Explicit modeling of a VVER-1000 radial reflector by a ring of assembly-size nodes - 15075

The problem of modeling the radial reflector of a VVER-1000 core is somewhat more complex in comparison with a PWR core. The complexity is caused by the hexagonal shape of the fuel assemblies and material variations in reactor vessel internals in the vicinity of the reactor core. While PWR core radial reflector can be modeled by color-set schemes (flat-side cell/corner cell), the VVER-1000 core provides an example of up to five different configurations in terms of geometry and material compositions. There are two common approaches to model VVER-1000 core radial reflector in three-dimensional diffusion codes. The first approach is an implicit modeling with core-reflector albedos while the second approach is based on an explicit representation of the reflector by a ring of assembly size nodes. The VVER-1000 core radial reflector model comprising of a ring of assembly size nodes is outlined in the paper. HELIOS lattice code calculations for VVER-1000/V320 from the OECD/NEA Kalinin-3 Coolant Transient Benchmark with taking into account baffle and barrel geometry and material compositions are performed. Radial reflector models of varying complexity are described as implemented into a customized version of the NEM (Nodal Expansion Method) code from the Pennsylvania State University. Results obtained for Kalinin 3 fuel cycle 1 bis are presented and compared. Kalinin 3 fuel cycle 1 bis pattern is used as an example but arrangement of various fuel assembly types in the outermost layer is quite common for any VVER-1000/V320 fuel cycle. (authors)