Protective Coatings on Hafnium Hydride and Their Effect on Its Thermal Decomposition

Abstract

Hafnium hydride is studied as an absorber for fast neutron reactors. A high value of the neutron absorption cross section is noted that is retained by all hafnium isotopes formed during neutron irradiation in reactor. However, there is a risk of hafnium hydride decomposing in the range of 600–700°C, which corresponds to the operating temperature of absorbers in fast neutron reactors. An approach is proposed to reduce hydrogen evolution from hafnium hydride that consists of applying a protective hafnium oxide coating to it. Hafnium hydride samples are annealed in helium at temperatures of 1200°C in a synchronic thermal analysis setup. Hydrogen desorption starts at a temperature of 640°C. Complete hydrogen evolution is observed at a temperature of 1200°C. A substantial drop in hydrogen evolution at low temperatures is seen when annealing samples with applied coatings. A special setup is developed that allows the thermal testing of hydride materials in a liquid sodium environment. Hafnium hydride is annealed in liquid sodium at 700°C. Synchronic thermal analysis of samples after exposure to sodium reveals a drop in the emission of gas that is associated with an increase in the thickness of the oxide layers on the surfaces of samples.