Experimental Evaluation of the Effect of Partial and Complete Spacer Grid Blockage on the Heat Power Supply Required to Reach the Critical Refrigerant Temperature of a Heated Non-Pressurized Fuel Rod Simulator in an Experimental Loop

This study investigates the influence of partial and complete blockage of the spacer grid on the power required to reach the critical coolant temperature in a vertical unpressurized experimental loop containing a single heated rod simulating a nuclear fuel element. Flow restrictors were designed to obstruct approximately one-third and three-quarters of the flow cross-section, respectively. The experimental setup included a glass channel, a directly heated stainless-steel rod, and advanced data acquisition via thermocouples and ultrasonic flow sensors. Experiments were carried out under various coolant flow rates, and critical heat flux conditions were evaluated. Results show that at higher flow rates, increased power was required to reach the set critical temperature of 180 °C, even in the presence of flow obstruction. Contrary to expectations, the introduced foreign objects (flow restrictors) enhanced local coolant mixing, delaying the onset of critical heat flux compared to the unobstructed case. The presence of the foreign element did not cause local overheating but showed a beneficial thermohydraulic effect under tested conditions. These findings are particularly relevant for understanding post-operational behaviour of impurities in spent fuel pools where flow rates and thermal conditions differ significantly from reactor cores.