Transport effects at control rod tips

Abstract

To analyse transport effects at the control rod tips in a nuclear reactor the more accurate SP3 approximation for neutron transport is compared to the usually applied mulitigroup diffusion equations. This has been done as a master thesis project at Studsvik Scandpower, and a previously two-dimensional solution method for SP3 theory has been generalised to three-dimensions and implemented in a independent code. Computations for one- to three-dimensional systems have been done and the results given for diffusion theory and SP3 theory compared. The thesis work has indicated that there are possible differences between the diffusion and SP3 solutions in an non-neglectable magnitude for computations in three-dimensions but with generally no negative implications in regards to reactor safety and fuel pin integrity. For the analysed partial cores the diffusion solution over estimated the power at and around control rod tips with around 2-8 % compared to the SP3 solution. Further studies will be necessary to show that this is the case in general as only a limited amount of cases was analysed. This order of magnitude is relatively large in comparison to the expected difference between diffusion theory and actual measurements in a real reactor. These usually differ by no more than a few percent. The exact order of magnitude for an actual nuclear reactor is not possible to determine with the partial cores and the code developed. For this a more sophisticated code computing with or without SP3 for a complete core would be necessary.