FROM A LOOP TO A FUKUSHIMA-LIKE SBO: ASSESSMENT OF THE EFFECT OF BWR3-MARKI SAFETY SYSTEMS

摘要

The Fukushima accident on March 11th 2011, largely affected nuclear community all over the world. Immediately, many R&D organizations and companies drew their attention to the facts that were being released by Japanese authorities, so that the most thorough understanding of the situation could be gained. CIEMAT joined this worldwide initiative by launching a specific activity on analyzing BWR3-MarkI behavior under "Fukushima-like" scenarios with the MELCOR 1.8.6 code. The focus has been given to the impact that availability of different safety systems would have had in the accident progression. Six scenarios have been analyzed based on an SBO sequence. The analyses performed do not allow setting exhaustive comparisons to the available data due to the specific differences between the plant modeled and the Unit-1 Fukushima Daiichi NPP (i.e., set-points, specific volumes ...). Nevertheless, all in all, predictions seem qualitatively consistent with the observations and interpretations of the accident, as the early core degradation or the substantial amount of H_2 production among others. According to RPV pressure, the most likely scenarios in Fukushima would have been those involving a short or no operation of IC and the SRV performance as designed. These two cases show also containment pressure peaks around 11 - 12 h, as recorded, but all the scenarios failed to keep a nearly steady containment pressure around 0.7 -0.8 MPa during the next 12 h, since in all of them containment loss of leak-tightness and/or venting have been predicted. Two other facts emerging from this analysis are the outstanding difference that even a reduced power availability might have had and, no less important, the key role played by the suppression pool as a source term mitigating feature.