RELAP5 analysis of OECD/NEA ROSA project experiment simulating a PWR loss-of-feedwater transient with high-power natural circulation

Takeda, Takeshi ; Asaka, Hideaki*; Nakamura, Hideo  

A ROSA/LSTF experiment was conducted for OECD/NEA ROSA Project simulating a PWR loss-of-feedwater transient with assumptions of failure of scram and total failure of HPI system. 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 SG secondary-side pressures were maintained respectively at around 16 and 8 MPa by cycle opening of PZR PORV and SG relief valves. 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 by employing a fine-mesh multiple parallel flow channel representation of SG U-tubes with a Wallis CCFL correlation. Problems remain in the predictions of the oscillative primary loop flow rate and SG U-tube liquid level as well as PZR liquid level.