Effect of fuel particle size on consequences of criticality accidents in water-moderated solid fuel particle dispersion system

ABSTRACT

In water-moderated solid fuel particle dispersion systems, such as fuel debris systems, the size of the particles can have a significant impact on the behavior of criticality accidents. However, this point, which is important for evaluators of criticality accidents, has not been discussed. This study aims to clarify the effect of fuel particle radius on the criticality transient behavior and the total number of fissions in water-moderated solid fuel dispersion systems. Neutronics/thermal hydraulics-coupled kinetics analysis was performed in a hypothetical fuel debris system, where small fuel particles aggregate in water and become supercritical. Results showed that the number of fissions is 10 times larger when the fuel particle radius is reduced by one order of magnitude under conditions where heat transfer, i.e. from fuel to water, is emphasized. Moreover, there is a possibility that lower reactivity could give a larger number of fissions when the fuel particle size is very small. In addition, the number of fissions may be overestimated or underestimated to an unexpected extent unless appropriate fuel particle size is set on the analysis.

KEYWORDS:

• nuclear criticality safety
• reactor kinetics
• criticality accident analysis
• fuel debris
• Fukushima Daiichi nuclear power plant

Acknowledgments

The authors greatly acknowledge Dr. Kenya Suyama in Japan Atomic Energy Agency and Prof. Ken Nakajima in Kyoto University for technical comments. The authors also acknowledge Dr. Shouhei Araki and Mr. Takeo Kikuchi in Japan Atomic Energy Agency for the management of the analysis environment. This paper includes results of the contract work funded by the Nuclear Regulation Authority (NRA)/the Secretariat of NRA of Japan.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

The work was supported by the Nuclear Regulation Authority.