Experimental evaluation of Sr and Ba distribution in ex-vessel debris under a temperature gradient

Abstract

ABSTRACT Dissolution behavior of Sr and Ba is crucial for evaluating secondary source terms via coolant water from ex-vessel debris accumulated at Fukushima Daiichi Nuclear Power Station. To understand the mechanism, knowing the distribution of Sr and Ba in the ex-vessel debris is necessary. As a result of reaction tests between simulated corium and concrete materials, two-layered structures were observed in the solidified sample: (A) a silicate glass-based phase-rich layer in the upper region and (B) a (U,Zr)O2 particle-rich layer at the inner region. Measurable concentrations of Sr and Ba were observed in layer (A) (approximately 1.7 times that in the layer (B)). According to thermodynamic analysis, (U,Zr)O2 is predicted to solidify, in advance, in the concrete-based melt around 2177°C. Then, the residual melt is solidified as a silicate glass, and Sr and Ba are preferentially dissolved into the silicate glass. During the tests, (U,Zr)O2 particles sank in the melt because of its higher density, and the silicate glass relocated to the upper layer. In the actual situation, the crust layer might form on the top surface with cracks and cavities, and therefore the water is possible to contact with upper silicate glass containing Sr and Ba.