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Fuel debris consisting of nuclear fuel and reactor structural materials generated in the accident in Fukushima Daiichi Nuclear Power Plant can deteriorate similar to rocks when subjected to changes in environmental temperature. Although the fuel debris has been cooled by water for 10 years, they are affected by seasonal and/or day-and-night temperature changes. Therefore, in evaluating the aging behavior of the fuel debris, we need to consider the changes in the environmental temperature. Assuming that the fuel debris was deteriorated, radioactive substances that have recently undergone micronization can be eluted into the cooling water, and such condition may affect the defueling methods. We focused on the effect of repeated changes in environmental temperature on the occurrence of cracks, and an accelerated test using simulated fuel debris was performed. The length of the crack increased with increasing number of heat cycles; therefore, the fuel debris became brittle due to the stress caused by thermal expansion and contraction. In conclusion, we confirmed that the mechanical deterioration of the fuel debris was similar to that of rocks or minerals, and predicting the changes in the length of the crack in the simulated fuel debris and environmental model is possible.

Establishing an evaluation method for the aging phenomenon by physical force in fuel debris