Improved Estimation of Anisotropic Neutron Diffusion Coefficients for Neutron Streaming in Advanced Gas Cooled Reactors (AGRs): Application to Steam I

The aim of this PhD project is to address numerical modelling challenges within Advanced Gas Cooled Reactor (AGR) Nuclear Power Plants (NPPs). In reactor physics analyses of AGRs one needs to determine power distribution within AGR fuel assemblies (stringers). In order to determine these power distributions one must solve the neutron diffusion or neutron transport equation. If one solves the neutron diffusion equation (an approximate form of the neutron transport equation) radial and axial neutron diffusion coefficients along with other nuclear cross-section data must be calculated and used as input data within the codes. These radial and axial homogenized neutron diffusion coefficients must be corrected to take into consideration the detailed effects of neutron streaming in AGR gas channels. The correction factors for these radial and axial neutron diffusion coefficients can be very significant for the AGR geometry; in the region of around 10-30%, and results in differing directional diffusion coefficients in the radial and axial directions being used in the whole core reactor analysis simulations. For AGR normal operation, these correction factors are fairly constant and produce diffusion parameters which agree well with measurements, e.g. of distorted radial/axial flux shapes. A current concern for AGRs is the reactivity effects of steam ingress into an operating or shutdown AGR core. As graphite weight loss increases, the reactivity effects of the hydrogen addition can change from net negative to net positive. In addition, the hydrogen fills the streaming voids in the cluster, resulting in a loss of neutron streaming, reduction in leakage and further reactivity addition. The latter effect is only partially represented in lattice physics calculations, and therefore a generous uncertainty is currently used to provide numerical bounds to this effect. The aim of this PhD project will be to determine new mathematical/computational methods for calculating the assembly homogenized radial and axial neutron diffusion coefficients. These new mathematical/computational methods will aim to reduce the pessimism in AGR reactor physics simulations, as compared against reference neutron transport theory simulations, in order to ascertain the accuracy of the newly developed methods. In addition these methods can be re-applied to generation IV nuclear power plants (NPPs) such as high temperature gas cooled reactors (HTGCRs) and gas cooled fast reactors (GCFRs).