Introducing a neutron detector model into the oscar 3 calculational code system and validation with Koeberg reactor calculations

Abstract

The objective of this research is to develop accurate and efficient computational methods of predicting neutron detector responses by introducing a neutron detector model into the OSCAR-3 code system. The importance of the nodal and homogenization schemes in reactor analysis is demonstrated by using a nodal diffusion code, MGRAC, to perform core calculations for the Koeberg reactor so as to validate the detector model that is being developed in this study. The measure of determining the detector response used in this study is referred to as the detector constant and is taken to be the 235U microscopic fission cross-section. Both the one group and two group detector constant methods are investigated and tested in developing a suitable detector model for this code in this study. Sensitivity studies are carried out to evaluate the sensitivity of the detector constant to changes in core parameters such as the moderator temperature, fuel temperature, boron concentration, assembly type and also to investigate the behaviour of the detector constant as a function of burnup. Two methodologies are used to calculate the detector responses in this study viz., the average flux method (which takes into account the average flux over the whole assembly) and the flux reconstruction method (which takes into account the flux at the centre of the assembly where the detector is inserted). The results obtained for both these methods (from the MGRAC code) are compared to the plant detector values and their accuracy is determined by calculating the maximum absolute % error and the root mean square (rms.) % error for each method. These calculations are performed for the two-group detector constant. The observation is that both the average and reconstructed flux models yield fairly accurate results, but owing to its affordability and less time-consuming nature, the average flux model is recommended for implementation into the OSCAR-3 code and for use in the production runs of the Koeberg Nuclear Power Station.