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MELCOR is developed at
Sandia National Laboratories
for the
U.S. Nuclear Regulatory Commission
MELCOR is a fully integrated, engineering-level computer code whose primary purpose is to model the progression of accidents in light water reactor nuclear power plants. A broad spectrum of severe accident phenomena in both boiling and pressurized water reactors is treated in MELCOR in a unified framework, Current uses of MELCOR include estimation of fission product source terms and their sensitivities and uncertainties in a variety of applications.

The MELCOR code is composed of an executive driver and a number of major modules, or packages, that together model the major systems of a reactor plant and their generally coupled interactions. Reactor plant systems and their response to off-normal or accident conditions include:
  • Thermal-hydraulic response of the primary reactor coolant system, the reactor cavity, the containment, and the confinement buildings
  • Core uncovering (loss of coolant), fuel heatup, cladding oxidation, fuel degradation (loss of rod geometry), and core material melting and relocation
  • Heatup of reactor vessel lower head from relocated fuel materials and the thermal and mechanical loading and failure of the vessel lower head, and transfer of core materials to the reactor vessel cavity
  • Core-concrete attack and ensuing aerosol generation
  • In-vessel and ex-vessel hydrogen production, transport, and combustion
  • Fission product release (aerosol and vapor), transport, and deposition
  • Behavior of radioactive aerosols in the reactor containment building, including scrubbing in water pools, and aerosol mechanics in the containment atmosphere such as particle agglomeration and gravitational settling
  • Impact of engineered safety features on thermal-hydraulic and radionuclide behavior
  • The various code packages have been written using a carefully designed modular structure with well-defined interfaces between them. This allows the exchange of complete and consistent information among them so that all phenomena are explicitly coupled at every step. The structure also facilitates maintenance and upgrading of the code.
Initially, the MELCOR code was envisioned as being predominantly parametric with respect to modeling complicated physical processes (in the interest of quick code execution time and a general lack of understanding of reactor accident physics). However, over the years as phenomenological uncertainties have been reduced and user expectations and demands from MELCOR have increased, the models implemented into MELCOR have become increasingly best estimate in nature. The increased speed (and decreased cost) of modern computers (including PCs) has eased many of the perceived constraints on MELCOR code development. Today, most MELCOR models are mechanistic, with capabilities approaching those of the most detailed codes of a few years ago. The use of models that are strictly parametric is limited, in general, to areas of high phenomenological uncertainty where there is no consensus concerning an acceptable mechanistic approach.

Current uses of MELCOR often include uncertainty analyses and sensitivity studies. To facilitate these uses, many of the mechanistic models have been coded with optional adjustable parameters. This does not affect the mechanistic nature of the modeling, but it does allow the analyst to easily address questions of how particular modeling parameters affect the course of a calculated transient. Parameters of this type, as well as such numerical parameters as convergence criteria and iteration limits, are coded in MELCOR as sensitivity coefficients, which may be modified through optional code input.

MELCOR modeling is general and flexible, making use of a "control volume" approach in describing the plant system. No specific nodalization of a system is forced on the user, which allows a choice of the degree of detail appropriate to the task at hand. Reactor-specific geometry is imposed only in modeling the reactor core. Even here, one basic model suffices for representing either a boiling water reactor (BWR) or a pressurized water reactor (PWR) core, and a wide range of levels of modeling detail is possible. For example, MELCOR has been successfully used to model East European reactor designs such as the Russian VVER, and RMBK-reactor classes.

This update of the MELCOR computer code manuals corresponds to MELCOR version 1.8.6, released to users in July 2005. Many new modeling enhancements have been added to the COR package to improve the capabilities of the code to better represent the late phase behavior of severe accidents. As part of this development, the Bottom Head (BH) package was eliminated and features formerly offered by the BH package that were missing from the COR package representation were added to the COR package. New models in the COR package include hemispherical lower head geometry, models for simulating the formation of molten pools both in the lower plenum and the upper core, crust formation, convection in molten pools, stratification of molten pools into metallic and oxide layers, and partitioning of radionuclides between stratified molten pools, reflood quench model, control rod silver release mode, new B4C control rod oxidation model, and capability to model the PWR core outer periphery. Improvements to other MELCOR packages include flashing of superheated sources and flows, and the extension of CORSOR-Booth release model to a second fuel type.

The MELCOR 1.8.6 code manuals are contained in three volumes. Volume 1 contains a primer that describes MELCOR‘s phenomenological scope, organization (by package), and documentation. The remainder of Volume 1 contains the MELCOR User‘s Guides, which provide the input instructions and guidelines for each package. Volume 2 contains the MELCOR Reference Manuals, which describe the phenomenological models that have been implemented in each package. Volume 3 contains a portfolio of sample demonstration problems. These problems are a combination of experiment analyses, which illustrate code model performance against data, and full plant analyses showing MELCOR‘s performance on larger realistic problems.