Main steam line break calculations using a coupled relapsiparcs model for the ringhals-3 pressurized water reactor

A Main Steam Line Break (MSLB) is an important transient for Pressurized Water Reactors (PWR) due to the strong positive reactivity introduced by the over-cooling of the core. Since this effect is stronger when the Moderator Temperature coefficient (JVITC) has a large amplitude, a conservative result will be obtainedfor a high burnup of the fuel due to the more negative MTC late in the cycle. The calculations have been performed at a cycle burnup of 12. 9742 GWd/tHM The Swedish Ringhals-3 PWR is a three loop Westinghouse design, currently with a thermal power of 3 000 MW The PARCS model has 157 fuel assemblies of 8 different types. Four different types of reflector are used. The cross sections, and kinetic data were obtained from cAsMO-4 calculations, using a cross section interface developed at the department. There are 24 axial nodes, and 2x2 radial nodes for each assembly. The transient option for calculating the effect of poisoning was used. The PARCS model has been validated against steady-state measurements from Ringhals-3 of the Relative Power Fraction (RPF) and of the core criticality. The RELAP5 model has 157 channels Jbr the core which means that there is a one to one correspondence between the thermal hydraulics model and the neutron ics model. There is eight axial nodes. Originally the intention was to have 24 axial nodes but this proved not to work because of some limitation in RELAP5. There is currently no mixing between the dfferent channels in the core. The feedwater and turbines are modelled as boundaty conditions. The stand-alone RELAP5 model has been validated against steady state measurements from Ringhals-3. A number of d/J\ue8rent cases were considered. In the first case, both the isolation of thefeedwater for the broken loop, and all the control rods were assumed to work properly. For the second case one of the control rods was assumed to be stuck. The stuck rod was located in the fuel assembly with the highest power This rod has also one of the highest rod worths. In the final case, the feedwater control valve for the broken loop was fully open. None of the cases led to any recriticality. The increase in power for each fuel assembly was also investigated. With the control rod located in the assembly with the highest power; the maximum power increase before scram turned out to be about 25% compared to the initial power