ROSA/LSTF test on nitrogen gas behavior during reflux condensation in PWR and RELAP5 code analyses

Abstract

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.