Thermal modelling of spent nuclear fuel pool storage during loss of external cooling system accident

Abstract

Spent nuclear fuel (SNF) is nuclear fuel that is no longer useful in sustaining a nuclear reaction in the nuclear reactor but still generates heat in term of decay heat, which is of concern for their disposal and transportation. For safety, it needs to be cooled adequately in spent fuel pool (SFP) to a safer level. At present, all SFP are equipped with an external cooling system to ensure the temperature and water level inside the SFP at a safe level. During the loss of an external cooling system accident, the SFP is fully dependent on the natural convection process to cool the SNF. It is important to predict and evaluate the SFP temperature and water level during this accident. Therefore, in this study, a computational model of SFP was developed in order to predict the thermal behaviour of the SFP, focusing on the SFP temperature and water level during the loss of an external cooling system accident. The computational model is based on a three-dimensional (3D) two-phases thermal fluid behaviour computed using the computational fluid dynamic software, Ansys Fluent 18.0. In order to validate the computational model, a small-scale SFP physical model with the ratio of 1 : 30 from the actual size of SFP was developed. Based on the validation process, the developed computational models were deemed applicable to predict the SFP water temperature and water level during the accident. From the computed results, it shows that for 10 MW decay heat, it took 20 hours for the water temperature to achieve the saturation condition and another 102 hours for the water level to decrease on the top part of the SNF. The computational model was further used to investigate the effect of SNF decay heat value and axial temperature distribution on the thermal behaviour of the SFP without an external cooling system. Computations for three different SNF decay heat values (5 MW, 1 MW and 0.1 MW) and three patterns of axial temperature distributions were carried out. The results show that SNF decay heat value affected the increase rate of SFP water temperature and the maximum SNF surface temperature. The result also shows that the effect of SNF axial temperature distribution was larger on the SFP water temperature distribution and its cooling capability. It can be concluded that both the decay heat value and SNF axial temperature distribution have significant effects on the SFP thermal behaviour; therefore, it should be considered in any SFP thermal analysis.