11.2.1 Plutonium incineration

Plutonium incineration represents a special case. As we have seen, the stabilization of the plutonium inventory, and even its slow decrease,

should be possible within the existing nuclear energy producing system. However, if nuclear energy were to come to a stop and if the solution of underground disposal is not considered satisfactory, the reduction of the plutonium inventory would take a very long time, if standard solid fuel fast or thermal reactors are used. This is due to the need to mix fertile and fissile nuclei to limit the reactivity decrease during irradiation. For example, in the CAPRA concept [38], liquid metal fast reactors (LMFR) are used as plutonium burners. A CAPRA burner producing 8 TWhe burns 800 kg of plutonium but only consumes 160 kg, because of Pu regen­eration. Similar figures could be obtained with PWR reactors [41, 47]. Table 11.4 shows the production of transuranic elements for different schemes proposed for plutonium incineration in thermal reactors. Aside from the once-through UOx and well known single MOx schemes, the three additional schemes are:

• the MIX [169] scheme in which the fuel elements involve a homogeneous mixture of uranium oxide and plutonium oxide

• the CORAIL [40] scheme where both UOx and PuOx rods are placed within the same fuel assembly

• the APA scheme [41] which is a heterogeneous arrangement of UOx and Pu on inert matrix fuel elements.

These figures show that plutonium incineration would be slower than theoretically possible. In view of this situation, Bowman [133] has recently proposed using a molten salt hybrid reactor. The reactor would have the fol­lowing characteristics:

• thermal power: 750 MWth

• molten salt fuel with an NaF-ZrF4 carrier, fission products and plutonium fluorides

• thermal flux: 2 x 1014n/cm2/s

• moderator: graphite

• ks = 0.96.

The reactor is fed with a mixture of fission fragments, zirconium and plutonium fluorides obtained through fluorization of the spent fuels and extraction by sublimation of the uranium hexafluoride. The annual input would be 300 kg of plutonium and minor actinides, 1200 kg of fission products and zirconium cladding.[56] The output would be 65 kg of plutonium and minor actinides, 1435 kg of fission products and carrier salt.

Following Bowman [133], the advantages of such a system would be:

• no weapons plutonium or other weapons materials in repository

• possibility of underground criticality in repository eliminated

• 80% of fission energy recovered before waste emplacement

• instant irreversible elimination of weapons potential upon entry into transmuter.

The emphasis is clearly put on the prevention of uncontrolled military use of the plutonium in spent fuels. One TIER reactor would be associated with every 3000 MWt reactor, thereby eliminating the need for radioactive material transportation.

Further incineration of the remaining plutonium and minor actinides would require more elaborate chemical processing in order to separate fission fragments. Special reactors would be devoted to the second stage of incineration. In this case, however, transportation would again be necessary, but with no risk of weapons materials smuggling.