Small break loss of coolant accidents: Bottom and side break

The prediction of the pressure transient and the fluid loss from a reactor due to a small break loss of coolant accident (LOCA) is a complicated problem. This is mainly due to the fact that the exit flow rate through such a small break is a function of geometry and location of the break as well as of the upstream fluid conditions. Analytical models and experimental results are presented here which are concerned with such an event if the break occurs either at the bottom or the side of a pressure vessel. As phase stratification is of significance, the mixture interface level location relative to the break is important for the evaluation of the fluid loss. Also the steam fraction in the flow at the break entrance has to be considered in the model. In the bottom break case, it depends strongly on the so-called ''vapor pull through''. Side breaks can be treated similarly to bottom breaks but at sufficient depressurization rates, swelling of the flashing liquid will influence the existing flow rate. The analytical models, based on these considerations, were successfully compared with blowdown data. There is good agreement of the predicted and measured pressure transients. It is also possible to compute the interface level and mixture void fraction history in a compatible way with the physical behavior observed during the experiment