Using the RELAP5/MOD2 Code Under Low-Pressure/Low-Temperature Loss-of-Coolant-Accident Conditions

The RELAP5 thermal-hydraulic transient code has been applied to a reactor safety analysis of a loss-of-coolant accident (LOCA) to a reactor with thin fuel plates operating at highly subcooled conditions, ∼75°C below boiling temperatures at operating pressure, 25°C below boiling conditions at atmosphere pressure that occurs immediately following the LOCA. Depressurization is not the critical issue in this case, but plate boiling and reflooding and mixing of vapor and fluid streams are the critical aspects of the calculation. The analysis of the results shows the sensitivity of the calculation to the time periods of the steam “chugging” effects in the water channels between fuel plates. This MOD2 version is the first version with which we found consistency and realism in the results calculated for these very difficult transient conditions.