Theoretical study of flashing and water hammer in a supercritical water cycle during pressure drop

During a loss of coolant accident (LOCA) the pressure of the coolant can drop significantly in the vicinity of the leak. It will be shown that unlike in pressurized water reactors (PWRs) where this pressure drop can cause only sudden vaporization – also called flashing – in supercritical water cooled reactors (SCWRs) it can cause sudden condensation (condensation-induced water hammer), too. The reason is that from supercritical state the system can go to metastable liquid as well as to metastable vapour state after LOCA. Relaxation from metastable fluid states is a fast process, followed by a local positive or negative pressure-jump, which might increase the damage around the leak. Conservative estimation will be given for the magnitude of these pressure jumps caused by the flashing or water hammer by assuming various initial pressure losses. In our calculations, three different equations of state are used: the simple van der Waals EoS; the Redlich–Kwong as an empirical development; and the more sophisticated non-cubic Deiters equation of state. These equations are able to describe metastable states qualitatively but with different accuracy. These calculations can help us to map the local immediate effect of any sudden pressure drop and therefore it can help to design better safety protocols.