Handledare på Ringhals: Mikael Fjällborg.
This thesis has been done at Ringhals AB and it focuses on the reactor coolant pumps on Ringhals third reactor (R3). The R3 is a pressurized water reactor and the reactor coolant pumps are one-stage vertical centrifugal pumps manufactured by Westinghouse.
The background for this thesis is that Vattenfall AB wants to increase its knowledge in the modelling of LOCAs and there is an ongoing project for modelling this in the thermo-hydraulic code TRACE. An important part in modelling a LOCA is how the reactor coolant pump will function when they have to handle a two-phase flow. The most important thing to evaluate is how degraded they will become by this two-phase flow.
In order to make the most correct model possible in TRACE one needs to input complete pump curves for all pump regimes for both one-phase and fully degraded two-phase flow. There is also need for a degradation curve that describes how the pump is degraded depending on how big the void is. The problem is that there are not enough information about the pumps at the R3. There are only curves available for four out of eight pump regimes for one-phase flow. Because of this it is very hard to correctly model how the pump will behave with a two-phase flow.
The purpose with this thesis is to improve the knowledge of how the reactor coolant pumps will behave when they are faced with a two-phase flow and how this can be modelled for the pumps on R3. The aim is to analyse and propose guidelines on how to best model the pumps.
The thesis has been divided into two main parts. A literature study that will investigate which pump characteristics should be used to model the pumps at R3 and it will also investigate why the pumps degrade in the first place. The second part will consist of modelling in TRACE to see how different pump curves and other parameters will affect the pump degradation.
When comparing the pump curves for one-phase flow the closest match is the curves from the KWU-pump. When it comes to the specific speed of the pumps the KWU-pump is a pretty good match. With the assumption that pumps with similar specific speed and similar one-phase characteristics also have similar two-phase characteristics the curves from the KWU-pump are the best candidates for modelling the R3s pumps, followed by the built in Westinghouse curves.
At low system pressures, as is the case during a LOCA, the pump will probably degrade more severely than what the curves of KWU and Westinghouse does. To not overestimate the ability of the pump one could either use the Semiscale curves or combine the Semiscale multiplier with either the KWU curves or the Westinghouse curves.