LBLOCA accident

As previously described, the RELAP5 and modified RELAP5 models and codes set the boundary conditions that are critical to the FLOWTRAN-TF assembly model. These codes have been subjected to an extensive benchmarking effort based on full-scale primary system hydraulic tests. While these tests do not fully simulate the transient nature of the LBLOCA accident, and no heating is... 

During 1988, new data was developed for the RVSS materials at Oak Ridge National Laboratory (ORNL) (References 2 and 3). This data indicated that low neutron flux at low temperatures caused greater embrittlement of the materials used in RVSS than previously anticipated. This increased material embrittlement or "upward shift" in NDTT reduces the fracture toughness of these materials and, under certain specific and conservative transient conditions such as an earthquake or Large-Break Loss-Of-Coolant Accident (LBLOCA), could conceivably result in the failure of the supports thus permitting the reactor vessel to move. 

LBLOCA Large-Break Loss of Coolant Accident... 

Discrepancies between code-set power levels for a very complicated problem like the LBLOCA are not anticipated to be excessive considering both code families are benchmarked to the same set of conditions and the accident transient is not significantly different from the benchmarking. In the case of the FI phase, the rapidity of the transient and timing remain critical. 

While this discussion focuses on the FI DEGB-LBLOCA analysis, because of the formative nature of the gamma heating and ECS analyses, the single-phase conditions in the primary system make the resultant methodologies and models developed for the FI phase of the LOCA applicable to the analyses of other DBEs. More specifically, as previously indicated, this technology has been applied to the LOPA, PSB, and GRW accidents. 

Analysis of the LOPA, pump shaft break (PSB), and gang rod withdrawal (GRW) accidents (Reference 19) DBAs, which traditionally defined the SRS reactors maximum power limit envelope, were analyzed using similar T/H criteria, modeling, and computational techniques (TRAC, FLOWTRAN-FI and physics code suites) as those developed for the LBLOCA FI phase analysis. Preliminary results are shown in Table 1. 

Currently, there are three DBAs that appear enveloping and could potentially limit reactor power the large break loss-of-coolant accident (LBLOCA) and the loss of pumping accident (LOPA), which are judged to very low probability events and the loss of septifoil cooling accident, which has occurred twice during the life of the SRS reactors. 

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