Both 238U and 232Th can be used as fertile materials for high temperature reactors. In case of 238U the ratio of fissile to fertile material in the fresh fuel corresponds to an enrichment of 5 to 8%. In the case of 232Th this fertile material has to be mixed with highly enriched (usually 93%) 235U. The Th cycle has thus the disadvantage of needing highly enriched uranium. On the other hand, the 233U produced in this cycle is a very good nuclear fuel with a very high tj value (number of fission neutrons produced per neutron captured in the fissile material).

Typical tj values, averaged over an HTR spectrum are:

233U tj = 2.29 235U tj = 2.05 239Pu tj = 1.80

These are only indicative values and can change from design to design according to the core composition. The high tj value of 233U allows a higher conversion factor and a value higher than 1 is theoretically possible. This means that it would be actually possible to breed fuel in an HTR. To achieve this it would be necessary to limit very greatly the fuel burn-up in order to reduce neutron losses in fission products, and to use at least in part Be as moderator in order to take advantage of its good moderating properties and of its n, 2n reactions. This turns out to be too expensive in practice so that breeding would be economically nonsensical in HTRs, nevertheless the Th cycle retains the advantage of a higher conversion factor and hence a better use of the natural fuel resources. In general it is possible to state that from the point of view of neutron economy the Th cycle is best suited to exploit the spectral characteristics of HTRs, but also other factors (enrichment, reprocessing, refabrication, existing industrial equip­ment, symbiosis with the fuel cycle of other reactor types) have an important weight in the choice of the fuel cycle.

As one can see from Tables 9.1 and 9.2 the breed fuel 233U or 239Pu is produced by the /З-decay of 233Pa or of 239Np. This means not only a certain delay in the production of the fissile isotopes, but also losses of neutrons and of fissile material because of parasitic absorption in 233Pa or 239Np. This fact is not very important in the case of 239Np because of its short half-life (2.33 d), while it is much more important in the case of 233Pa (half-life 27.4 d).

The losses in 233Pa are proportional to the thermal flux level, i. e. to the fuel rating (power per unit weight of fissile material). The losses due to decay of M1Pu in 241 Am are negligible during reactor operation because of the long half-life of “‘Pu (13.2 y), but can become important if plutonium fuel is stored for several years.