ON THE SUBCOOLED CRITICAL FLOW MODEL IN RELAP5 MOD3/

An analysis of an anomaly in the subcooled critical flow model in the RELAP5/MOD3 computer code is presented. Specifically, the code produce s a discontinuity in going from unchoked subcooled liquid flow (i.e., subsonic flow) Co subcooled choked flow (i.e., sonic flow). The same a nomaly has been reported elsewhere. The root cause for this anomaly ha s been analyzed, and it is found that the user-supplied junction loss coefficient and discharge coefficient play an important role in the oc currence of this anomaly. The analysis is verified by assessment again st a test problem simulating single-phase liquid flow through a conver gent nozzle with a fixed upstream pressure and a varying downstream pr essure. A corrective measure to eliminate the discontinuity is suggest ed.