In order to make full use of the advantages of 233U it is necessary to extract the fissile material still present in the spent fuel and to reinsert it into the fresh fuel elements. This reprocessing operation implies of course further costs and complications.

A small part of the fuel will be lost during reprocessing and a reprocessing efficiency has to be taken into account in the fuel-cycle calculations. In the case of low enriched uranium cycle reprocessing is less interesting because the cross-sections of Pu being higher than the one of U, most of the bred Pu is burnt before the fuel element is discharged from the reactor. It must be noted, however, that because of increasing restrictions in disposing of irradiated fuel it might be necessary to reprocess also low enriched fuel. During reprocessing the uranium (or plutonium) present in the spent fuel is chemically separated from the fission products and other materials. It is not possible by these means to separate the fissile from the non-fissile isotopes of the same element. Continuing in this way to recycle the fuel one gets an accumulation of the parasitic absorbers like 236U and 237Np, as one can see from the chain of Table 9.1. This fact is very important in the case of the Th cycle because of the continuous introduction of 235U in the chain.

The problem is not important in the case of low enriched uranium cycle because, even in the case of reprocessing, only Pu would be reused and the parasitic absorber 242Pu, being the result of a long chain, has always a low concentration. In order to avoid this problem it is possible, in the Th cycle, to discharge selectively from the system those portions of the fuel that have experienced the largest burn-up. In order to achieve this it is necessary to separate physically the fertile from the fissile part of the fuel. This

can be done using different fertile and fissile particles (e. g. with different coating) so that it is possible to separate the bred 233U from the residual feed fuel. In particular the method chosen by General Atomic is to use a TRISO SiC coating on the fissile particles (containing the feed highly enriched uranium) using BISO coating for the fertile particles (containing 233U and Th). Another separation method would require the use of different diameters for the two types of coated particles.

The use of different elements containing the feed and the breed fuel has also been considered, in which case it might be possible to have a higher residence time in the reactor for the breed elements. This is very difficult in prismatic fuel because of resulting distortion in power distribution, but it has been seriously considered for pebble bed reactors.