The use of breeding or converter[6] reactors would change the picture consid­erably. Converters and breeders allow full use not only of the fissile 235U isotope, but also of the fertile U and Th isotopes. Thus, in principle, a 1 GWe reactor requires only 1 ton of natural uranium or an equivalent amount of thorium. This means that, at the current market cost, assuming a production capacity of 9000 GWe, corresponding to the most nuclear­intensive scenario of table 2.13, the reserves would amount to 2000 years for natural uranium and about four times more for thorium. In fact, the very efficient use of the uranium and thorium would allow the use of very low grade ores, including sea water uranium, which means that the resources would be practically unlimited. The mill tailings would also be considerably reduced by a factor more than 100.

While the plutonium present in spent fuels has to be considered a waste, it is the fissile material for breeders and converters. Only long-lived fission products (LLFPs) and minor actinides (MAs)^ can thus be considered as nuclear wastes. In the absence of specific transmutation of these wastes, their radiotoxicity, after a cool-down period of 300 years,* would be at least one order of magnitude smaller than that of the PWR spent fuels, for an equivalent energy production. Since fuel reprocessing is a prerequisite for any breeding or converting cycle, it is quite logical to consider the trans — mutating of LLFPs and MAs. We shall discuss this possibility in some detail below. It has been shown that the incineration of MAs and the transmutation of some of the most significant LLFPs are feasible. Nuclear wastes would then be reduced to the reprocessing losses. Modern reprocessing is claimed to have 99.9% efficiency in the recovery of plutonium and 99% in the recovery of MAs [38]. It would then be possible to reduce the total radiotoxi­city of the wastes by several orders of magnitude after a few hundred years of cooling. With such a reduction, long-term disposal might not be necessary, or at least will be considerably reduced.

While the reliability and safety of PWR reactors has been widely demon­strated in industrialized western countries,§ the experience with breeder or converter reactors is limited and ambiguous. By far the best known breeders are of the liquid metal fast reactor type. Practically all of these liquid metal reactors have used sodium as their coolant,[7] with the exception of several recent Russian submarine propulsion reactors which are cooled with a liquid lead-bismuth eutectic. While it seems that the records of the Russian sodium cooled reactors like BOR60, BN350 and BN600 are very good, the records of such western reactors are much more questionable. The small American reactor EBR2 worked satisfactorily until its final shutdown in 1995. Another American reactor, Enrico Fermi, could never run. The small French reactor RAPSODIE ran very nicely until its final shutdown. The 250 MWe Phenix reactor ran satisfactorily for ten years until unexplained reactivity fluctuations led to its shutdown. It has been started again recently at reduced power, pending safety improvements. The large, 1200 MWe Superphenix reactor was plagued by sodium leaks and administrative imbroglio until the decision was made to stop it indefinitely. The Japanese Monju reactor is, also, suffering from sodium leaks. It seems that the combination of increasingly stringent safety constraints and the use of the very reactive liquid sodium led to difficult running conditions. Further­more, the investment costs for a reactor like Superphenix are about twice as large as those necessary for a standard PWR reactor with the same power. The cost of electricity which would have been produced by Superphenix in normal running conditions would have been twice that produced by a PWR. Of course, it can be argued that, for an industrial-scale series of reactors, the investment cost as well as the fuel cost would have decreased.

In conclusion, while the interest of breeder reactors is clear in the hypoth­esis of an extension of nuclear power, one cannot consider the present type of sodium cooled reactor to be the only solution. In this context it is possible that hybrid reactors may help develop alternatives to sodium cooled reactors. They may also facilitate a switch to the thorium breeding cycle which would lead to a much reduced production of minor actinides.