Spent nuclear fuel radiotoxicity

Less is known about the fate of the fission and neutron capture products that could result in the precipitation of unique alteration phases depending on the availability of these species in the fuel matrix. Burns et al. (1997) theorized that many of the U(VI) alteration phases may be capable of incorporating the long-lived radiotoxic isotopes, including 237Np, 99Tc, and 239Pu. In this chapter, we will discuss the evidence for Np incorporation into U(VI) phases and the behaviour of Pu in corroded spent nuclear fuel (SNF). 

Some countries, e.g., France, Japan, Russia, and the United Kingdom have chosen to reprocess their spent nuclear fuel to recycle uranium and plutonium as nuclear fuel and to obtain a high active waste (HAW) firaction that is less radiotoxic than the spent fuel itself. In this process, very high separation factors are necessary. The fission product activity has to be reduced by a factor of > 10 and the separation factor between uranium and plutonium must be at least 2 x lO. All full-scale reprocessing processes are based on solvent extraction, and today the plutonium uranium redox extraction (PUREX) process dominates the market completely. 

One of the major issues related to the expanded use of nuclear power is the fate of plutonium and actinides. Although there are a number of fission product radionuclides of high activity ( Cs and °Sr) and long half-life ( Tc, 200,000 years I, 1.6 x 10 years) in spent nuclear fuel, actinides account for most of the radiotoxicity of nuclear waste because, after several hundred years, the radiotoxicity is dominated by Pu (half-life = 24,100 years) and Np (half-life = 2,000,000 years). Thus, a major part of the long-term risk is directly related to the fate of these two actinides in the geosphere. 

The specific radiotoxicity was defined as the radiotoxicity of the spent nuclear fuel under a given power production divided by the power produced. 

Figure XIX-5 presents the specific long-lived radiotoxicity of spent nuclear fuel within the nuclear fuel cycle as a function of energy produced by the SVBR-75/100 reactors. These calculations show that specific radiotoxicity of the technetium-99, iodine-129 and caesium-135 before the final disposal is 0.014 km /GW(e)/year, without taking into account the losses in reprocessing. It is nearly equal to the specific radiotoxicity of natural uranium extracted from Earth and added to the fuel cycle each year.

The MSR (see Fig. 2.7) embodies the very special feature of a liquid fuel. MSR concepts, which may be used as efficient burners of transuranic elements from spent LWR fuel, also have a breeding capability in any kind of neutron spectmm ranging from thermal (with a thorium fuel cycle) to fast (with a uranium—plutonium fuel cycle). Whether configured for burning or breeding, MSRs have considerable promise for the minimization of radiotoxic nuclear waste. 

Another benefit for the SFR fuel cycle system is the reduction of environmental burden by recycling all actinide nuclides and partitioning selected fission products (FPs). The spent fuel contains minor actinides (MAs ie, neptunium, americium, curium, etc.) as well as uranimn and plutonium. In the conventional nuclear fuel cycle, those MAs and FPs are disposed of in a deep geological repository as high-level radioactive wastes. Because of the long-lived radioactive MAs such as Am (half-life 433 years) and Np (half-life 2.1 million years), it takes several hundred thousand years to reduce the radiotoxicity of high-level radioactive waste to the level of natural uranium. 

Nuclear waste, either in the form of spent fuel reprocessing, is associated with a radiotoxicity potential due to minor actinides (MA) and aon products (FP). The possibility of partitioning minor actinides out of the waste and transmuting them into less hazardous nuclides has been proposed and its technical feasibility is being studied. 

In the Reprocessing Fuel Cycle (RFC) option, the unused uranium and the plutonium produced in the reactor are recovered leaving the minor actinides with the fission products as HLW. (The radiotoxicity of these wastes will be significantly less than that of the spent fuel although the toxic lifetime is determined by the minor actinides - neptunium, americium, curium - and, to a lesser extent, by some of the long-lived fission products content of the waste.) As mentioned previously, this was the scenario initially envisioned by the nuclear power industry to reprocess fuel for two reasons ... 

Nonetheless, many proponents of the thorium cycle promote this feature, and indeed, many detractors of tiie nuclear industry cite the long-term radiotoxicity in spent uranium-based fuels as a major issue. 

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