Although some attempt of standardization of the fine group structure had been made,<21) there still remain considerable differences as we have seen in the preceding sections.

These differences have not only historical but also physical reasons. A code with a very fine group structure can be used for any sort of calculations, but requires a longer running time, while codes having a limited energy detail are very fast but their library is valid only for a more restricted range of calculations. In HTR survey analysis where many spectrum calculations have to be performed both approaches are possible.

The first approach,<22) used by General Atomic, consists in using a rather sophistica­ted code (MICROX) for a limited number of spectrum calculations in which the broad group cross-sections are obtained as a function of fuel temperature and C/Th ratio in the fast range, and as a function of moderator temperature and C/235U ratio in the thermal range. These results are stored and a subsequent interpolation is made for the actual composition of each single case.

The second approach consists of using a simplified code like MUPO, repeating the spectrum calculation for each case.

The fine energy structure for the spectrum codes most frequently used for HTR calculations is shown in Table 8.1.