Advanced Moderator Material Handbook

Abstract

High hydrogen density moderators such as metal hydrides are an important research topic within the DOE NE Microreactor Research, Development, and Deployment (RD&D) Program due to their ability to retain hydrogen to much higher temperatures than other hydrogenous media. This class of moderators, which includes yttrium dihydride (YH2), thermalizes neutrons in the system such that the overall fuel mass or the required uranium enrichment in the system can be significantly reduced. Knowledge of material properties, both in the as-fabricated and irradiated state, are important to understanding moderator performance during steady-state and transient reactor operation. Provided in this document is the Advanced Moderator Material Handbook, which provides a detailed summary of the literature data on yttrium dihydride, thermomechanical and other property data, and a critical evaluation of that data. This handbook also provides a description of ongoing experiments to understand in-reactor performance, such as irradiations in ATR, as well as nuclear data from an integral critical experiment at NCERC. The majority of this report focuses on measured values but also includes some modeling results for comparison where applicable.

A New Era of Nuclear Criticality Experiments: The First 10 Years of Planet Operations at NCERC

“…Microreactor designers at LANL are considering the use of yttrium hydride (YH X ) due to its ability to retain hydrogen within the material at increased temperatures, in comparison to other hydrided materials. 34 However, no integral experimental data on YH X could be found. The Hypatia experiment utilized uranium fuel, YH X , and a beryllium metal reflector to design a critical system.…”

Hydrogen and its detection in fusion and fission nuclear materials – a review

“…Given the size and mass constraints to be transportable and the high operating temperatures needed to maximize power conversion, the reactor core requires a highly efficient, high-temperature moderator. Yttrium hydride is of particular interest due to having the highest known temperature stability of the metal hydrides [19]. Figure 2 shows the plateau pressures, or minimum hydride dissociation pressure at a given temperature, of various metal hydrides [24].…”

“…Water is an extremely effective moderator, readily available, and simultaneously serves as the reactor coolant. One method of comparing different moderators is through their slowing down power (product of lethargy and scattering cross section, 𝑃 = 𝜉Σs) and moderating ratio (quotient of slowing down power and absorption cross section, 𝑅 = 𝑃/Σa) [19]. High values for both slowing down power and moderating ratios are desirable, and values for some hydride moderators are shown in Table 2.…”

Helium and Hydrogen Effects in Structural Materials for Nuclear Applications

“…Several options, such as temperature stability, radiation tolerance, heat transfer properties must be considered when choosing a moderator. Yttrium hydride is of particular interest since it has the highest known temperature stability among the metal hydrides [106]. However, even stable hydrides may eventually lose hydrogen at the high temperatures anticipated for microreactors (about 800 °C).…”