Abstract :
Energeticmelt-waterexplosionsareawell-establishedcontributortoriskfornuclearreactors,andevenmoresoforthemetalcastingindustry.In-depthstudieswereundertakeninanindustry-nationallaboratorycollaborativeefforttounderstandtherootcausesofexplosiontriggeringandtoevaluatemethodsforprevention.Thesteamexplosiontriggeringstudies(SETS)facilitywasdevisedandimplementedforderivingkeyinsightsintoexplosionprevention.Dataobtainedindicatedthatonsetofbasesurface-entrapmentinducedexplosiveboiling-causedtriggershocksisaresultofcomplexcombinationofsurfacewettability,typeofcoating(organicversusinorganic),degreeofcoatingwearoff,existenceofbypasspathwaysforpressurerelief,charringandnon-condensablegas(NCG)releasepotential.OftheseparametersNCGswerefoundtoplayapreeminentroleonexplosionpreventionbystabilizingthemelt-watersteaminterfaceandactingasashockabsorber.TheroleofNCGswasexperimentallyconfirmedusingSETSfortheireffectonstablefilmboilingusingadownwardfacingheatedbodythroughwhichgaseswereinjected.ThepresenceofNCGsinthesteamfilmlayercausedasignificantdelayinthetransitioningoffilm-to-nucleateboiling.TheroleofNCGsonexplosionpreventionwasthereafterdemonstratedmoredirectlybyintroducingmoltenmetaldropsintowaterpoolswithandwithoutNCGbubbling.WhereasspontaneousandenergeticexplosionstookplacewithoutNCGinjection,onlybenignquenchingoccurredinthepresenceofNCGs.GravimetricanalysesoforganiccoatingswhichareknowntopreventexplosiononsetwerealsofoundtoreleasesignificantNCGsduringthermalattackbymeltinthepresenceofwater.Thesefindingsofferanovel,simple,cost-effectivetechniqueforderivingfundamentalinsightsintomelt-waterexplosionsaswellasforexplosionpreventionundermostconditionsofinteresttometalcasting,andpossiblyfornuclearreactorsystemsduringsevereaccidentconditions.Energeticsofentrapmentboilinginducedshocksforexplosiveandnon-explosiveconditionswerequantifiedusingamodifiedzero-crossingtechnique.InhonorofProfessorR.T.LaheyJr.thenon-dimensionalparameter“LT”wasproposedtodelineatetheexplosion-onsetboundary.ExperimentalevidencesuggeststhatasystemwithLTaboveathresholdvalueof∼65leadstothegrowthofperturbationsandonsetofpropagatingmelt-waterexplosions.Thedataappeartooffervaluableinsightsintoexplosionpreventioninnuclearreactorsduringbeyond-designbasisaccidentconditions.Anunresolvedissueconcernsthepotentialfortriggershocksfromchemicalignitionreactionsbetweenreactivemetalsincontactwithoxidecoatingssuchasrust.
