Evaluation of the neutronic boundary conditions used in the diffusion analysis model of the PBMR core

Abstract

The definition of boundary conditions are an important part of all core neutronics models. In the CITATION model, used as part of the VSOP core simulation, the neutronic boundary condition is defined in terms of an extrapolation constant called a. In this work, the definition of a, the sensitivity of the neutronic results to changes in a and the appropriate value of a for the PBMR 400MW core design is determined. The value a = 0.4692 used in the current model of VSOP has been shown to represent a vacuum boundary condition. Decreasing this value in the VSOP calculations to 0.3 does not change the reactivity significantly, but decreasing it further below that shows a significant increase in the calculated core reactivity. The appropriate value of a was determined from a transport 1-0 model using XSDRNPM calculations and yielded a value of a = 0.11414. In this case, the core barrel and the reactor pressure vessel were included in the neutronic calculation, so that the influence of these structures which lie beyond the neutronic boundary in the VSOP analysis would be included. The change in reactivity in the VSOP model when the default (a = 0.4692 representing vacuum) and newly determined (a = 0.11414) are used compares well with similar MCNP and XSDRNPM calculations where either a vacuum boundary condition or the actual geometry (barrel and reactor pressure vessel) was modelled. These values are 249 pcm (VSOP), 150 & 40 pcm (MCNP) and 219 pcm (XSDRNPM) respectively. In this work both a corrected extrapolation constant a =0.11414 and a methodology to calculate it from a reference 1-D transport solution was determined.