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Severe accident progression in the BWR lower plenum and the modes of vessel failure
KTH, School of Engineering Sciences (SCI), Physics, Nuclear Power Safety.
KTH, School of Engineering Sciences (SCI), Physics, Nuclear Power Safety.
2015 (English)In: International Topical Meeting on Nuclear Reactor Thermal Hydraulics 2015, NURETH 2015, 2015, 8035-8045 p.Conference paper (Refereed)Text
Abstract [en]

Most of our knowledge base on the severe accident progression in the lower plenum of LWRs is based on the data obtained from the TMI-2 accident. It should be recognized that the lower plenum of a BWR is very different from that of a PWR. Unlike the PWR, the BWR plenum is full of control rod guide tubes (CRGTs) with their axial structural variations. These CRGTs are arranged in a cellular fashion with each CRGT supporting 4 rod bundles. There are also a large number of instrument guide tubes (IGTs), each generally placed in the middle of 4CRGTs. Both the CRGTs and IGTs traverse the thick vessel bottom wall and are welded to their extensions which come to bottom of the core. The core-melt progression in the lower plenum is controlled by the structures present and they, in turn, influence the timings and the modes of vessel failure for a BWR. The Uranium oxide-Zirconium oxide core melt formed in the 4 fuel bundles is directed by the structure below towards the water regions in-between the 4 CRGTs. The FCI will take place in those water regions and some particulate debris will be created, although there is insufficient water for quenching the melt. A FCI may occur inside a CRGT if and when the melt breaches the wall of the CRGT. The important issue is whether the welding holding the IGT inside the vessel will fail and the bottom part of the IGT falls out creating a hole in the vessel with release of melt/particulate debris to the containment. Similarly the failure of CRGT could have melt/particulate debris coming out of the vessel. These modes of vessel failure appear to be credible and they could occur before any large-scale melting and melt pool convection takes place. These modes of vessel failure and the melt release to the containment will have very different consequences than those generated by the other modes of vessel failure. Such BWR plenum melt progression scenarios have been considered in this paper. Some results of analyses performed at KTH have been described. We believe that the issues raised are important enough to consider a set of experiments for verification and validation of the melt progression in a BWR plenum. Such experiments are proposed.

Place, publisher, year, edition, pages
2015. 8035-8045 p.
Keyword [en]
Severe accident, Accidents, Boiling water reactors, Debris, Hydraulics, Hydrophilicity, Knowledge based systems, Nuclear reactor accidents, Nuclear reactors, Pressurized water reactors, Welding, Welding rods, Control rod guide tubes, Knowledge base, Structural variations, Uranium oxides, Verification-and-validation, Vessel failure, Water regions, Failure analysis
National Category
Mechanical Engineering
URN: urn:nbn:se:kth:diva-187544ScopusID: 2-s2.0-84964000379ISBN: 9781510811843OAI: diva2:936990
16th International Topical Meeting on Nuclear Reactor Thermal Hydraulics, NURETH 2015, 30 August 2015 through 4 September 2015

QC 20160614

Available from: 2016-06-14 Created: 2016-05-25 Last updated: 2016-06-14Bibliographically approved

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Sehgal, Balraj R.Bechta, Sevostian
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