Measurement of non-condensable gas in a PWR small-break LOCA simulation test with LSTF for OECD/NEA ROSA Project and RELAP5 post-test analysis

Abstract

An OECD/NEA ROSA Project experiment with the large scale test facility was conducted simulating a PWR 0.5% cold leg small-break loss-of-coolant accident with steam generator (SG) secondary-side depressurization as an accident management measure to clarify such complex phenomena as non-uniform flow among the SG U-tubes under natural circulation (NC) with non-condensable gas. Non-condensable gas inflow to the primary system from accumulator tanks was simulated with air. The gas concentration was measured at the SG outlet plenum by using a gas measurement device to measure oxygen concentration in steam–gas mixture that is developed employing a Zirconia sensor. Non-uniform flow behaviors were observed in the SG U-tubes with non-condensable gas ingress depending probably on gas accumulation rate as well as asymmetric NC between two loops. Air concentration in steam–gas mixture was estimated to be about 40-50% at the SG outlet plena after the U-tube downflow side became empty of liquid, which was a bit larger than the estimated value based on the gas phase temperature. Post-test analysis by JAEA-modified RELAP5/MOD3.2.1.2 code revealed that the code underpredicts the primary pressure a bit after the gas inflow with some discrepancies in the SG U-tube liquid level and the NC mass flow rate.