16.2 - Severe Accident Analysis (RRC-B) - EDF Hinkley Point

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SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 15 / 295Document ID.No.UKEPR-0002-162 Issue 041.3.1.1.4. Recent European StudiesThe second steam explosion review group (SERG2) established in 1995 [Ref] deliberated onthe phenomenology of steam explosions and provided expert estimates on the probability ofcontainment failure because of an in-vessel steam explosion. Although there were somedifferences of opinion, the vast majority of the experts concluded that the conditionalprobability (i.e. if there is a core melt) is less than 0.001 [Ref], i.e. the containment failure isvery unlikely. Recent European tests in the BERDA program at FZK [Ref], also have shownthat, for a scaled upper vessel head subjected to impact loads simulating those from a verystrong steam explosion, the head and the bolts survived the impact energy of a corium slugabove 0.8 GJ.Much experimental and analysis-development work is in progress presently on in-vesselsteam explosions. Experiments have been performed with several kilogram quantities ofheated particles and molten materials. Elaborate three-field analysis codes: MC3D [Ref], IVA[Ref], ESPROSE.m [Ref] and PM-ALPHA [Ref] have been developed. Some of the insightsgained are:1) steam explosion probability is much reduced due to the extensive water-depletionthat occurs around the fragmented particles of a jet in the pre-mixture;2) super-critical steam explosions, however, cannot be excluded.The most remarkable observations derived from the experimental program employingprototypic corium melt (UO 2 –ZrO 2 ) in the FARO and (UO 2 –ZrO 2 ) and A1203 in the KROTOS[Ref] facilities at Ispra, Italy are:• UO 2 –ZrO 2 melt jets dropped in sub-cooled and saturated water at low pressuredo not generate spontaneous steam explosions;• Strongly-triggered UO 2 –ZrO 2 melt jets in sub-cooled and saturated water at lowpressure may develop a propagating event, although of very low efficiency(≤ 0.15%);• Al 2 O 3 melt jets (serving as a stimulant for the corium fuel) generally experiencespontaneous strong steam explosions when dropped in low pressure sub-cooledwater;• Al 2 O 3 melt jets dropped in saturated water at low pressure, in general, have to betriggered to experience strong steam explosions.These significant observations point to the important role that the melt physical propertiesmay be playing in the steam explosion process. Much research on this aspect is beingpursued in Europe under the auspices of the European Commission. Some physicalmechanisms have been identified. Nevertheless, it appears that the prototypic coriummixtures may not be as explosive (very low efficiency and/or explosiveness) as previouslyassumed.
SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 16 / 295Document ID.No.UKEPR-0002-162 Issue 041.3.1.1.5. Resolution StatusThe conclusion from the NRC studies by SERG1 and SERG2 is that the α-mode failure dueto explosive corium-water interactions is not sufficient to rupture the primary system or thecontainment vessel and therefore this failure mode is very unlikely. This conclusion isconsistent with the results from several experimental studies performed in Europe andJapan.Therefore, this issue of in-vessel steam explosion is considered as definitely resolved,especially in new reactor designs such as the EPR, and hence additional measures withrespect to Accident Management Evaluations are not justified.
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16.2 - Severe Accident Analysis (RRC-B) - EDF Hinkley Point

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