Temperature reactivity effects in mixed TRIGA cores

Calculations of the fuel temperature reactivity coefficient αf are treated in this paper for three types of TRIGA fuel: standard (8.5 w/o U), FLIP and LEU. Uniform and mixed cores are considered. αf of a uniform core is calculated in unit-cell approximation with effective buckling using the WIMSD-4 computer programme. The original library of WIMS is extended by the authors for several nuclides specific for TRIGA fuel: H in ZrH, Er-166 and 167, etc. Temperature reactivity effects of TRIGA fuels cannot be reproduced without using temperature-dependent cross sections of hydrogen bound in a zirconium lattice. αf of a mixed core is calculated for TRIGA-MARK II geometry in diffusion approximation using the TRIGAP computer programme. The code is provided with an effective temperature dependent few-group library based on WIMS results for unit cells. It is shown the that temperature coefficient of a mixed core depends not only on the number of different fuel elements, but also on their loading pattern. Diagrams for predicting αf of mixed cores are presented for practical use in safety analysis for most common combinations of different fuel types