To evaluate upper head injection system (UHIS) performance during a postulated loss-of-coolant accident (LOCA) in a pressurized water reactor (PWR), ten UHIS tests were conducted at the ROSA-II test facility. The experimental results were different from the expected UHIS performance in the following points. First, flashing took place in the upper head and a mixture level was formed before UHIS actuation. Second, emptying of the upper head was observed immediately after UHIS shut off. Third, part of the water which flowed down from the upper head, penetrated into the core and contributed to core cooling at the top part of the core, however, most of the water flowed out through the broken loop hot leg. In the case of higher injection water temperature (∼120°C), the fluid behavior in the pressure vessel differed significantly from the results for the low injection water temperature (∼20°C), and the core cooling was remarkably improved. Therefore, high-temperature UHIS water is recommended for effective core cooling. The results described above are due to the following physical phenomena:

  1. Fluid mixing in the upper head is not good.
  2. Subcooled water, which flows into places such as the upper plenum where steam exists, causes strong condensation-depressurization which affects the flow behavior and core cooling.
Although the magnitude of the thermal-hydraulic effects observed in the ROSA-II/UHI tests may be unique to this facility, the above two physical phenomena observed are applicable to all PWRs with a UHIS. Therefore, these two phenomena must be included in a LOCA analysis of a PWR with a UHIS.