Analysis of pressure- and temperature-induced steam generator tube rupture during PWR severe accident initiated from station blackout

Severe accident studies showed that the direct containment heating issue was resolved for PWRs because a creep rupture at pressurizer surge line would occur prior to the model-through of reactor pressure vessel during high pressure sequences such as total station blackout (TMLB'). However, it was recently concerned that, if the secondary system is depressurized during TMLB', the integrity of SG U-tubes would be threatened earlier than the surge line because of their heat up due to the hot leg countercurrent flow and pressure difference between primary and secondary systems. This pressure- and temperature-induced SG U-tube rupture (PTI-SGTR) could increase significantly the source terms due to containment bypass of fission products (FP). The SCDAP/RELAP5 analyses for Surry plant by USNRC showed that the surge line would fail earlier than the U-tubes during TMLB' with secondary system depressurization However, the analyses assumed a homogeneous steam mixing at the SG inlet plenum that plays an important role in determining the temperature of steam in the U-tubes. In order to investigate the effect of incomplete steam mixing, an analysis was performed at JAERI with MELCOR 1.8.4 for TMLB' of Indian Point 3. The analysis showed that the surge line would fail earliest during TMLB' without secondary system depressurization while the PTI-SGTR could occur earliest during TMLB' with secondary system depressurization. (author)