Iztok Parzer

The work presents the possibilities for animation of MELCOR 1.8.3 predictions with NPA graphical interface software for the case of a hypothetical LB LOCA accident scenario for Krško NPP, a two-loop Westinghouse plant. The model of Krško consists of 17 hydrodynamic volumes, 22 flow paths and 38 heat structures. SG1 and SG2 secondary side and environment are modelled each with only 1 volume, while containment, including reactor cavity, is represented by 4 volumes. Reactor core is modelled with 13 axial segments in 3 radial rings, initially occupying upper 10 axial layers with the possibility to later relocate into 3 lower axial segments, before penetrating through the reactor vessel bottom head.

MELCOR is a modular, fully integrated engineering-level computer code for modelling progression of severe accidents in light water reactor nuclear power plants. It was developed at SNL (Sandia National Laboratories) as the successor to the Source Term Code Package. The phenomena treated by MELCOR code are thermal-hydraulic response of the reactor coolant system, heat conduction within intact, solid structure and energy transfer across its boundaries into control volumes. Core and lower plenum structures relocation during melting, slumping and debris formation, including failure of the reactor vessel and debris ejection into the cavity, is treated by MELCOR code. The thermal response of the core and lower plenum internal structures including the lower portion of the reactor vessel is also taken into account. Hydrogen production is assumed during the cladding interaction with coolant. The attack of the basemat concrete by hot or usually molten core materials is modelled. The effects of heat transfer, concrete ablation, cavity shape changes and gas production are included. Radionuclide release and transport behaviour of fission product vapours and aerosols are also described.

In parallel with the MELCOR model development, the graphical interface has been upgraded, using NPA (Nuclear Plant Analyzer) software, developed at INEL (Idaho National Engineering Laboratory). In the beginning phase some masks have been developed for observing main phenomena during the severe accident progression. These include thermal-hydraulic phenomena during the initial blowdown phase, core melt and relocation after the core heatup and cavity shape changes during the interaction of concrete and molten core material. During the process of animation mask creation some drawbacks of MELCOR code have showed. Many variables in MELCOR can only be used as parameters to control functions or as plot variables, but not for both purposes. So the coding had to be modified and the list of the recognized variables was extended.