Large-break loss of coolant accident

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. 

Large pipes are eliminated by design, thus eliminating the large-break loss of coolant accident (LOCA) design basis accident. 

The safety I C systems located inside the containment were environmentally qualified (see also SP (182)) for large break, loss of coolant accident conditions according to the original design. However, formal documentation from this quahfication was not available. The parts of the system located inside the I C rooms but outside the containment were not environmentally qualified. The valves of the safety systems located inside the containment were also not fully qualified. The plant launched a wide environmental qualification programme. In the first step the items to be environmentally qualified were identified. Environmental criteria were set up... 

By adopting the integrated primary system design and the HHTS without reactor coolant pumps and main coolant pipelines, the possibility of accidents that may cause fuel failure, such as a large-break loss of coolant accident (LOCA), rod ejection (R/E), loss-of-flow (LOF) and locked rotor (L/R), is essentially eliminated. During the normal operation, the water level in the reactor vessel is controlled by injection from the charging pumps. However, since the diameter of the pipes connected to the primary system (reactor vessel) is less than 10 mm, water level can be maintained to submerge the top of the core without any injection. 

LBLOCA Large-Break Loss of Coolant Accident... 

A typical large dry containment is shown in Fi re 4.1-3. A large dry containment is designed to contain the blowdown mass and energy from a large break Loss-of-Coolant Accident (LOCA), assuming any single active failure in the containment heat removal systems. 

R. Chambers et al.. Assessment of FRAP-T6 Code Capabilities During Large-Break Loss-of-Coolant Accidents, EGG-CAAD-5829, March 1 982. 

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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