In order to limit the computer time, the number of groups used in diffusion and transport calculations should be kept as low as possible, as long as a sufficient accuracy can be guaranteed. This may lead to different group structures for the treatment of different problems.

A higher number of groups can be easily used in fast running one-dimensional diffusion calculations, while in the treatment of three-dimensional cases practical considerations limit the number of groups. The effect of the reduction of groups on accuracy can be tested in the one-dimensional case. While ксЯ can be calculated with sufficient accuracy with relatively few groups, an accurate power distribution in regions with strong space-dependence of the neutron spectrum (e. g. core-reflector boundary) requires a high number of groups particularly in the thermal range. An example of the effect of the number of thermal groups on power distribution is given in Fig. 8.3<20) (the example of this figure is rather extreme because of the very high fuel loading of this case). Usually group structures are defined by trying to separate, if possible, different

types of nuclear reactions or other significant phenomena. This separation, which is sometimes necessary for accuracy reasons, provides better information on the effect of different reactions. It is then possible to distinguish a range of the fission source spectrum, a range of unresolved and of resolved resonances, a thermal energy range, etc. Particular energy partitions are used in the interpretation of experiments in which threshold detectors are used. In the case of high Pu loading particular groups may be chosen to include the low-lying Pu resonances.

A high number of fast groups may be required if the core calculation has to provide a neutron source to be used in subsequent shielding calculations. Considerable detail in the fast spectrum may also be required for the calculation of neutron damage in reactor materials. In the case of calculations with a very limited number of groups care must be taken to avoid the artificial transfer of neutrons to energy ranges they cannot physically reach. A typical example is given by neutrons up-scattered above the boundary between thermal and fast range. As this boundary is usually assumed to be around 2 eV for HTR calculations, a certain up-scatter exists above it. If the fast group above this boundary is broad enough to reach the resonance range, the up-scattered neutrons can be artificially absorbed in these resonances while in reality the probability of being scattered up to that energy is negligibly small. This means that there should be a group between the thermal and the resonance range, otherwise it would be more accurate to neglect up-scattering altogether. As an example Tables 8.2 and 8.3 give the group structures used for the calculations of the Fort St. Vrain and of the THTR reactors. The

boundary at 17.6 eV has been chosen to keep the up-scattered neutrons outside the resonance range.