(A) study on evaluation method of impact by fission gas release during control rod ejection accident

Abstract

The Control Rod Ejection (CRE) is one of the most important accidents among Design Basis Events (DBE) in terms of fuel rod integrity and radiological consequences. Since the regulatory guides for this accident were issued in 1979, new regulatory guides were issued in June 2020, RG 1.236 including the experimental results performed from 1979 to 2020. According to the new regulatory guides, the fission products release during CRE accident should be taken into account for evaluation of safety in terms of fuel rod integrity and radiological consequences. Since the current CRE accident analysis method was established to check whether the regulatory requirements presented in RG 1.77 are satisfied, the current analysis method cannot evaluate the CRE based on the new regulatory requirements. Therefore, a study has conducted on a methodology to quantitatively calculate the impact of fission products release during CRE accident in terms of the integrity of the fuel rod and radiological consequences.

In this study, in order to develop a methodology that meets the new regulatory requirements, the correlation for the fission product release fraction during the accident presented in the new regulatory requirements was installed in SPACE code, the accident analysis code. Using this modified code version, the quantitative accident analysis was performed to evaluate the integrity of the fuel rod according to the release of fission products during the accident. The verification of the results was performed by comparing them with the results obtained when applying the CORSOR-M model, which is approved by the NRC and generally used. Additionally, in order to consider the fission products release during the accident as well as the steady-state fission products before the accident in the source term for dose assessment, the enthalpy increase of the fuel rods and the related fission products release during the accident for all the rods were calculated based on the information on all fuel rods of the core produced by the ASTRA code, three dimensional kinetics code. Using this method, additional radiological source term for dose assessment is calculated quantitatively.

Based on the this study, by establishing a method for evaluating quantitatively the impact of fission gas release during an accident on fuel rod integrity and radiological consequences, the evaluation method in accordance with new regulatory requirements is suggested.