Speaker
Description
Many transition metals react with hydrogen to form stable metal hydrides. Metal hydrides are technologically attractive materials because of their ability to store large amounts of hydrogen. There is considerable interest in zirconium hydrides in the nuclear industry because they are used as neutron moderators in nuclear reactors. TRIGA reactors have a unique fuel composition: a homogeneous mixture of 20 % enriched uranium and zirconium hydride (ZrH ratio close to 1.6). This is the main reason for the prompt negative temperature reactivity coefficient. Since the hydrogen in the zirconium hydride acts as a moderator, most of the moderation occurs in the fuel element itself and only a small part in the water surrounding the fuel elements. Therefore, any change in power and fuel temperature directly affects the moderator in the fuel element. In this case, the fuel and the moderator directly affect the reactivity of the core. This underlines the need to understand the basic atomistic properties of zirconium hydrides.
In the present work, we adopt first-principles calculations to obtain thermal neutron scattering law data (TSL) of ZrH2 in the ε phase (at room temperature dominant for x > 1.74). This work aims at evaluating our calculation method and compare our results with the current thermal neutron scattering law data from the ENDF/B-VIII.0 library. The current ENDF/B-VIII.0 ZrHx nuclear data evaluations do not distinguish between phases. The thermal neutron scattering law data for ZrHx from the ENDF/B-VIII.0 library were generated using historical phonon spectra derived from a central force model and assume incoherent elastic scattering for both bound hydrogen and zirconium, neglecting the effects of crystal structures that are important for the scattering from zirconium bound in ZrHx.
The system is modelled and relaxed to its ground state using the density functional theory (DFT) capable computer code VASP. The positions of the atoms are then perturbed and the interatomic force constants are calculated. Once the force constants are obtained, they are transferred to the Phonopy code, which performs lattice dynamics calculations that seek solutions to the dynamical matrix problem. The solutions constitute the dispersion relations of the system, from which the atomic vibrational density of states (DOS) are computed using a geometrical sampling procedure. Once the density of states is known, the scattering law and all related quantities can be calculated using the LEAPR module of the NJOY data processing system. The scattering law data are the basis for the analysis of thermal reactor systems, which are frequently modelled by Monte Carlo transport codes to obtain the desired physical reactor parameters. The thermal nuclear data of our calculated ZrH2 are compared to the ZrHx from the ENDF/B-VIII.0 library. The calculated thermal nuclear data is validated by comparing the calculated physical reactor parameters from the Monte Carlo transport code with measurements of the physical reactor parameters from the TRIGA Mark II research reactor at Jožef Stefan Institute.