Numerical investigation of in-vessel core coolability of PWR through an effective safety injection flow model using MELCOR simulation

Abstract

A safety injection (SI) flow model predicting target depressurization was developed in the previous study. The model estimated the sum of the decay heat and oxidation heat using the core exit temperature increase rate and core water level decrease rate during the accident progression. However, in the old model only the heat transfer to the coolant was considered but the heat accumulation in the structures was not included in detail. To resolve this issue, therefore, a new mechanistic model was developed by considering heat sources accumulated in the core heat structures. The accuracy of the new model was validated through the prediction of core total heat using the MELCOR 1.8.6 code. It was confirmed that the new model resulted in a relatively small error less than 10% in almost all sections while the old model exhibited a large error exceeding 50% since the start of oxidation for postulated SBO severe accident scenario. Through the model validation, an improved SI flow map was developed to predict more accurate target depressurization of the reactor coolant system (RCS) needed for maintaining core coolability. This study suggests that new SI flow map can effectively assist operator's execution related to the RCS depressurization and SI injection into the RCS implemented in the severe accident management guideline under various severe accident scenarios.