Experimental verification of integrated reactor head structure development

Scholars have extensively researched the development of the reactor head structure. Most studies focus on numerical analysis and partial experimental verification of temperature and flow fields in local areas surrounding specific reactor types. However, few studies examine the entire reactor pit. There is a lack of full-scale numerical simulations and experimental validation of natural convection flow fields and temperature fields covering the integrated reactor head structure, clusters of control rod drive mechanisms (CRDMs), reactor head cables, and reactor pit ambient conditions. This scarcity leaves a research gap in the comprehensive performance evaluation of the entire reactor pit. Based on the high-temperature-resistant control rod drive structure, this paper proposes an innovative design for the integrated reactor head structure. The proposed scheme eliminates the forced ventilation system for the reactor head. This approach enables modular installation and integral hoisting of the reactor head structure and components, which greatly simplifies refueling and overhaul of nuclear reactors.

In this research, the computational fluid dynamics (CFD) method is adopted. Following the coupling theory of heat conduction and convection with internal heat sources, the heat dissipation performance of three-phase media, including air, fluid, and metal inside the reactor head structure, is analyzed under two operating modes: natural convection and reactor cavity pool auxiliary ventilation. The flow field distribution of the reactor head structure and the heat dissipation effect of CRDM coils under diverse working conditions are thoroughly explored. Furthermore, a full-scale 1 : 1 test bench for the integrated reactor head structure is established to perform high-temperature ventilation tests to verify the accuracy and reliability of the numerical simulation results. The high-temperature ventilation test for the integrated reactor head structure realistically simulates the high-temperature ambient and heat dissipation conditions. The temperatures of CRDM coils, in the RGL (rod control and guide system) cables area, and in the RIC (reactor in-core instrumentation system) cables area of the reactor head structure comply with the design requirements. Compared with natural circulation, auxiliary ventilation markedly reduces the coil temperature and the air temperature around the reactor upper plenum. A comparison between test data and CFD simulation results shows that the overall variation trends of key reactor head structure components (including CRDM coils and air temperature distribution) are in good agreement, while certain deviations exist in the maximum temperature values. Under natural circulation conditions, the maximum temperature of high-temperature-resistant CRDM coils remains within the allowable range. The coil temperatures obtained from auxiliary ventilation tests match well with CFD predictions. However, the CFD-calculated coil temperatures under natural circulation are relatively low. Sensitivity analysis reveals that this discrepancy is associated with the setting of coil thermal resistance in the CFD model. The relatively low cable temperature in the RGL area under auxiliary ventilation is mainly attributed to the lower ambient temperature during the test. An obvious temperature difference exists between the test environment and the actual operating environment of a nuclear power plant.

The results of CFD simulations and high-temperature ventilation tests on the integrated reactor head structure indicate that auxiliary ventilation through the reactor cavity pool delivers significantly superior heat dissipation performance compared with natural convection cooling. Coil temperature is reduced substantially, and the temperatures of all components in the reactor head structure are kept within permissible limits. The integrated reactor head structure adopts a truss structure, eliminating the forced ventilation system and seismic tie rods.

This design realizes modular installation and integral hoisting of the reactor head structure and simplifies the operation of upper plenum components during the opening and closing of the reactor head for refueling and overhaul. The integrated reactor head structure with auxiliary ventilation is proven effective and feasible. It meets all heat dissipation requirements, simplifies structural design, reduces refueling and overhaul times, and further enhances the operational reliability and economic efficiency of nuclear power plants.