Comparison of different energy group models in evaluating radial and azimuthal xenon transients at the example of HTR-cores

The investigation concerns the influence of different few-group energy models for the neutron flux in computing xenon-induced spatial power instabilities in thermal reactors with the aid of the multi-group theory. Comparative transient calculations with a 1-, 2-, 4- and 7-group model have been performed at two typical HTR-models using a finite difference program. We considered the first azimuthal and the first radical oscillation mode of the cylindrical cores. The dependence of essential oscillation parameters like damping factor and period on the number of energy groups and on the choice of the energy intervals is determined. Trends are explained and the increased expenditure of computer work at larger group numbers is discussed in relation to the profit to accuracy recognizable therefrom. Essential results are, that the one-group formalism proves to be totally unsuitable for such calculations and that not unimportant but considerably case-dependent differences are also found to exist between the multigroup calculations. These latter effects are not merely due to differently calculated steady-state power density profiles. (orig.)