In the SAPFIR_95&RC_VVER software package attested in the Rostechnadzor of the RF for calculation of VVER reactor neutronic characteristics, the pin-to-pin power density calculation is based on superposition of neutron flux micro-distribution in the fuel assembly (FA) and macro-distribution in the reactor on the whole (hereinafter “superposition method”). Micro-distributions are calculated with the SAPFIR_95 code 95, based on the transport equation solution in the infinite FA lattice by the FCP method, and variations of macro-fluxes in the core are calculated in the diffusion approximation with the RC_VVER code.
This method provides prompt pin-to-pin power density calculation with the acceptable accuracy. However it has some restrictions related first of all to the fact that the reactor modeling is carried out through homogenization of reactor structures (FA, inter-assembly gap water, reflector). In particular, this approach cannot correctly consider the effect of structural specific features of reflector elements on power density in fuel pins at the core.
In order to improve the accuracy of pin-to-pin parameters calculation and to make it possible to estimate the uncertainty of these calculations, the option was included into the SAPFIR_95&RC_VVER software package with the aim to solve the neutron diffusion equation on the regular hexagonal finite-difference mesh, which corresponds to the fuel pin lattice in the core (hereinafter “direct method”).
The direct pin-to-pin method of calculation is more complicated and bulky, but allows the core structure and lateral blanket to be vividly dawn in. This approach uses the diffusion equation solution on the fine mesh that corresponds to the fuel pin lattice pitch. It results in the increment of time required to calculate one state of the reactor. The direct method has a more serious drawback related to methodological uncertainty of diffusion approximation calculation in the course of modeling of BPR and fuel pins burn-up. So, for the SAPFIR_95&RC_VVER software package a combined computation scheme was offered and implemented to calculate pin-to-pin power density, where burn-up distributions are calculated by the superposition method. And these data are used as input data to calculate power density micro-fields. Besides burn-up distributions, the superposition method is used to calculate distributions of fuel temperature and coolant density, which are also used as input data to calculate pin-to-pin power density by the direct method. The paper presents the computation scheme to calculate pin-to-pin power density by the combined method, which was implemented in the SAPFIR_95&RC_VVER software package. Consideration is given to the testing results that illustrate the advantages of the combined method in comparison with direct calculations and calculations with the superposition method.
SAPFIR_95&RC_VVER software package, VVER-reactor, pin-to-pin power density, fuel pin linear rating, neutron flux, combined method, superposition method