Properties of Lithium Hydride-V Vacancy Formation, Cavitation, and Lithium Precipitation in Irradiated Lithium Hydride

Abstract

Expansion of crystals and compresses of LiH containing up to 40% LiT was observed during exposure to tritium β particles at temperatures from −196° to 400°C. Lithium metal precipitation was observed by electron paramagnetic resonance in small samples of similar composition. The results of auxiliary measurements of optical absorption, electrical conductivity and diffusion, nuclear magnetic resonance, and x-ray lattice parameter were used to interpret the data. Expansion was primarily related to the introduction of vacancies into the LiH crystal lattice to accommodate the products of radiation damage and tritium decay, to permit the formation of bcc Li metal, and to produce cavities in the crystals at high temperatures. Radiation damage was quickly reversed at the lowest temperatures, and the primary defects observed were interstitial He3 atoms and F centers. Warming to room temperature caused aggregation of the F centers and introduced vacancies which destroyed the interstitials Periodic warming for observation further increased the net expansion to allow the formation of bcc Li metal. At higher storage temperatures, vacancy diffusion stabilized a portion of the defects produced by radiation damage with the net result that increased expansion, increased Li formation, and H2 molecular species were observed in the crystals. The expansion rates in the temperature range of increasing radiation damage gave an activation energy consistent with that obtained for vacancy diffusion. An intercomparison of expansion and bcc Li formation at 23°C gave results consistent with the radiation-damage yield and with the formation of Li metal by a diffusion-controlled process. Above a 300°C minimum a second increase in expansion related to cavitation was observed.

A review of irradiation stability of lithium hydride neutron shielding material

“…[17][18][19][20][21] Irradiated lithium hydride as determined by dimensions and mass was studied, and it showed three distinct phases at 323 K and above: region I -rapid initial growth; II -slower, near linear swelling; III -erratic and unpredictable growth with eventual sample breakup. 22 Accompanying the swelling is the formation of aggregates of metallic lithium and gaseous hydrogen within the lattice. The state of these decomposition products has been characterised.…”

“…Lithium hydride is the best neutron moderator and absorbent by far and has been selected by the Aircraft Nuclear Propulsion (ANP) as a primary shielding material because of the combination of high hydrogen density (,5.87610 22 H atoms cc 21 ), high melting point (,956 K), low density (,0.775 g cc 21 ), high absorption cross-section and low molecular weight, all of which are ideal for use as a low mass, high efficiency neutron shield. 1 Naval Research Laboratory compared the radiation exposure rate for various materials as a function of their thickness in mass per unit area.…”