1. 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

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

3. 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

4. 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

5. 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

6. 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

7. 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

8. History and brunching 8

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

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

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

12. 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

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

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

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

16. 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 pcm