Mixing Conditions in the Lower Plenum and Core Inlet of a Boiling Water Reactor

A thermal mixing analysis of the downcomer, main recirculation pumps (MRPs) and lower plenum of Forsmark's Unit 3 has been carried out with three separate computational fluid dynamics models. Several difficulties with the boundary conditions have been encountered, particularly with the MRP model. The results obtained predict stable temperature differences of around 8 K at the core inlet. Such large temperature differences have never been observed at Forsmark nuclear power plant (NPP). Temperature measurements at four positions above the reactor pressure vessel (RPV) bottom give the mean value used as the core inlet temperature for core analyses. Even if the temperature transmitters used are rather slow and inaccurate, they should be able to detect such large temperature differences that may lead to fuel damage. The only damage reported at Forsmark NPP since the implementation of liner cladding in fuel design is that caused by mechanically induced debris fretting (threadlike particles). Also, the difficulties with the connection of the models throw some doubt on the accuracy of these predictions. A completely connected model of the same RPV volume covered by the separate models predicts temperature differences at core inlet that are almost one fourth of those mentioned above, i.e., approximately 2.5 K. Most of the mixing occurs downstream of the MRP diffusers, at the lower plenum "inlet. " This prediction divergence seems to arise from an impossibility of a correct transfer of complete three-dimensional flow field properties by means of boundary conditions defined at a two-dimensional inlet section.