Thermal-hydraulic and solid mechanics safety analysis for VVER-1000 reactor using analytical and CFD approaches

Neutronic, thermal-hydraulics, and solid mechanics computations were used to study the different VVER-1000 reactors' operational conditions for the steady-state and the transient state. The main objective of these studies is to investigate the safe operating conditions, which is essential in determining the reactor safety limits. The coupling between different physics enables a better understanding of reactor phenomena and enhanced accuracy by reducing the assumption needed for separate calculations for different physics. MCNPX code is used to provide the source power distribution for two themes of calculations. The first theme is the analytical solution using MATLAB software, and the second theme is the finite element coupled physics using COMSOL Multiphysics. Analytical solutions using MATLAB software and results from COMSOL-Multiphysics were used to simulate the uncoupled analytical models and coupled models, respectively. The coupled theme provide feedbacks between different physics by iterating the solution between different physics; such as solving the heat transfer with taking into consideration the variation of the coolant velocity along the coolant channel and solving solid mechanics with taking into consideration the change of either the fuel and the clad temperatures distribution (thermal load) or the change in the coolant pressure (pressure load). While in the uncoupled solutions, certain assumptions reflect an average behavior for the effect of other physics on the physics under concern, such as the average pressure of the coolant and the average value for the coolant velocity. The obtained results are verified by comparing it to WIMS D-4 for neutronic calculations and COPERA-EN code for the thermal-hydraulic analytical solution. Also, thermal-hydraulic results by both coupled and uncoupled themes are verified against results from the final safety analysis report. The Departure from Nucleate Boiling Ratio (DNBR) and the Minimum Departure from Nucleate Boiling Ratio (MDNBR) were applied for the investigation of VVER-1000 reactor safety parameters. Solid mechanics, fluid dynamics and heat transfer were used to compute the maximum stress/maximum volumetric strain acting on both fuel and cladding material. Results from both methods satisfied the limits addressed by the designer and shows a good agreement with the other published works that used WIMS D-4 for neutronic calculations and COBRA-EN code for thermal-hydraulic calculations analytically. in addition, the maximum stress occurring on the fuel and clad materials didn't exceed the yield stress. Furthermore, the fuel material displacement didn't exceed the Helium gap thickness, which means the surface contact between the fuel and the clad material is impossible that prevents the chemical interaction between both fuel and clad at 1135 K.