MACCS (MELCOR Accident Consequence Code System), WinMACCS, and MelMACCS now facilitate a multi-unit consequence analysis. MACCS evaluates the consequences of an atmospheric release of radioactive gases and aerosols into the atmosphere and is most commonly used to perform probabilistic safety assessments (PSAs) and related consequence analyses for nuclear power plants (NPPs). WinMACCS is a user-friendly preprocessor for MACCS. MelMACCS extracts source-term information from a MELCOR plot file. The current development can combine an arbitrary number of source terms, representing simultaneous releases from a multi-unit facility, into a single consequence analysis. The development supports different release signatures, fission product inventories, and accident initiation times for each unit. The treatment is completely general except that the model is currently limited to collocated units. A major practical consideration for performing a multi-unit PSA is that a comprehensive treatment for more than two units may involve an intractable number of combinations of source terms. This paper proposes and evaluates an approach for reducing the number of calculations to be tractable, even for sites with eight or ten units. The approximation error introduced by the approach is acceptable and is considerably less than other errors and uncertainties inherent in a Level 3 PSA.
This document summarily provides brief descriptions of the MELCOR code enhancement made between code revision number 14959and 18019. Revision 14959 represents the previous official code release; therefore, the modeling features described within this document are provided to assist users that update to the newest official MELCOR code release, 18019. Along with the newly updated MELCOR Users Guide and Reference Manual, users are aware and able to assess the new capabilities for their modeling and analysis applications.
The U.S. Nuclear Regulatory Commission initiated the state-of-the-art reactor consequence analyses (SOARCA) project to develop realistic estimates of the offsite radiological health consequences for potential severe reactor accidents. The SOARCA analysis of an ice condenser containment plant was performed because its relatively low design pressure and its reliance on igniters make it potentially susceptible to early containment failure from hydrogen combustion during a severe accident. The focus was on station blackout accident scenarios where all alternating current power is lost. Accident progression calculations used the MELCOR computer code and offsite consequence analyses were performed with MACCS. The analysis included more than 500 MELCOR and MACCS simulations to account for uncertainty in important accident progression and offsite consequence input parameters. Consequences from severe nuclear power plant accidents modeled in SOARCA are smaller than previously calculated. The delayed releases calculated provide more time for emergency response actions. The results show that early containment failure is very unlikely, even without successful use of igniters. The modeled behavior of safety valves is very important to this conclusion, but there is sparse data and a lack of established expert consensus on the failure rates under severe accident conditions. Even for scenarios resulting in early containment failure, the calculated individual latent fatal cancer risks are very small. Early and latent-cancer fatality risks are one focus of this paper. Regression results showing the most influential parameters are also discussed.
This paper is the third paper in a special session on the State-of-the-Art Reactor Consequence Analyses (SOARCA) Uncertainty Analyses (UAs), and summarizes offsite consequence insights from the three SOARCA UAs. The U.S. Nuclear Regulatory Commission (NRC) with Sandia National Laboratories has completed three UAs for particular station blackout scenarios as part of the SOARCA research project: for a boiling-water reactor with a Mark I containment in Pennsylvania State (Peach Bottom), for a pressurized-water reactor (PWR) with an ice condenser containment in Tennessee State (Sequoyah), and for a PWR with subatmospheric large dry containment in Virginia State (Surry). The Sequoyah and Surry SOARCA UAs focused on an unmitigated short-term station blackout (SBO) scenario involving an immediate loss of offsite and onsite AC power. In the Surry UA, induced steam generator tube rupture was also modeled. The Sequoyah study focused on issues unique to the ice condenser containment and the potential for early containment failure due to hydrogen combustion. The Peach Bottom UA focused on an unmitigated long-term SBO scenario, where battery power is initially available. The MELCOR Accident Consequence Code System (MACCS) suite of codes was used for offsite radiological consequence modeling. This paper presents the offsite consequence results, individual latent cancer fatality risk and the individual early fatality risk, for the three SOARCA UAs and summarizes some of the insights and features of the analyses.