Improved S(α, β) tables for TRIGA criticality and reactivity feedback calculations

The accurate evaluation of the reactivity feedback in TRIGA cores gained additional importance within the TRADE project. In this sub-critical system coupled with an external source the operational temperatures could reach 150 C degrees and more. The suggested reactivity feedback curve provided by General Atomics which is sufficient for core shut down analysis appears to be inaccurate in comparison with new experimental data, in particular in the range of 80-140 C degrees. The hydrogen bound in zirconium within the TRIGA fuel is the main cause for the negative reactivity safety mechanism. An immediate neutron up-scattering by hydrogen as a result of a fuel temperature increase shifts the neutron spectrum to lower fission cross-section energy range. The up-scattering of neutrons is governed by the scattering kernel which is a measure of the probability of a neutron to be scattered from its incident energy to another one and from its original angular direction to a new one after a collision. Those probabilities also known as the scattering law are presented within probability tables based on so call S(α,β) formalism which should conserve the energy and momentum laws. For bound isotopes like hydrogen in zirconium, the structure of the lattice and the intermolecular forces influence immensely the scattering probability. The current study looks at various models and parameters concerning the generation of the molecule structure dependent S(α,β) tables and their impact on the reactivity feedback temperature coefficient of the TRIGA fuel. It is shown that different computational handling of the models for internal forces within the ZrH molecule in particular for the so called phonon spectrum description leads to variation of the reactivity feedback as well as the criticality value. The influence of adopting a new scattering kernel on the fuel pin criticality value is also presented for the first 8 S resonances of U238 within the TRIGA fuel pin. (authors)