Radiation-Induced Decomposition of Thorium Nitrate Solutions

Concentrated thorium nitrate solution has been proposed as a blanket material in power-breeder reactors. The radiation stability, especially of the nitrate group, is therefore of considerable importance. The radiation-induced decomposition of thorium nitrate solutions was studied as a function of concentration, type of radiation (fission recoils, pile radiations, gamma rays), temperature, and total energy absorbed. The principal products were H₂ and O₂ from decomposition of the water, and N₂, O₂, and oxides of nitrogen from decomposition of the nitrate. Hydrogen yield decreased with increasing thorium nitrate concentration, a behavior similar to that for uranium solutions. Nitrogen yield was independent of temperature, but increased with increasing nitrate concentration and with increasing linear energy transfer along the paths of the ionizing particles. The 100-ev yield of N₂ in 2.73 molal solution was 0.06 for fission particle decomposition, 0.006 for pile radiation (mixed fast neutrons and γ rays) and 0.001 for γ radiation alone. The oxide of nitrogen produced with the largest yield was N₂O and amounted to about ten per cent of the N₂ yield. In-pile autoclave measurements indicated little radiation-induced back reaction of the nitrogen.