A novel MELCOR modeling approach for BWR applications

Even though improvements in modeling the phenomena that occur in the lower plenum have been made, the MELCOR user must decide on the physical events that are to be modeled and create the corresponding input. In addition to the standard modeling techniques, the flexibility of MELCOR allows for nontraditional modeling as well. For typical BWR applications, the user establishes concentric radial rings for the COR Package. In theory, each ring should be composed of a channel (within the channel box walls) and a bypass (the volume region outside the channel box walls) with each region containing both fluid and structures. However, in MELCOR 1.8.3 and earlier versions, the COR Package allows particulate debris to relocate only inside the channel box walls, using a volume equivalent to the channel and bypass fluid volumes. The consequences of this modeling approach with respect to the combined channel and bypass volumes produces two anomalies in the BWR calculation, particularly for boildown sequences. The first is that the CVH Package will calculate inaccurate channel and bypass water levels for the core region, which can significantly affect predicted metal-water reaction rates. The other is that lack of separation of the channel and bypass volumes will cause an artificially large flow area through the core plate, which can lead to inaccurately determined core plate failure times and unrealistic pressure transients in the lower plenum. In an effort to mitigate the consequences of the current COR package treatment of BWR channel and bypass volumes, an atypical way of modeling the core region by doubling the number of rings so that each ring represents only a channel or bypass region has been developed and has been shown to be beneficial.