First principles calculations of thermodynamics of high temperature metal hydrides for NGNP applications

Abstract

In addition to their potential use at low to moderate temperatures in mobile fuel cell technologies, metal hydrides may also find application as high temperature tritium getterers in the U.S. DOE Next Generation Nuclear Plant (NGNP). We use Density Functional Theory to identify metal hydrides capable of sequestering tritium at temperatures in excess of 1000 K. First we establish the minimum level of theory required to accurately capture the thermodynamics of highly stable metal hydrides and determine that isotope effects can be neglected for material screening. Binary hydride thermodynamics are largely well established, and ternary and higher hydrides typically either do not form or decompose at lower temperatures. In this thesis we investigate anomalous systems with enhanced stability in order to identify candidates for the NGNP application beyond the binary hydrides. Methods implemented in this work are particularly useful for deriving finite temperature phase stability behavior in condensed systems. We use grand potential minimization methods to predict the interstitial Th−Zr−H phase diagram and apply high throughput, semi-automated screening methodologies to identify candidate complex transition metal hydrides (CTMHs) from a diverse library of all known, simulation ready ternary and quaternary CTMHs (102 materials) and 149 hypothetical ternary CTMHs based on existing prototype structures. Our calculations significantly expand both the thermodynamic data available for known CTMHs and the potential composition space over which previously unobserved CTMHs may be thermodynamically stable. Initial calculations indicate that the overall economic viability of the tritium sequestration system for the NGNP will largely depend on the amount of protium rather than tritium in the metal hydride gettering bed feed stream.