considered here. 

Figure 29 An outline process for irradiated fuel reprocessing by solvent extraction...

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