Assessment of MELCOR 1.8.5 Versus Different Versions of SCDAP/RELAP5 MOD 3.3 With Lower Head Creep Rupture Analysis of Alternative Accident Sequences of the Three Mile Island Unit 2

Abstract

The objective of this analysis is to assess MELCOR 1.8.5-RG against SCDAP/RELAP5 MOD 3.3kz (SR5m33kz), and SCDAP/RELAP5 MOD 3.3bf (SR5m33bf). This lower head creep rupture analysis considers: (1) Three Mile Island Unit 2 (TMI-2) alternative accident sequence-1, and (2) TMI-2 alternative accident sequence-2. SCDAP/RELAP5 model of TMI-2 alternative accident sequence-1 includes the continuation of the base case of the TMI-2 accident with the reactor coolant pumps (RCP) tripped, and the High Pressure Injection System (HPIS) throttled after approximately 6000 s accident time, SCDAP/RELAP5 model of TMI-2 alternative accident sequence-2 is derived from the TMI-2 base case accident by tripping the RCP after 6000 s, and the HPIS is reactivated after 12,012 s. MELCOR model of TMI-2 alternative accident sequence-1 is based on MELCOR TMI-2 phase-2 model by tripping the RCP and throttling back the makeup flows to zero from 6000 s onward. In MELCOR model of TMI-2 alternative accident sequence-2, the RCP are tripped from 6000 s and the constant makeup flow rate of 3.75 kg/s — including pump seal flow rate, but without HPIS flow rate — is activated from 6000 s and beyond 10440 s. The simulation is run until the lower head wall ruptures. In addition, the lower head penetration failure is also calculated with MELCOR for both TMI-2 alternative accident sequences. Lower head temperature contours calculated with SCDAP/RELAP5 are visualized and animated with open source visualization freeware ‘OpenDX’. Significant findings of the analysis include: (1) the TMI-2 lower head wall fails by creep rupture with either deactivations or activations of the HPIS; (2) for the TMI-2 alternative accident sequence-1 the time to creep rupture calculated with MELCOR 1.8.5-RG, SR5m33kz, and SR5m33bf agrees reasonably; (3) the calculation with MELCOR for the TMI-2 alternative accident sequence-1 predicts that the lower head wall failure occurred earlier than penetration failure, while MELCOR predicts the opposite for the TMI-2 alternative accident sequence-2; (4) calculation with MELCOR for TMI-2 alternative accident sequence-2 shows that when the lower head wall fails the temperature calculated with MELCOR is 1810.9 K, which exceeds the melting temperature of 1789 K for carbon steel; (5) calculations with both SR5m33kz and SR5m33bf for both TMI-2 alternative accident sequences indicate that different lower head wall locations fail rapidly one after another by a delay of a few seconds, while this is not the case for MELCOR.