# 3-D SIMULATION OF VVER CORE POWER DENSITY BASED ON SAPFIR_95&RC_VVER ALGORITHMS

24th Symposium of AER on VVER Reactor Physics and Reactor Safety (2014, Sochi, Russia)

Advances in spectral and core calculation methods

## Abstract

3-D SIMULATION OF VVER CORE POWER DENSITY BASED ON SAPFIR_95&RC_VVER ALGORITHMS

Artemov V.G., Kuznetsov A.N., Shemaev Yu.P. FSUE «Alexandrov NITI», Sosnovy Bor, Russia

ABSTRACT

SAPFIR_95&RC_VVER is certified by Federal Environmental, Industrial and Nuclear Supervision Service of Russia for calculation of VVER reactor neutronic characteristics. The calculation of subchannel power density is based on the superposition of micro-distribution of neutron flux in the fuel rod assembly and the macro-distribution in the complete reactor core (referred to as superposition method). The micro-distributions are calculated by the SAPFIR_95 program on the basis of the solution of the transport equation in an infinite lattice of fuel rod assembly using the first-collision probability method. Nonuniformity of the neutron flux macro-distributions across the core is calculated by the RC_VVER program in the diffusive approximation.

In order to increase the calculation accuracy of individual rod characteristics and to obtain an error estimation of the calculations, the SAPFIR_95&RC_VVER package includes the possibility of solving the equation of neutron diffusion on hexagonal finite-difference grid corresponding to the fuel rod lattice (referred to as direct method).

The direct method of calculation is more complex and cumbersome, but it gives the detailed description of the core structure and side reflector. This method uses the solution of the equation of fine-grid diffusion corresponding to the real lattice pitch, leading to longer time required for calculation of one reactor state. Another and more important limitation of the direct method is a systematic accuracy of diffusive approximation in simulating the burnup of burnable poison rods and gadolinium-containing fuel rods.

Therefore, a combined calculation method of subchannel power density is developed and implemented in the SAPFIR_95&RC_VVER package. In this combined method, the superposition method calculates the burnup distributions which are used as input data for power density micro-field calculation. The superposition method also calculates the distributions of fuel temperature and coolant density which are also input data for the calculation of the subchannel power density by the direct method.

The paper presents the scheme of subchannel power density calculation by the combined calculation method implemented in the SAPFIR_95&RC_VVER. Results of comparative assessment are given which demonstrate the advantages of the combined method in comparison with the direct calculation and the superposition calculation method.