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A ROSA/LSTF experiment was conducted for OECD/NEA ROSA Project simulating a PWR loss-of-feedwater (LOFW) transient with specific assumptions of failure of scram that may cause natural circulation with high core power and total failure of high pressure injection system. Auxiliary feedwater (AFW) was provided to well observe the long-term high-power natural circulation. The core power curve was obtained from a RELAP5 code analysis of PWR LOFW transient without scram. The primary and steam generator (SG) secondary-side pressures were maintained, respectively, at around 16 and 8 MPa by cycle opening of pressurizer (PZR) power-operated relief valve and SG relief valves for a long time. Large-amplitude level oscillation occurred in SG U-tubes for a long time in a form of slow fill and dump while the two-phase natural circulation flow rate gradually decreased with some oscillation. RELAP5 post-test analyses were performed to well understand the observed phenomena by employing a fine-mesh multiple parallel flow channel representation of SG U-tubes with a Wallis counter-current flow limiting correlation at the inlet of U-tubes. The code, however, has remaining problems in proper predictions of the oscillative primary loop flow rate and SG U-tube liquid level as well as PZR liquid level.

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The effectiveness of an operator-initiated steam generator (SG) secondary-side depressurization on the core cooling performance during small-break loss of coolant accidents (SBLOCAs) in a pressurized water reactor (PWR) with total failure of the high pressure injection (HPI) systems is studied. The study is based on experiments conducted in the ROSA-V Large Scale Test Facility (LSTF) and analyses with the RELAP5/Mod3 code. The sensitivity of the core minimum liquid level and peak cladding temperature (PCT) to the secondary-side depressurization rate and the initiation time of the depressurization is evaluated analytically for various break sizes. It is shown that the P C T takes a maximum value for break areas between 1.0% and 1.5% of the cold leg cross-sectional area. The conditions which the depressurization rate and the initiation time should satisfy to limit the maximum P C T are derived.