A model of side ionization chambers for the VVER-1000 as part of the PROSTOR software complex

Abstract

Estimating reactor power based on neutron flux monitoring system parameters is the fastest method for power determination among those employed at the VVER-1000 reactor. Therefore, the power value calculated by neutron flux monitoring system is utilized in the reactor control and protection system. Consequently, accurately modeling neutron radiation transport to the locations of the ionization chambers is a crucial and relevant task for computational codes that provide a comprehensive description of the reactor facility's dynamic operating modes. This paper describes the models of side ionization chambers of the neutron flux monitoring system used in the PROSTOR software. The initial simplified model of the neutron flux monitoring system qualitatively described the dependence of the neutron flux at the ionization chamber locations on the angular positioning of the chambers. However, this simplified model failed to accurately reproduce the axial dependence of the neutron flux within the ionization chambers channels, as it did not account for neutron flux attenuation caused by the supporting concrete cantilever. To address this shortcoming, a new improved neutron flux monitoring system model is developed. To implement this improved model within the OpenMC Monte Carlo code, a three-dimensional model of the reactor and its biological shielding is created. Subsequently, assessments are made of the contributions from two peripheral rows of fuel assemblies to the neutron flux at the detector placement radius, as a function of the detector's angular and vertical position. Based on the resulting data, an improved model of the neutron flux monitoring system is developed and is currently utilized within the PROSTOR software. PROSTOR software is used to simulate a scram measurement previously conducted at the Kalinin NPP to evaluate the quality of the neutron flux monitoring system models.