Study on the chemical structure and actinide leaching of MCCI debris

To understand the chemical structure and stability of molten core-concrete interaction (MCCI) debris generated by the Fukushima Daiichi nuclear power plant accident in Japan in 2011, simulated MCCI debris consisting of the U–Zr–Ca–Si–O system and other simpler systems were synthesized and characterized. 237Np and 241Am tracers were doped for the leaching tests of these elements and U from the simulated debris. The MCCI debris were synthesized by heat treatment at 1200 °C or 1600 °C, in reductive (Ar + 10% H2) or oxidative (Ar + 2% O2) atmospheres. Subsequently, the debris were used for actinide leaching tests with water. Zr and Ca formed a solid-solution with the UO2 matrix, such as (ZryU1-y)O2+x, (CayU1-y)O2+x, and (CazZryU1-y-z)O2, which stabilized the matrix and suppressed actinide leaching from the simulated debris. On the other hand, the cement components (CaO and SiO2) in the debris formed a glass-like coating on the debris, which also remarkably suppressed the leaching of actinides. •Simulated molten core concrete interaction debris with actinide tracers were synthesized.•Subsequently, these debris were used for leaching tests.•The solid-solution phases (ZryU1-y)O2, (CayU1-y)O2, and (CazZryU1-y-z)O2 were formed.•The cement components formed a glass-like coating on the debris, which suppressed actinide leaching.