MELPROG-PWR/MOD0: A mechanistic code for analysis of reactor core melt progression and vessel attack under severe accident conditions

Since the Three Mile Island accident, increased emphasis has been placed on the analysis of severe reactor accidents because such accidents represent the dominant public risk from nuclear reactors. The US Nuclear Regulatory Commission has made the development of mechanistic models for severe accident progression a major priority. The purpose of these models is to provide detailed, best-estimate, coupled analyses of all the major phenomena involved in the reactor vessel and coolant system in the course of the accident. To meet this objective, the MELPROG computer code is being developed. The initial version of this code, MELPROG-PWR/MOD0, is described in this report. The purpose of MELPROG is to provide at any time during an accident sequence a description of (1) the state of the reactor core and surrounding in-vessel environment, and (2) the disposition of core materials (in particular, fission products) contained within the reactor coolant system boundary. MELPROG is coupled to the TRAC-PF1 RCS thermal-hydraulics code to provide an integrated analysis of the behavior of core, vessel, and reactor coolant system during severe accidents. MELPROG treats core degradation and loss of geometry, debris formation, core melting, attack on supporting structures, slumping, melt/water interactions and vessel failure. The key elementmore » in MELPROG is the use of detailed modeling for the entire damage progression and failure sequence. Emphasis is also placed on the rates of hydrogen, steam and fission product formation, and transport to containment during damage progression.« less