International Conference on Topical Issues in Nuclear Installation Safety: Safety Demonstration of Advanced Water Cooled Nuclear Power Plants 6–9 June 2017, Vienna, Austria Comparative analysis of WWER-440 Reactor Core with PARCS/HELIOS and PARCS/SERPENT Codes S. Bznuni, N. Baghdasaryan, A. Amirjanyan Nuclear and Radiation Safety Center, Armenia G. Baiocco, A. Petruzzi Nuclear and Industrial Engineering (NINE), Italy P. Kohut Brookhaven National Laboratory, USA J. Ramsey Nuclear Regulatory Commission, USA

Introduction (1/2) 2D codes (HELIOS, CASMO, TRITON, WIMS): Mature codes Established technique Multigroup model Geometric limitations Axial discretization 2

Introduction (2/2) 3D Monte Carlo codes (SERPENT) Continuous energy neutron-nucleus interaction cross-section libraries self-shielding effects are automatically accounted for, without relying on various approximations used by deterministic codes No-geometric limitations, fuel assembly, group of assemblies (super-cell), reflector or entire reactor core More precise modelling of the isotopic composition system-specific neutron spectrum 3

Used codes (1/3) HELIOS-2 (version 2.0.01) CCCP (current coupling and collision probabilities) and MOC (Method of Characteristics) models CCCP applied in this work Multi-group Model 49 group neutron and 18 group gamma group cross- sections library k=3 coupling order Verified and validated code 1/12 symmetry 4

Used codes (2/3) SERPENT (2.1.28 version) Monte Carlo method conventional surface-tracking and the Woodcock delta-tracking method 3D geometry universe-based constructive solid geometry (CSG) model Continuous-energy cross sections from ACE format data libraries ENDF/B-VII data files Classical collision kinematics 5

Used codes (3/3) Probability table sampling in the unresolved resonance region Built-in  burnup model Bateman depletion equations Transmutation Trajectory Analysis Chebyshev Rational Approximation Method Parallelization Full Assembly Model 6

Methodology The B1 methodology - consistent with HELIOS The thermal cutoff energy is 1.84 eV to ignore upscattering Fission yields and delayed neutron data - consistent with PARCS code Reflective boundary conditions Burnup discretization: 0, 0.15, 1, 3, 6, 10, 12, 14, 16, 18, 20 MWt*day/kgU 3 history cases accounting axial moderator density gradient 7

History and brunching 8

Results: kinf Hystory-1 reference case For brunches difference varies between 0.67pcm to 587pcm 9

Results: XS (1/2) Hystory-1 reference case 10

Results: XS (2/2) Hystory-1 reference case 11

Discussion: XS (1/2) Cross-sections relative difference varies within 1- 6%. Transport cross-sections simplified approach [1] (out-scatter approximation) used in SERPENT which is prone to significant errors for light nuclides like, hydrogen bound in water Increase of differences with increase of burnup absence of leakage (B1) correction [1] to the transmutation cross sections during burnup calculation kinf differences Approximations [1] used in B1 leakage correction in case of using reflective boundary condition 1. Leppänen J., Pusa M., Fridman E., Overview of methodology for spatial homogenization in the Serpent 2 Monte Carlo code, Annals of Nuclear Energy, 96 (2016) 126–136

Discussion: XS (2/2) Delayed neutron fractions and precursors decay constants methodological differences of their calculations

Full Core Model (1/2) PARCS coupled with RELAP 60-degree rotational symmetry 59 fuel and 2 reflector types

Full Core Model (2/2) Assembly-Average Burnup Distribution Difference – 161.7 pcm

Conclusions Helios-2 and Serpent 2 generated group cross sections for WWER-440 fuel shows fair agreement Maximal differences in XS – 5% PARCS calculated k-eff values using HELIOS and SERPENT generated cross-sections for WWER-440 full core 3D model - 161.7 pcm