Role of Ba(NO3) pretreatment in reducing the yellow phase formation during vitrification of nuclear waste

Abstract During vitrification of sulfate bearing high-level liquid waste (HLLW), Na2SO4 is easy to decompose at high temperatures and a water soluble “yellow phase” containing mainly alkali metal sulfates generally forms on the surface of glass melts. Herein, we report the SO42− ions present in the HLLW as Na2SO4 can be sequestrated as the form of BaSO4 by addition of Ba(NO3)2 solution and the formation of yellow phase can be effectively reduced during vitrification. XRD and SEM-EDX show that the main constituents of yellow phase consist of BaSO4, BaCrO4, and BaMoO4 by adding excess Ba(NO3)2, and the alkali metal sulfates phases almost disappear. In addition, the sulfate retention of waste glass is increased by the Ba(NO3)2 pretreatment, especially for vitrification of HLLW with relatively high sulfate content. The SO3 content in the waste glass remains almost unchanged in temperature range of 850‒1050°C. These results suggest that the Ba(NO3)2 pretreatment may provide an alternative new way to solve the yellow phase problem during the vitrification of sulfate bearing HLLW.