We conducted an experiment to clarify nitrogen gas behavior at low pressures during reflux condensation in a pressurized water reactor (PWR) with the rig of safety assessment/large scale test facility (ROSA/LSTF) at Japan Atomic Energy Agency. The primary pressure was lower than 1 MPa under the constant core power of 0.7% of the volumetric-scaled (1/48) PWR nominal power, unlike a previous related test with the LSTF. Steam generator (SG) secondary-side collapsed liquid level was maintained at a certain liquid level above the SG U-tube height. Nitrogen gas was injected stepwise into each SG inlet plenum at a certain constant amount. The primary pressure and the degree of subcooling of the SG U-tubes were largely dependent on the amount of nitrogen gas accumulated in the SG U-tubes. Nitrogen gas accumulated from the outlet towards the inlet of the SG U-tubes. Non-uniform flow behavior was observed among the SG U-tubes with nitrogen gas ingress. The RELAP5/MOD3.3 code indicated remaining problems in the predictions of the primary pressure and the degree of subcooling of the SG U-tubes depending on the number of nitrogen gas injection. We investigated further the applicability of the RELAP5 code with different models for the SG U-tubes to the prediction of the non-uniform flow behavior through sensitivity analyses.
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