Validational exercises with RELAP5/PARCS coupled codes using Ringhals-3 plant data

The present study is part of a series of validation exercises, in which features of a coupled RELAP5/PARCS model is being assessed against various steady-state and transient plant data obtained at the Swedish Ringhals-3 NPP. The current work is focusing on the simulation of two specific off-normal situations, such as a Load Rejection and a Loss of Normal Feedwater case occurred in 2005. At first, the stand-alone RELAP5 and PARCS models are presented. All the 157 fuel assemblies are modelled individually in both codes. The coupling between the two codes is touched upon, with emphasis on the mapping between the hydrodynamic/heat structures and the neutronic nodes. The load rejection transient was initiated by disconnection of the electric grid, which resulted in a reduced power generation. The other analysed transient started with a malfunction of the feedwater valve at the 2. steam generator. Consequently, the turbines were tripped and the reactor was scrammed. Activation of the auxiliary feedwater provided proper amount of cooling from the secondary side, resulting in safe shutdown conditions. Capabilities of the RELAP5 code were more challenged in these transient, where the influences of the feedback from the neutron kinetic side were also taken into account in the analyses. The calculated values of the parameters show good agreement with the measured data. (authors)