Simplified model for efficient calculation of debris bed melting process in the hypothetical severe accident of HPR1000

The mechanistic analysis of debris bed melting is particularly important to understand the severe accident scenario and to improve the development of integral severe accident analysis codes. In previous work, we had successfully predicted the transient melting process of debris bed in the hypothetical severe accident of HPR1000. However, there was a problem of high computation cost by using large eddy simulation (LES) model, which was not conducive to the fast calculation of analysis codes. Therefore, a simplified calculation model based on the phase change effective convectivity model was developed to improve calculation efficiency, and both the COPRA experiment and the numerical study using LES model were utilized to validate the reliability of the developed model. Results indicated that the predictions by the simplified calculation model agreed well with the data of COPRA experiment and the numerical study using LES model with the maximum deviation of 10.93%. Moreover, the calculation efficiency of the simplified calculation model was improved by at least 320 times compared to the previous LES model, suggesting that the simplified calculation model was effective to enhance the calculation efficiency and ensure the reasonable simulation accuracy. Additionally, sensitivity analysis of stainless-steel content in debris bed indicated that the increasing stainless-steel content resulted in the lower maximum temperature, smaller peak heat flux and thicker final crust thickness. Sensitivity analysis of zirconium oxidation fraction in debris bed indicated that the increasing zirconium oxidation fraction rendered the maximum temperature and peak heat flux of debris bed to rise, but there was no large change in the final crust thickness. The debris bed with 50% stainless-steel content and 80% zirconium oxidation fraction achieved the overheat with superheat of 96.7 K and the highest heat flux of 0.44 MW m(-2). These findings provided a reference for the research of debris bed melting and the developments of integral severe accident analysis codes.