Evaluation of end of cycle plutonium isotopic content in a VVER-1000 reactor using a 3D full-core simulator

Detailed three-dimensional (3D) spent nuclear fuel (SNF) isotopic composition analysis using a full-core 3D nodal simulator is not a widespread practice within production-level nuclear analysis fuel management codes. A 3D full-core pin-wise detailed isotopic analysis can be very time consuming, so more often than not, reduced geometries are used, either based on two-dimensional (2D) lattice-physics, single fuel pins, or point-reactor (0D) estimations. In fact, the typical macroscopic cross section based codes do not usually have the ability to evaluate the full detailed composition of SNF, but instead may track only a few isotopes in spatially homogenized regions. Accordingly, a microscopic cross section based depletion sequence was revisited within the otherwise macroscopic-based NESTLE code. To illustrate this capability, the plutonium content of SNF within a VVER-1000 benchmark was calculated and its non-proliferation risk was estimated. The total plutonium masses and 239Pu fractions were calculated on a 3D node-by-node basis using an established VVER-1000 NESTLE benchmark model. The total plutonium mass after the first cycle is estimated at 502.6 kg, and it increased to 523.4 kg after 90 days of decay. The mass of plutonium with a 239Pu content greater than 80% at the end of cycle is about 7.8 kg, while all remaining content shows to have a 239Pu fraction below 87%. The plutonium content with a 239Pu fraction greater than 80% after 20 days of the end of cycle is less than 7.9 kg. This article highlights a feature of the NESTLE code that can help make isotopic assessments radially and axially for fuel assemblies located in different regions of the core, thus accounting for the varying 3D conditions and flux distributions as a function of exposure.