Quicklook overview of model changes in Melcor 2.2: Rev 6342 to Rev 9496

Analysis of the Effect of Severe Accident Scenario on Debris Properties in Lower Plenum of Nordic BWR Using Different Versions of MELCOR Code

“…The results obtained with MELCOR code versions 1.86 and 2.1 look very consistent with each other, and the differences in predictions can be explained by the differences in fuel failure modeling between MELCOR 1.86 and 2.1. MELCOR 1.86 uses the temperature threshold (TRDFAI, oxidized fuel rod collapse temperature, defined as sensitivity coefficient SC1132-1 = 2500K, [18]), while MELCOR 2.1 uses "timeat-temperature" model, where the fuel assembly lifetime at 2500K equals 1h and at 2700K equals 30sec [14,25]. The effect of this difference can be also observed in the mass averaged temperature of the debris in core (all axial levels above the core support plate) shown in Figure 9, which is significantly higher in MELCOR 2.1 compared to MELCOR 1.86, especially in the domain of scenarios with late depressurization.…”

“…The remaining discrepancy in the results can be attributed to the changes in the MELCOR models. Based on the literature review [14,25], the possible explanations for the discrepancy in the results between MELCOR code version 1.86/2.1 and MELCOR 2.2 are the changes in the process of early core degradation, in particular canister material candling as was discussed in Section 2.2. Furthermore, in MELCOR 2.2 (all revisions after 7874, see Section 2.2 and [25] for details), the Lipinski dryout model is not used above the core support plate, which may result in higher convective heat removal rate from the core in MELCOR 2.2 compared to MELCOR 1.86/2.1.…”

“…In MELCOR code, new modeling approaches and parameter selections that support the best-estimate analyses were frequently introduced over the last few years [14,25]. In Table 1 we outline the main changes in selection of the parameters that were introduced in MELCOR code throughout rev2911 (MELCOR 1.86) up to rev9541 (MELCOR 2.2), which can be relevant to the in-vessel phase of accident progression analysis presented in this paper.…”

“…Furthermore, we list several new models and improvements that can affect in-vessel accident progression, which were introduced with different revisions of MELCOR code [1,2,25]: (i) Fuel rod collapse model (oxidized fuel rod collapse temperature) in MELCOR 1.86. It is the temperature at which intact fuel rods are assumed to transition a limiting maximum heat transfer rate from a particulate debris bed.…”

“…It is the temperature at which intact fuel rods are assumed to transition a limiting maximum heat transfer rate from a particulate debris bed. This model was also applied above the core support plate with upward flow of coolant and would lead to problems when the occurrence of particulate debris along with intact components would stop convective heat removal from the intact components in a COR cell [25].…”