Mixing Conditions in the Lower Plenum and Core Inlet of a BWR

Abstract

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 CFD 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 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 with codes such as POLCA. The temperature transmitters used are rather slow and inaccurate. Still, they should be able to detect large stable temperature differences such as those predicted by the aforementioned computations. Indirect indication of the incongruity of these predictions is the possibility of fuel damage caused by such large temperature differences. Fuel damage other than the one caused by debris fretting (thread-like particles) through mechanical influence has not been reported at Forsmark NPP since the implementation of liner cladding in fuel design. Also, the aforementioned difficulties with the connection of the models throw some doubt upon 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 a fourth of those mentioned above, i. e. approximately 2.5 K. Most of the mixing occur downstream of the MRP diffusers, at the Lower Plenum “inlet”. The reason for this prediction divergence is 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.