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The fatal event occurred in Fukushima Daiichi NPP, focused the e orts of many R&D organiza- tions towards the comprehensive study of the accident sequence, in order to better understand the phenomenology which took place throughout the whole accident. The development of powerful analytical tools and advanced plant models, underwent a surge since the event occurred in Three Mile Island NPP. From Fukushima Daiichi accident on, plant modeling has been notably enhanced, and furthermore it has pointed out the necessity of having tools capable to provide emergency preparedness and emergency response against this kind of events. This necessity has given rise to the creation of the European FASTNET project, within the Horizon 2020. Nowadays, advanced plant modeling combined with powerful analytical tools, have allowed to obtain more accurate results of severe accident at nuclear power reactors than at any time in the past. FASTNET gathers all these previous e orts, in order to develop tools capable to obtain fast response concerning source term evolution which eventually reaches the environment. The foundation of FASTNET is a large database of accident scenarios. This project is enclosed in the creation of the mentioned database for a Station Blackout scenario in a GE BWR 3 Mark 1 reactor design, by means of the analytical code MELCOR 2.1. Given the fact that source term evolution cannot be conceived without thermal-hydraulic evolution, present work focuses on both elds of study, and attempts to provide a comprehensive analysis of the accident for a reactor technology quite similar as the one of the Unit 1 in Fukushima Daiichi.
This report documents the results from MELCOR calculations of the Long-Term Station Blackout Accident Sequence, with failure to depressurize the reactor vessel, at the Peach Bottom (BWR Mark I) plant, and presents comparisons with Source Term Code Package calculations of the same sequence. STCP has calculated the transient out to 13.5, hours after core uncovery. Most of the MELCOR calculations presented have been carried out to between 15 and 16.7 hours after core uncovery. The results include the release of source terms to the environment. The results of several sensitivity calculations with MELCOR are also presented, which explore the impact of varying user-input modeling and timestep control parameters on the accident progression and release of source terms to the environment. Most of the calculations documented here were performed in FY1990 using MELCOR Version 1.8BC. However, the appendices also document the results of more recent calculations performed in FY1991 using MELCOR versions 1.8CZ and 1.8DNX.
This paper presents the results from MELCOR (Version 1.8BC) calculations of the Long-Term Station Blackout Accident Sequence, with failure to depressurize the reactor vessel, at the Peach Bottom (BWR Mark I) plant, and presents comparisons with Source Term Code Package (STCP) calculations of the same sequence. This sequence assumes that batteries are available for six hours following loss of all power to the plant. Following battery failure, the reactor coolant system (RCS) inventory is boiled off through the relief valves by continued decay heat generation. This leads to core uncovery, heatup, clad oxidation, core degradation, relocation, and, eventually, vessel failure at high pressure. STCP has calculated the transient out to 13.5 hours after core uncovery. The results include the timing of key events, pressure and temperature response in the reactor vessel and containment, hydrogen production, and the release of source terms to the environment. 12 refs., 23 figs., 3 tabs.
Long-term station blackout analyses in Peach Bottom were first carried out using MELCOR 1.8BC, and later with 1.8DN, as part of an overall program between the US Nuclear Regulatory Commission (NRC) and Brookhaven National Laboratory (BNL), to provide independent assessment of MELCOR as a severe accident/source term analysis tool. In addition to the reference MELCOR calculation, several sensitivity calculations were also performed to explore the impact of varying user-input modeling and timestep control parameters on the accident progression and radionuclide releases to the environment calculated by MELCOR. An area of concern that emerged from these studies was the impact of the selection of maximum allowable timestep (?t{sub max}) on the calculational behavior of MELCOR, where the results showed significant differences in timing of key events, and a lack of convergence of the solution with reduction of?t{sub max}. These findings were reported to the NRC, SNL, and the MELCOR Peer Review Committee. As a consequence, a significant effort was undertaken to eliminate or mitigate these sensitivities. The latest released version of MELCOR, Version 1.8.2, released in April 1993, contains several new or improved models, and has corrections to mitigate numerical sensitivities. This paper presents the results of updating the earlier sensitivity studies on maximum timestep, to more properly represent the abilities of the improved MELCOR version 1.8.2. Results are presenter in terms of timing of key events, thermal-hydraulic response of the system, and environmental release of radionuclides. The impact of some of the newer models, such as falling debris quench model, and ORNLs̀ new BH model, is also evaluated.