Minor actinide production is largely responsible for the radiotoxicity of the wastes of fuel reprocessing. However, the Th/Pu system seems to be the most efficient if the asymptotic thorium cycle is to be reached, starting from present fissile materials. In order to have a quantitative description of this scenario, we choose to represent the radiotoxicity after 200 years of energy generation, taking into account the actinide losses incurred during reproces­sing and the final inventory of the reactor. We compare these results with the two other simple and realistic scenarios: the U/Pu cycle used in fast reactors and PWR continuation over the same period. Moreover, we consider two different types of reprocessing. Type-A fuel recycling has been described in the previous section and consists of removing every 5 years all the fission products from the fuel and replenishing with fertile matter. All the heavier elements are kept in the fuel, the minor actinides remaining as part of the reactor inventory. This is an optimistic scenario which requires chemical separation techniques that are not yet available. Type-B fuel recycling con­sists of keeping only the main fissile element, uranium or plutonium, in the fuel, while fission products and all other actinides flow into the wastes. The new fuel is topped up with fresh fertile material. This simple scenario corresponds to the techniques used today at existing reprocessing plants [66].

Figure 11.7 shows the calculated radiotoxicity of the total wastes produced during 200 years of energy production and of the final inventory generated by the simulated hybrid reactors (Th/Pu and U/Pu systems), per unit of installed thermal capacity, assuming type-A or type-B fuel reproces­sing. Also shown is the radiotoxicity of PWR uranium oxide spent fuel, assuming 200 years of continuous energy production, with no reprocessing. For the hybrid systems this scenario requires two fuel loads each running over 20 fuel cycles. Figure 11.7 displays the values of the radiotoxicity R[1000y], 1000 years after discharge. Several conclusions can be drawn from the quantities displayed. First, the radiotoxicity associated with the initial Pu loading (about 10 tons of Pu for the two loads of fuel in the 1 GWe system) is always smaller than 18% of the total, and represents a rela­tively weak source of radiological risk. Second, if type-B fuel reprocessing is assumed, leaving all the minor actinides in the wastes, the U/Pu fast-neutron reactor does not offer a significant advantage, in terms of the radiotoxicity 1000 years after discharge, with respect to a PWR, after 200 years of energy production; a 232Th/Pu reactor, on the other hand, reduces the total radiotoxicity by a factor of five as compared with a PWR. Third, the implementation of a type-A fuel regeneration scheme, where all actinides are reprocessed, causes a decrease of the radiotoxicity generated by the U/ Pu system by a factor of three with respect to type-B recycling, and a consid­erable reduction for the 232Th/Pu system, whose radiotoxicity becomes 70 times smaller than the PWR reference.

In conclusion, it appears that the U/Pu fuel cycle does not offer a signif­icant radiotoxicity reduction when compared with a PWR system. Both innovative options, plutonium and minor actinide reprocessing (type-A) plus a thorium-based fuel cycle, give together an optimum scenario which meets three of the basic requirements for an acceptable future use of

nuclear energy, namely the incineration of heavy actinides from PWR plants, the use of abundant fuel resources, and the minimization of radiotoxic waste production. Subcritical reactors would help to develop the thorium cycle in optimized conditions of 233U breeding.

The flow of radioactive nuclei exiting the layer being known, it is possible to determine the maximum dose delivered to the critical population. The

assumption is that the radioelements coming out of the clay layer are dispersed in the ground water used by the critical population. Let Jmax be the maximum value of the flow of nuclei. Admitting that an equilibrium of the flows is reached, an equal flow is obtained at the outlet of the ground water. Each member of the population ingests a fraction a of the water available so that the flow of radioelements ingested is aJmax. Only a fraction f of radioelements ingested settles in the body. The radioelement is character­ized by its biological period Tb that corresponds to the time during which it remains in the body and, hence, a biological half-life Ab = 0.693/Tb. The amount r of radioelement present in the body is thus such that, at equilibrium, rAb = afJmax. The radioactivity inside the body due to the radioelement is Ar = AafJmax/Ab. The radioelement is assumed to settle in target organs whose mass is m,. If the radioelement’s decay energy (neutrinos excluded) is є, the annual dose received is A = A"(afJmaxv/m, Ab) where, time being measured in seconds, v is the duration of a year. If the energies are expressed in Joules and masses in kg, the absorbed dose is in Gray/year. To account for the differing biological effectiveness of radiations, each is given a quality factor, which is 1 for photons and electrons and approximately 20 for a particles. The committed dose is then expressed in Sieverts. Moreover, as the radioelement settles preferentially in one or several organs, the mean dose delivered to the whole body is less than that delivered to the organ(s) considered. To account for this effect, a weighting factor w is associated with each organ. Finally, the effective committed dose is