Treatment of Staggered Mesh for BWR Pin-by-Pin Core Analysis

Abstract

In this paper, the neutron current interpolation (NCI) method is proposed to treat the staggered mesh on the basis of the Simplified P3 (SP3) theory code. In the NCI method, the response matrix formulation is applied to the SP3 theory code, and neutron partial currents at assembly boundary are interpolated using low-order polynomial functions, i.e., flat, linear or quadratic function. The pin-by-pin fine-mesh core analysis method is considered as a candidate next-generation core calculation method for BWR. Since the number of fuel rods in BWR assembly is increased with the higher burn-up, the different types of assembly may be adjacent in a BWR core. Therefore, when the cell-homogenized pin-by-pin analysis method is applied to the BWR core analysis, it must have a capability to treat the staggered mesh. Our previous study indicates that the SP3 theory is appropriate for pin-by-pin BWR calculations. However, the SP3 theory code that can treat the staggered mesh has not been developed so far. For these reasons, the treatment method of the staggered mesh is investigated to develop such SP3 theory code. The performances of the NCI method are evaluated through comparison with the cell-heterogeneous detailed transport calculations by the method of characteristics (MOC). Two-dimensional, 2 Â 2 multi-assembly geometries are used to compare the prediction accuracy of the NCI method. The 2 Â 2 multi-assembly geometries consist of 9 Â 9 UO 2 and 10 Â 10 MOX fuel assemblies. The calculation results indicate that the prediction accuracy of the NCI method is similar to that of the detailed transport calculation method.

An optimization approach to establish an appropriate energy group structure for BWR pin-by-pin core analysis

“…However, the number and the structure of energy groups, which are suitable for the pin-by-pin core analysis, have not been extensively investigated yet. For example, the validity of the 8-group structure used in the previous study for the BWR pin-by-pin core analysis is not fully confirmed [5,6]. Especially, in BWR, there is a significant variation of energetic and spatial distribution in neutron spectrum in a fuel assembly since BWR fuel assemblies have complicated geometries (e.g.…”

“…In Equation (6), D is an index to represent the variation of neutron spectrum. The subscripts of i and g represent the mesh and the energy group indexes, respectively.…”

“…Recently, a pin-by-pin approach is studied and is applied for BWR core calculations as a candidate of the next generation core analysis methods [5][6][7][8]. However, the number and the structure of energy groups, which are suitable for the pin-by-pin core analysis, have not been extensively investigated yet.…”

A new technique for spectral interference correction on pin-by-pin BWR core analysis

“…Recently, as a next generation core analysis method, the pin-by-pin fine mesh core calculation method for BWR core analysis has been studied [7,8]. The previous study indicates that the pin-by-pin approach for BWR core analysis has good accuracy to evaluate important parameters for highly heterogeneous cores (e.g., mixed-oxide (MOX) or high-burnup fuel loaded core).…”

“…In order to capture significant variation on energetic distribution of neutron spectrum in BWR fuel assemblies or in highly heterogeneous cores and to obtain higher prediction accuracy, utilization of finer energy group structure in core calculations is desirable. For example, eight-or nine-group structure is used in the previous study of the pin-by-pin approach for light water reactor core analysis [7][8][9]. However, even if we utilize such finer energy group structure, the spectral interference effect on the energy collapsing should be still taken into account and several correction techniques considering this effect are necessary.…”