12.1.1 Critical PWR and FR

The main restriction to introducing minor actinides into critical reactors is linked to their impact on the core’s reactivity and kinetic parameters [3-5]. This would produce:

• a drop in the fuel temperature coefficients (Doppler effect),

• an increase in reactivity effect linked to the coolant voiding,

• a reduction in delayed neutron yield.

For instance, the impacts of these factors as a function of minor actinides content loading are given for the case of a large fast reactor in Fig. 12.3. The minor actinides content limit is around 3% of total heavy nuclides for large sodium cooled reactors. The gas cooled fast reactor allows a higher

loading in MA than liquid metal fast reactor due to the disappearance void effect constraint: 5% of MA seems acceptable even for a large core.

The void effect is also actually the most restricting criterion for MA loading in PWR from the point of view of its impact on physics and safety. Without the coolant, the neutron spectrum moves to higher energy and the minor actinides contributions of the thermal and epithermal resonances vanish. The minor actinide content must then be limited to about 1% of total heavy nuclides.

For critical reactors, some conclusions can be proposed:

• The admissible quantities of minor actinides in the core have to be kept low (about 1% for PWR type and 3% to 5% for FR type).

• The maximum MA fission rates are about 5 to 10% in PWRs and from 15 to 30% in FRs. In both cases, a multi recycling is necessary in order to reach satisfactory overall performance.

• In PWRs, curium recycling is to be avoided as it produces by itself and by producing californium, an intense source of neutrons. In FRs, curium recycling also produces upper elements, but they stabilize at a far lower level than in the PWRs and therefore do not pose any specific new problem.

Fast spectrum reactors can operate as breeders or burners, or in a self­sufficient breakeven mode. Breeders incorporate external blankets, both axial and radial. When reflectors replace blankets, FRs become net burners of fissile material. If an appropriate amount of blanket is incorporated, then a self-sufficient mode can be maintained. In whichever mode they operate, FR discharged fuel contains a large fraction of fissile inventory, and hence recycling is mandatory. As a matter of fact, resource utilization improve­ment is the primary rationale for fast reactors and recycling is required to achieve that goal.

Historically, only uranium and plutonium have been recovered from LWR spent fuels, and only plutonium recycling has taken place or been envisioned for the fast reactor fuel cycle. However, we have seen that minor actinides can be also recycled along with plutonium in fast reactors. Minor actinides and even-mass isotopes of plutonium may not be attractive as fuel for thermal reactors because they have unfavourable ratios of fission to capture, as demonstrated in the previous chapter. These same materials, as well as odd-mass isotopes of plutonium, are fissionable in fast spectrum, where we have seen that the fission to capture ratio is much more favour­able. Furthermore, the high content of recycled plutonium may require remote fabrication and hence minor actinides can be more easily incorpo­rated into continuous recycling.

For economic and political reasons and because of proliferation concerns in some countries, fast reactors and their fuel cycle development pro­grammes have been curtailed since the 1990s, except in China, France, India, Japan and the Russian Federation. Fast reactor concepts for actinide trans­mutation have been of interest in recent international initiatives such as the International Project on Innovative Nuclear Reactors and Fuel Cycles (INPRO) or Generation IV International Forum (GIF). Apart from these developments, the value of preserving the large technology base developed in Japan, France, Germany, the Russian Federation, the United Kingdom and the USA, as well as information developed in other countries, has been a subject of essential scientific interest.