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

SUB-CHAPTER : 16.2PRE-CONSTRUCTION SAFETY REPORTCHAPTER 16: RISK REDUCTION AND SEVEREACCIDENT ANALYSESPAGE : 14 / 295Document ID.No.UKEPR-0002-162 Issue 041.3.1.1.2.3. Influence of Particulate Debris and Steam GenerationParticulate Debris and in-vessel steam generation are also part of the understanding relatedto steam explosions. In particular, significant work has been performed with respect to thebreak-up of molten jets as they pour through water. Moreover, the FARO experiments [Ref]provide a substantial scale, real material demonstration of the steaming rate during thisprocess. From the information accumulated to date, including the TMI-2 Vessel InspectionProject (VIP) [Ref], some particulates may occur, but the assessment of debris within thereactor vessel lower head must also consider that there is a substantial material layer whichdoes not particulate. Particulate debris causes a net steam generation to the reactor coolantsystem with some potential for additional pressurisation as was observed in the TMI-2accident. These integral system details are part of the lower plenum modelling in the MAAP4code described in Appendix 16A. With the extensive information available on particulatedebris and the net steaming rate from molten material draining into the lower plenum, themajor issue is how much material does not particulate since this results in a potential threatto the RPV wall integrity.1.3.1.1.3. US-NRC StudiesIn 1985, the NRC formed the Steam Explosion Review Group (SERG) [Ref] and tasked thisgroup to assess the likelihood of α-mode failure. The consensus was that explosiveinteractions sufficient to rupture the primary system and therefore the containment were veryunlikely. During the 1993 CSNI-FCI Special Meeting in Santa Barbara [Ref], similar questionswere asked, and again, the consensus was that the work performed since the NRC 1985meeting supported the conclusions made by the SERG. In many instances, additional workhad further refined key arguments related to the inability to establish the necessary initialconditions. In June of 1995, the NRC sponsored a workshop to update the understandingwith respect to steam explosions. With the additional experiments provided in the FARO [Ref]and ALPHA facilities, as well as the additional analyses performed in 1999 [Ref] it wouldappear that there is a developing consensus on the α-mode failure issue.In the thorough review of the probability of α-mode failure, the group of experts (SERG)performed independent analysis and examined available experimental data. The spectrum ofopinions indicated that the probability of α-mode failure is considered to be much less likelythan was estimated in WASH-1400 [Ref]. The SERG divided the fundamental processes ofsteam explosion into three general areas:1) Initial conditions: this involves the geometrical configuration of the reactor vessel atthe time of fuel–coolant contact and the amount of fuel and coolant available for theinteraction.2) Mixing and conversion ratio: this involves the basic physics of the vapour explosionsuch as the fuel–coolant mixing, triggering, propagation, and the resultantconversion of fuel thermal energy to the slug kinetic energy.3) Slug–missile dynamics: this involves the expansion characteristics of the slug withinthe specific reactor geometry, and the coupling to solid missile generation andcontainment penetration.The consensus reached by the SERG was that the occurrence of an in-vessel steamexplosion of sufficient energy, which could lead to containment failure, was sufficiently low inprobability to allow its elimination as a credible threat. The recent 1995 second SERGWorkshop [Ref] concluded that the α-mode issue was resolved from a risk perspective.