Coupled RELAP5/PARCS main steam line break calculations before and after a power uprate of a Pressurized Water Reactor

Paper in proceeding, 2008

This paper reports on the analysis of a hypothetical Main Steam Line Break (MSLB) at the Swedish Ringhals-3 Pressurized Water Reactor. Ringhals-3 is of a three-loop Westinghouse design with an original design power of 2783 MWth. A power uprate to 3142 MWth is planned for this reactor, and the consequences of this uprate on different MSLB scenarios are investigated in this paper. The codes used in the analysis are PARCS and RELAP5. The PARCS model accounts for the full heterogeneity of the core, with explicit modelling of the top, bottom, and radial reflectors. The RELAP5 model accounts for each individual fuel assembly in the core, which means that there is a radial one-to-one correspondence between the thermal-hydraulic and neutronic models. Both the stand-alone PARCS model and the stand-alone RELAP5 model were earlier validated against steady-state measurements at the Ringhals-3 unit. Validations for the coupled model using measured transients that occurred at Ringhals-3 were also successfully carried out in the past. In the MSLB simulations reported in this paper, a number of different cases are considered, where the influence of core burnup, of flow mixing in the lower plenum, and of the power level at Hot Full Power conditions is investigated. Simulations performed at Hot Zero Power conditions demonstrate the time between the reactor scram and the break has a strong influence on the behaviour of the system (effect due to the delayed neutrons). For longer shutdown times, a return to criticality is possible. The simulations also showed that the system returns to criticality if the safety injection fails.