Experimental investigation of the effect of operating conditions on internal flow behavior in APR1000

Understanding the behavior of the primary coolant in a nuclear reactor is crucial for evaluating thermal margins and ensuring reactor safety. This study examines the behavior of the primary coolant using the ACOP experimental facility, a 1/5 scale model of the APR1000 reactor, to evaluate the effects of flow rate and flow unbalance on coolant distribution. Experiments were performed at flow rates of 60 %, 80 %, 100 %, and 110 % under balanced conditions, as well as under 15 % unbalanced and extreme unbalanced conditions with three pumps. The coolant flow characteristics were assessed by measuring the core inlet flow distribution, core outlet pressure, upper plenum pressure, and hot leg velocity using 177 core simulators. The results showed that the core inlet flow distribution ranged between 88 % and 112 % across all conditions. Under extreme unbalanced conditions, a maximum deviation of 5.3 % in single core flow distribution was observed. The outlet pressure distribution remained stable under four-pump conditions but showed greater variation with three pumps. The study concludes that coolant flow rate and flow unbalance had minimal impact under balanced four-pump conditions, while significant effects on outlet pressure, upper plenum pressure, and hot leg velocity were evident under extreme unbalanced conditions.