MELPROG-PWR (pressurized water reactor) MOD1 analysis of a TMLB' accident sequence
The first complete, coupled, and mechanistic analysis of a reactor core meltdown sequence has been made with MELPROG-PWR/MOD1. The sequence analyzed was a station blackout accident sequence (TMLB') for the Surry plant. The MELPROG calculation was initiated at the point where boiling began in the vessel and was run through the point that the reactor vessel failed. Between the beginning and the end, all important aspects of the meltdown sequence were calculated with MELPROG. This version of MELPROG permits a full two-dimensional treatment of the in-vessel phenomena. As such, the important effects of in-vessel natural circulation can be accurately modeled. To assess the importance of natural circulation and other two-dimensional effects, the current calculation was compared with one-dimensional MELPROG and MARCH calculations of the same accident scenario. This comparison shows that natural circulation reduces the rate of core heating, but increases the rate of heating of upper plenum structures. This implies that a significant amount of the core energy is deposited in the plenum and primary piping. This increased heating can inhibit fission product deposition and may lead to an early failure of the primary system. Hence, natural circulation alone can completely change the course of a meltdown sequence (relative to one-dimensional calculations). Limited sensitivity studies were performed to assess the relative importance of various modeling assumptions. One of the key models assessed was the modeling of the initial fuel rod melting and relocation. Variations in the modeling assumptions were found to strongly affect hydrogen production and the subsequent course of the accident. The magnitude of hydrogen source could be varied by a factor of two through variations in fuel rod modeling. This result implies that accurate and mechanistic modeling is important for severe accident sequence analysis.
- Research Organization:
- Sandia National Labs., Albuquerque, NM (USA); Nuclear Regulatory Commission, Washington, DC (USA). Div. of Reactor Systems Safety
- DOE Contract Number:
- AC04-76DP00789
- OSTI ID:
- 6681768
- Report Number(s):
- NUREG/CR-4742; SAND-86-2175; ON: TI87006737
- Country of Publication:
- United States
- Language:
- English
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