Development and Modification of Pool Scrubbing Decontamination Factor Evaluation in Severe Accident Code

When severe accidents in boiling water reactor and laser cutting in fuel retrieval during nuclear power plant decommissioning release the fission products in the form of aerosols into the containment, which may cause severe accidental damages, the gas mixed with radioactive aerosol particles can be injected into the suppression chamber. Aerosols are retained in the liquid while the gas passing through the pool and this process is called pool scrubbing. The removal effect of the aerosols is represented by decontamination factor (DF) and it is evaluated by severe accident codes such as MELCOR. In MELCOR, SPARC-90 model is used. The hydrodynamics variables such as bubble rise velocity, bubble aspect ratio and bubble diameter are calculated with many assumptions and by empirical formula. The previous comparisons between the calculation results of MELCOR and experimental data show that the physical models used in the MELCOR code have many limitations and need to be modified. In this paper, a code for evaluating the decontamination factor of pool scrubbing was developed. Based on former experimental studies and CFD simulation analysis on the decontamination process of one single bubble, modified bubble hydrodynamic and surface velocity were applied to improve the prediction of DF. The modified simulation results were compared with calculation results with original model and the experimental data. The comparison showed larger discrepancy with new surface velocity model, which indicates that surface velocity model might not be crucial for the underprediction of DF in MELCOR.