MELCOR

I

University of Maribor, Faculty of Mechanical Engineering

Smetanova 17, 2100 Maribor, Slovenia

The severe accident including loss of coolant scenario using computer program MELCOR 1.8.3 was modeled. There have been number of analysis using other integral computer packages performed in the past including MAAP and STCP programs as well as mechanistic computer packages such as RELAP5.

MELCOR is state of the art program maintained by United States Nuclear Regulatory Commission and is currently used at University of Maribor to assess some of the accident sequences.

At the University of Maribor the database describing Krško Nuclear Power Plant was developed to be used with MELCOR 1.8.3 computer code. The database was developed using mainly Updated Safety Analysis Report, and other available documentation collected both through Slovenian Nuclear Safety Administration and Krško nuclear power plant. This database optimization and further development is subject of this work.

The special attention was given to optimization of nodalization of Reactor Coolant System. There are number of tasks involved in this work including the nodalization study to determine the most suitable cost (number of nodes) vs. benefit (accuracy of the results) ratio. Also, the database is currently modified for loss of off site power scenario used as benchmark as it has been studied previously using MAAP integral code so several assumptions embedded therein were contemplated.

An attempt at consistent nodalization analysis with different numbers of control volumes on the primary side was made. This work was primarily motivated by a desire to compare results to results of other computer programs. Several thermodynamics parameters were followed: time to open safety valves on steam generator, time to rupture disk failure, time for the core to uncover, time to reactor vessel failure and time to containment failure.

Additionally, initialization criteria were addressed. In particular, the system pressure, the temperature of primary medium and the energy transfer through the steam generators were chosen.

For melting criteria, the temperature of zircalloy cladding at different levels in core, the temperature of melted material in cavity and the concentrations of hydrogen and carbon monoxide in the containment were selected.

Advantages and shortcomings of the nodalization are discussed and future plans are presented.

Several test results to show the influence of nodalization of Krsko NPP simulation are presented.