7.1 INTRODUCTION

In Chapters 4-6 we have discussed hypothetical and actual accident conditions in reactors. Now we return to the discussion of the next phase of normal oper­ation, namely, the removal of the used fuel from the reactor and its subsequent processing.

In a nuclear reactor, the fissile material is gradually used up and converted to energy and fission products. During the nuclear reaction there are changes in the microstructure of the fuel due to the release of fission products, which ei­ther combine with the fuel or are released inside the fuel can. These changes have two effects: (1) a gradual deformation of the fuel and in some cases the can and (2) the release of fission products (such as xenon and iodine), which are themselves strong absorbers of neutrons, leading to a reduction in neutron population and a less efficient nuclear reaction. For these reasons, the fuel ele­ment must be removed from the reactor after a period of time and before all the fissile material is used up. Typically this period will be between 3 and 5 years for thermal reactors and 1 year to 18 months for fast reactors. For thermal reac­tors, 60 to 75% of the original fissile material is used up at the time of fuel re­moval. For the fast reactor, the utilization is much less, of the order of 25%. The fraction utilized is often referred to as the burn-up.

The fuel removed from a nuclear reactor contains three kinds of valuable material:

1. The unused proportion of the fissile material that was originally introduced with the fresh fuel.

2. New fissile material that has been bred as a result of the nuclear reactions, in particular, the reaction between neutrons and 238U to form 239Pu. The pluto­nium produced can be used as a fissile material in both thermal and fast re-

actors. Note that bred material also participates in the fission reaction while the fuel is still in the reactor; in a thermal reactor system, 25% of the heat production might arise from fission of material bred in situ.

3. Much of the original 23HU, the nonfissile isotope of uranium, still remains. This material is valuable as a fertile material for use, particularly in the blan­kets of fast reactors, where it is converted to 239Pu.

Of course, these valuable materials are mixed with a range of highly ra­dioactive fission products that form the waste from the nuclear cycle. Basically, there are two choices facing the nuclear power operator:

1. To discharge the fuel and store it safely without making any attempt to sep­arate the useful fissile-fertile materials from the fission products in the fore­seeable future. If such storage is regarded as permanent, this approach is colloquially referred to as the throwaway cycle in the sense that valuable re­sources are being disposed of. Such a cycle could be practical only if it was felt that the world’s uranium resources were adequate to operate thermal re­actors for a sufficiently long period. As discussed in Chapter 1, this would be a highly inefficient use of these resources.

2. To discharge the fuel, store it for a relatively short time (typically 1-5 years) to allow the more active fission products to decay and the decay heat to drop to manageable levels, and then to process the fuel chemically to sepa­rate the valuable fissile and fertile materials from the fission products, which can then be stored in a safe form.

If a program of fast reactor operation is envisaged, the second option is manda­tory; otherwise, far too much of the fissile material in the cycle will be wasted. Reprocessing the fuel is more expensive in the short term than simply storing it, and the decision about whether to reprocess in the case of thermal reactor fuels is closely related to the overall utilization strategy for nuclear energy in individ­ual countries. Where a program of fast reactors is envisaged, reprocessing of thermal reactor fuel is necessary in order to produce the initial inventory of plu­tonium for such a program. Typically, it would take about 15 years’ worth of spent fuel from a thermal reactor to produce the initial inventory for a fast re­actor of similar size.

In this chapter we discuss the removal of spent fuel elements from reac­tors, their transport to a long-term storage location or a reprocessing plant, and the problems of the reprocessing plant itself. The questions of long-term storage of nuclear waste products will be discussed in Chapter 8. This chapter concentrates on the thermal aspects of these operations, in line with the rest of this book.