An uncertainty analysis of the hydrogen source term for a station blackout accident in Sequoyah using MELCOR 1.8.5

Abstract

A methodology for using the MELCOR code with the Latin Hypercube Sampling method was developed to estimate uncertainty in various predicted quantities such as hydrogen generation or release of fission products under severe accident conditions. In this case, the emphasis was on estimating the range of hydrogen sources in station blackout conditions in the Sequoyah Ice Condenser plant, taking into account uncertainties in the modeled physics known to affect hydrogen generation. The method uses user-specified likelihood distributions for uncertain model parameters, which may include uncertainties of a stochastic nature, to produce a collection of code calculations, or realizations, characterizing the range of possible outcomes. Forty MELCOR code realizations of Sequoyah were conducted that included 10 uncertain parameters, producing a range of in-vessel hydrogen quantities. The range of total hydrogen produced was approximately 583kg  131kg. Sensitivity analyses revealed expected trends with respected to the parameters of greatest importance, however, considerable scatter in results when plotted against any of the uncertain parameters was observed, with no parameter manifesting dominant effects on hydrogen generation. It is concluded that, with respect to the physics parameters investigated, in order to further reduce predicted hydrogen uncertainty, it would be necessary to reduce all physics parameter uncertainties similarly, bearing in mind that some parameters are inherently uncertain within a range. It is suspected that some residual uncertainty associated with modeling complex, coupled and synergistic phenomena, is an inherent aspect of complex systems and cannot be reduced to point value estimates. The probabilistic analyses such as the one demonstrated in this work are important to properly characterize response of complex systems such as severe accident progression in nuclear power plants.

Total loss of AC power analysis for EPR reactor

“…(Humphries et al, 2010;SNL, 2014;Gauntt et al, 2001). Hence, principal recommendation is to perform sensitivity analysis to get wider perspective.…”

“…Code developers from Sandia recommends (it is not demanded) to develop CVH core model with one CVs per two core axial levels (SNL, 2014). In general MELCOR in-vessel hydrogen production is very sensitive to huge set of uncertain parameters and for same reactor it can give wide span of the results (Gauntt et al, 2001). MAAP and MELCOR have different oxidation and core degradation models and it is also potential source of differences.…”

A Conceptual Approach to Eliminate Bypass Release of Fission Products by In-Containment Relief Valve under SGTR Accident

“…MELCOR can simulate various SA phenomena such as thermal-hydraulic response of the primary RCS, core uncovering, fuel heat-up, cladding oxidation, heat-up of the reactor pressure vessel (RPV) lower head by the molten core, lower plenum penetration, hydrogen production, and fission product release. Because of this usefulness in analyzing SA, MELCOR is usually used for uncertainty analysis and sensitivity studies [13]. In addition, many countries have used MELCOR for the regulation of LWR [14].…”

Effect of Molten Corium Behavior Uncertainty on the Severe Accident Progress

“…Table 1 shows the selected uncertain parameters and distributions. The lower/upper bound of zircaloy melt breakout temperature (SC1131(2)) was defined by melting point of zircaloy (2100 K) and the value of hydrogen uncertainty study [12] based on Phebus experiments [13]. The distribution was updated from the distribution of Peach bottom plants with the S/Q simulation results [14].…”