Investigation of Coolant Mixing in the VVER-1200 Reactor Core at Energy Levels of Power

3/25/2025 2025 - #01 Thermal physics and thermal hydraulics

Baykov A.V. Stepanova A.O. Dubov A.A. Kotsarev A.V. Gusev S.S. Bedrinov A.A.

https://doi.org/10.26583/npe.2025.1.02

This study focuses on the investigation of coolant mixing in the VVER-1200 reactor core at 90 % of nominal power. To investigate the mixing processes, a transient was simulated using the ATHLET/BIPR-VVER code, which allows undertaking coupled neutron kinetic and thermal-hydraulic calculations. In the process of testing the emergency boron injection system, a cold boric acid solution with a high concentration was injected into one of the primary circuit loops, while all four reactor coolant pumps were in operation. The coolant mixing process is analyzed by calculating local and integral inter-loop mixing coefficients based on the fuel assembly inlet and outlet temperature values, as well as based on the power peaking factors. These mixing coefficients are used to quantify the distribution non-uniformity of coolant flows within the reactor core. The simulation results are compared with the experimental data obtained in the process of tests at the Novovoronezh NPP-2’s unit 1. The analysis of the calculation and measurements results has shown that the injection of a cold boric acid solution leads to major non-uniformities in power and coolant temperature distributions. Furthermore, the results has revealed that the coolant flow from the fourth loop has a localized effect on specific reactor core sectors, as evidenced by differences in the mixing coefficients. The comparison results have shown that the intensity of simulated coolant mixing is lower than obtained based on measurement data. It emphasizes good utility of the ATHLET/BIPR-VVER for conservative safety assessments in simulation of non-symmetrical thermal-hydraulic transients. Further validation of the computational model using comprehensive experimental data is recommended to improve the accuracy of realistic (non-conservative) simulations.

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coolant mixing ATHLET/BIPR-VVER VVER-1200 emergency boron input system (JND) energetic level of power Novovoronezh NPP-2 thermocouple

Link for citing the article: Baykov A.V., Stepanova A.O., Dubov A.A., Kotsarev A.V., Gusev S.S., Bedrinov A.A. Investigation of Coolant Mixing in the VVER-1200 Reactor Core at Energy Levels of Power. Izvestiya vuzov. Yadernaya Energetika. 2025, no. 1, pp. 24-36; DOI: https://doi.org/10.26583/npe.2025.1.02 (in Russian).