In-vessel Coolability and Steam Explosion in Nordic BWRs

Transcription

1 Nordsk kerneskkerhedsforsknng Norrænar kjarnöryggsrannsóknr Pohjosmanen ydnturvallsuustutkmus Nordsk kjerneskkerhetsforsknng Nordsk kärnsäkerhetsforsknng Nordc nuclear safety research NKS-219 ISBN In-vessel Coolablty and Steam Exploson n Nordc BWRs Wemn Ma, Roberta Hansson, Langxng L, Pavel Kudnov, Francesco Cadnu, Ch-Thanh Tran Royal Insttute of Technology (KTH), Sweden March 2010

2 Abstract The INCOSE project s to reduce the uncertanty n quantfcaton of steam exploson rsk and n-vessel coolablty n Nordc BWR plants wth the cavty floodng as a severe accdent management (SAM) measure. Durng 2009 substantal advances and new nsghts nto physcal mechansms were ganed for studes of: () n-vessel corum coolablty development of the methodologes to assess the effcency of the control rod gude tube (CRGT) coolng as a potental SAM measure; () debrs bed coolablty characterzaton of the effectve partcle dameter of mult-sze partcles and qualfcaton of frcton law for two-phase flow n the beds packed wth mult-sze partcles; and () steam exploson nvestgaton of the effect of bnary oxdes mxture s propertes on steam exploson. An approach for couplng of ECM/PECM models wth RELAP5 was developed to enhance predctve fdelty for melt pool heat transfer. MELCOR was employed to examne the CRGT coolng effcency by consderng an entre accdent scenaro, and the smulaton results show that the nomnal flowrate (~10kg/s) of CRGT coolng s suffcent to mantan the ntegrty of the vessel n a BWR of 3900 MWth, f the water njecton s actvated no later than 1 hour after scram. The POMECO-FL expermental data suggest that for a partculate bed packed wth mult-sze partcles, the effectve partcle dameter can be represented by the area mean dameter of the partcles, whle at hgh velocty (Re>7) the effectve partcle dameter s closer to the length mean dameter. The pressure drop of two-phase flow through the partculate bed can be predcted by Reed s model. The steam exploson experments performed at hgh melt superheat (>200oC) usng oxdc mxture of WO3-CaO ddn t detect an apparent dfference n steam exploson energetcs and precondtonng between the eutectc and noneutectc melts. Ths ponts out that the next step of MISTEE experment wll be conducted at lower superheat. Key words severe accdent, debrs coolablty, steam exploson NKS-219 ISBN Electronc report, March 2010 NKS Secretarat NKS-776 P.O. Box 49 DK Rosklde, Denmark Phone Fax e-mal nks@nks.org

3 Contract AFT/NKS-R(09)75/3 Mdterm Report In-vessel Coolablty and Steam Exploson n Nordc BWRs Wemn Ma, Roberta Hansson, Langxng L, Pavel Kudnov, Francesco Cadnu, Ch-Thanh Tran Dvson of Nuclear Power Safety Department of Physcs Royal Insttute of Technology (KTH) Aprl 2010

4 Executve Summary The INCOSE project s to reduce the uncertanty n quantfcaton of steam exploson rsk and n-vessel coolablty n Nordc BWR plants wth the cavty floodng as a severe accdent management (SAM) measure. Durng 2009 substantal advances and new nsghts nto physcal mechansms were ganed for studes of: () n-vessel corum coolablty development of the methodologes to assess the effcency of the control rod gude tube (CRGT) coolng as a potental SAM measure; () debrs bed coolablty characterzaton of the effectve partcle dameter of mult-sze partcles and qualfcaton of frcton law for two-phase flow n the beds packed wth multsze partcles; and () steam exploson nvestgaton of the effect of bnary oxdes mxture s propertes on steam exploson. An approach for couplng of ECM/PECM models wth RELAP5 was developed to enhance predctve fdelty for melt pool heat transfer. MELCOR was employed to examne the CRGT coolng effcency by consderng an entre accdent scenaro, and the smulaton results show that the nomnal flowrate (~10kg/s) of CRGT coolng s suffcent to mantan the ntegrty of the vessel n a BWR of 3900 MWth, f the water njecton s actvated no later than 1 hour after scram. The POMECO-FL expermental data suggest that for a partculate bed packed wth mult-sze partcles, the effectve partcle dameter can be represented by the area mean dameter of the partcles, whle at hgh velocty (Re>7) the effectve partcle dameter s closer to the length mean dameter. The pressure drop of two-phase flow through the partculate bed can be predcted by Reed s model. The steam exploson experments performed at hgh melt superheat (>200 o C) usng oxdc mxture of WO 3 -CaO ddn t detect an apparent dfference n steam exploson energetcs and precondtonng between the eutectc and non-eutectc melts. Ths ponts out that the next step of MISTEE experment wll be conducted at lower superheat. 2

5 Contents Executve Summary Introducton Results and Analyss In-vessel coolablty The effectveness of CRGT coolng Coolablty of n-vessel debrs bed Steam exploson energetcs Bubble and melt dynamcs Steam exploson energetcs Metallc (tn) versus Oxdc (WO 3 -CaO) materals Concluded Remarks References

6 1. Introducton The goal of the severe accdent research at Kunglga Teknska Högskolan (KTH) s to create knowledge base for resoluton of two long-standng severe accdent ssues, namely steam exploson and corum coolablty n severe accdent scenaros of Nordc BWRs whch adopt cavty floodng as the cornerstone of Severe Accdent Management (SAM) measures. For ths objectve, the resecrh project INCOSE (In-vessel Coolablty and Steam Exploson n Nordc BWRs) s focused on two areas of hghest return: () n-vessel coolablty wth svere accdent management (SAM) acton (e.g., CRGT coolng) snce t has more chances to contan the corum n the reator pressure vessel (RPV), and () steam exploson energetcs n case of vessel falure, whch may threaten the contanment tegrty. Ths report summarzes the progress and achevements of the project durng Year The resesrch actvty has a synergc collaboraton wth Swedsh APRI-7 research program, EU SARNET2 Excellence Network for severe accdent research and OECD SERENA-II project dedcated to understanng of steam exploson n reactor safety. 2. Results and Analyss 2.1. In-vessel coolablty The n-vessel coolablty study at KTH s focused on a) quantfcaton of the effectveness of control rod gude tubes (CRGT) coolng as a potental severe accdent management (SAM) measure n BWRs; b) reducton of the uncertanty n coolablty analyss of a debrs bed formed n the RPV durng a postulated severe accdent The effectveness of CRGT coolng To perform a mechanstc mult-dmensonal analyss of heat transfer n the corum-flled lower head, Effectve Convectve Model (ECM) and Phase-change Effectve Convectve Model (PECM) were developed at KTH [1-5], whch ncorporate the advantages of the modern CFD method and the avalable correlaton-based method, beng able to smulate melt pool behavor n a complex geometry as the BWR's lower plenum wth a forest of CRGTs. The ECM and PECM were appled to smulaton and analyss of melt pool heat transfer n the lower plenum durng a severe reactor accdent of a BWR of 2500 MWth. The key fndngs are as follows [5]: In case of formaton of melt pool wth a thckness (heght, or depth) less than 0.7 m n the BWR lower plenum, the CRGT coolng at nomnal water flow rate,.e g/sec per CRGT, s suffcent to remove the decay heat generated n the melt pool, and protect the vessel wall from thermal attack. In the case of a melt pool wth depth hgher than 0.7 m, the CRGT coolng s nsuffcent, and the vessel wall s predcted to fal n the secton connected to the uppermost regon of the melt pool; Addtonal coolng measure (e.g. external coolng, and/or ncrease of CRGT water flow rate) s needed for protecton of the vessel from falure. The ECM/PECM smulaton results and key fndngs suggest that CRGT coolng possesses a hgh potental as an effectve and relable

7 mechansm to remove the decay heat from a melt pool formed n the BWR lower plenum. Thus, the CRGT coolng presents a credble canddate for mplementaton as a SAM measure n BWRs. Durng 2009, further work on extenson and enhancement of the ECM/PECM predctve capabltes was performed [6-7]. ECM/PECM models for smulatons of molten metal layer heat transfer are developed n [6] for consderaton of scenaro wth stratfed melt pool heat transfer wth CRGT coolng. The PECM smulaton results revealed no focusng effect n the metal layer on top of a debrs pool formed n a BWR lower plenum [6] n presence of CRGT coolng. On the other hand, very hgh heat fluxes were obtaned on the surface of the CRGTs durng rapd soldfcaton of the metal layer. The reason for that was the fxed temperature boundary condton on the nner sde of the CRGTs whch ddn t take nto account properly feedback between nner and outer heat transfers n ths partcular case. In order to capture ths feedback, a method has been developed [7] for coupled smulatons of the melt pool heat transfer (wth PECM model) and the flow and heat transfer nsde the CRGTs (wth the system thermal-hydraulc code RELAP5). The PECM model of the metal layer heat transfer, mplemented on the base of the CFD code Fluent, calculates the transent heat fluxes on the CRGTs walls to be used, as a boundary condton, by RELAP5. Conversely, RELAP5 provdes the temperature of the CRGTs external wall as a boundary condton for the smulaton of the core melt heat transfer. The couplng between the two codes, whch run concurrently, s performed by a scrptng nterface that organzes the exchange of nformaton at each tme step of the PECM smulaton (Fg. 1). Fg. 1: Implementaton of the ECM/RELAP5 couplng scheme. The coupled PECM/RELAP5 smulatons results suggest that even n the case of remote, hypothetcally boundng scenaro of ntally hot (2000K) heterogeneous melt pool confguraton, wth lqud metal layer floatng atop of a debrs bed, just a fourfold ncrease of the standard mass flow rate of the CRGT flow appears to be suffcent to prevent CRGT creep. More detaled results can be found n [7]. Whle the ECM/PECM method provdes an effcent and computatonally affordable tool for mechanstc mult-dmensonal analyss of melt pool heat transfer n the lower head, the effects of system dynamcs and accdent progresson on melt pool heat transfer were not drectly consdered n the analyss. The nfluence of coolant njecton on other phenomena 5

8 was also neglected. Ths s because ECM/PECM smulatons were only focused on the melt pool n the lower head, wthout any lnk and nteractons wth other systems and phenomena. For nstance, the core degradaton and relocaton are mportant for resultng melt pool n the lower plenum. But ths cannot be captured n the ECM/PECM method; nstead the boundary condtons (e.g., melt mass, compostons and ntal temperatures) have to been assumed n the analyss. To lft ths lmtaton, the whole accdent progresson (from core degradaton, to melt relocaton, and to melt pool formaton n the lower head) must be smulated. Such a systematc analyss s realzed here by usng MELCOR code for the severe accdent smulaton [8]. Based on the MELCOR smulaton results, the followng ponts can be concluded. The nomnal flowrate (~10kg/s) of CRGT coolng s suffcent for n-vessel coolablty and to mantan the ntegrty of the vessel n a BWR of 3900 MWth, f the water njecton s actvated no later than 1 hour after scram. For late recovery of CRGT coolng (later than 1 hour after scram), a hgher flowrate than the nomnal s needed to contan the melt n the vessel. In case the water njecton to the CRGTs s actvated n 2 hours, for nstance, much hgher flowrate (~40kg/s) s requred for n-vessel coolablty and retenton. Although we are aware of the lmtaton of MELCOR modelng for melt pool heat transfer n the lower plenum, the frst-cut analyss n the present study hghlghts the mportance of tmng and flowrate of the CRGT coolng when chosen as a SAM measure for n-vessel corum coolablty and retenton. For early actuaton of the CRGT coolng, there s lttle corum relocated to the lower plenum for melt pool formaton and the predcton by MELCOR code at system level s therefore more credble. For late recovery of the CRGT coolng, a mechanstc mult-dmensonal analyss of heat transfer n the corum-flled lower plenum s necessary for confrmaton and quantfcaton. Ths s why we at KTH developed the Effectve Convectve Model (ECM) for smulaton of corum behavor n a complex geometry lke the BWR's lower plenum wth a forest of CRGTs. The next step s to complete methodology formulaton for nformaton transfer from MELCOR output to the ECM method, and to perform ECM analyss for the late njecton scenaros. The dual approach leverages on the strength of the two methods (MELCOR/ECM), and therefore ncreases the relablty of the assessment Coolablty of n-vessel debrs bed The research on ths topc s concerned wth reducng the uncertanty n coolablty analyss of a debrs bed whch may be formed from fuel coolant nteractons (FCIs) n the reactor pressure vessel (RPV) durng varous stages (cf. Fg. 2a & 2b) of a severe accdent scenaro. Due to ts nternal pores whch facltate coolant ngress, the debrs bed provdes more chances to remove the decay heat than a molten corum pool where coolant access s very lmted (only to surface). Thus, debrs bed coolablty plays an mportant role n the termnaton and stablzaton of a severe accdent. Towards the quanttatve understandng of debrs bed coolablty, many experments [9-14] have been conducted to nvestgate two-phase flow and heat transfer n partcle beds. We at KTH also addressed the nfluences of the bed s characterstcs (prototypcaltes) on coolablty [15]. To analyze the experments and fnally assess debrs coolablty n reactor applcaton, a great number of analytcal models and emprcal correlatons were developed 6

9 for predcton of two-phase flow (frcton) and heat transfer (dryout heat flux) n packed beds. The central pont n modelng was to provde the formulaton of the frcton laws for momentum equatons of sngle and two-phase flow n porous meda. (a) n the core (b) n the lower plenum (c) n the reactor cavty Fg. 2: Debrs bed formaton durng dfferent stages of a severe accdent scenaro. The momentum equaton of sngle-phase flow through porous meda can be expressed by Ergun s equaton [16]: dp μ ρ 2 = J + J (1) dz K η where dp dz s the pressure gradent along the bed, ε s the porosty, μ s the dynamc vscosty of flud, d s the dameter of partcles, ρ s the densty of the flud and J s the superfcal velocty of flud. For unform sphercal partcles bed, the permeablty K and passablty η are taken as ε d ε d K = η = (2) 2 150(1 ε ) 1.75(1 ε ) where, d and ε are partcle dameter and porosty of the debrs bed, respectvely. Ergun s equaton was adapted to two-phase flow n porous meda by the ncluson of relatve permeablty, relatve passablty and nterfacal frcton: dpl dz μl = ρl g + K K r, l ρl J l + η η r, l J J l l F 1 α (3) dpg dz μ g = ρ g g + K K r, g ρ g J l + η η r, g J g J g F + α (4) where J l and J g are the superfcal veloctes of fluds; and K r and η r are relatve permeablty and relatve passablty that dffer from model to model as lsted n Table 1, where the nterfacal frcton F s also defned. 7

10 Table 1. Relatve permeablty and passablty n models for coolablty analyss. Parameter Model K r η r F Lpnsk K r,l = s 3 η r,l =s 3 0 (1981) [17] K r,g =α 3 η r,g =α 3 Reed K r,l = s 3 η r,l = s 5 0 (1982) [18] K r,g =α 3 η r,g =α 5 Hu & Theofanous (1991) [13] K r,l = s 3 K r,g =α 3 η r,l = s 6 η r,g =α 6 0 Schulenberg & K r,l = s 3 η r,l = s 5 Műller (1987) K r,g =α 3 η r,g =α 6 7 ρ J l K g J l F 350 ( ) = s α ρ l ρ g g, α>0.3 ησ α s [19] η r,g =0.1α 4, α From the above equatons, one can see the partcle dameter s an mportant parameter to determne flow frcton (and therefore dryout heat flux) of a porous bed. Nevertheless, the dentfcaton of partcle dameter s not straghtforward when the bed s composed of multdameter partcles, as the case for rector applcaton where the debrs partcles have a wde range of sze dstrbuton. For the mxture of partcles wth a sze dstrbuton, the mean partcle dameter s very dfferent even for the same combnaton of mult-sze spheres, dependng on whch sze dstrbuton functon (mass, area, length, number, etc.) to be chosen [20]. Accordngly, there exst among others the mass mean dameter, area mean dameter, length mean dameter and number mean dameter, defned as follows. 3 4 d x f x = = f m xm x = (5) 3 x f x 3 f 2 3 d x f x f a = xa = ( x ) = (6) 2 x f x 2 f d l = xl = x ) = x f 2 x f x f ( (7) x f f d n = xn = x (8) f where f s the number of partcles wthn the gven sze range (x, x +Δx), and the pararmeters m, a, l and n are sze dstrbuton functons by mass, area, chord length, and number of the partcles, respectvely. If the partcles are non-sphercal, shape factor should be appled n the equatons as well. It can be seen from Eqs. (5)-(8) that the contrbuton of small sze partcles s more and more pronounced to the mean dameter from d m through d n. In other words, compared wth the mass mean dameter, the small sze partcles play a more mportant role n the determnaton of number mean dameter. For nstance, for a packed bed wth spheres of three dameters (1.5mm, 3mm and 6mm) at the mass rato of 1:1:1, the resultng mean dameters are as shown n Table 2. In ths case, mass mean dameter s two tmes of number mean dameter. From the mass mean dameter (3.5mm) to the number 8

11 mean dameter (1.73mm), t s gettng closer to the dameter of the smallest spheres (1.5mm). Table 2. Dfferent mean dameters of a packed bed Spheres Mass rato d m (mm) d a (mm) d l (mm) d n (mm) 1.5mm+3mm+6mm 1:1: When confronted wth so vared mean dameters, a natural queston s that whch one s sutable for coolablty analyss of a debrs bed? So far there has not been a clear answer yet. On the one hand, mass and area mean dameters were wdely appled n coolablty studes. For nstance, Konovalkhn [12] used mass mean partcle dameter n the tests on packed beds wth sand grans, and found that the expermental dryout heat flux agrees wth the predcton of the Lpnsk s model. Smlarly, mass mean partcle dameter was employed by Schmdt [14], whle area mean partcle szes were used by Dhr [21]. On the other hand, a so-called effectve dameter was proposed and used n coolablty studes [10-11]. The effectve partcle dameter s usually derved from expermental data of flud flow n porous meda provded that the above-mentoned frcton laws are stll vald. For DCC-2 rubbles bed [10], the effectve dameter (1.42mm) s much smaller than the mass mean dameter (2.43mm). For the STYX partcle bed [11], t was found the effectve partcle dameter s close, but not equal to the number mean dameter. Zesberger & Maynger [22] performed an nvestgaton on a porous bed flled wth steel balls (4mm) and glass spheres (0.95mm), yeldng pressure gradent closer to the model predcton by usng number mean dameter rather than area mean dameter. In general, the lterature survey shows that number mean dameter s seemngly more sutable for debrs characterzaton than mass and area mean dameters, but the data are stll qute lmted. The present study s to advance the understandng of the effectve partcle dameters of partculate beds packed wth vared sze dstrbutons of partcles. The work conssts of three folds: ) to obtan the effectve dameter of a so-packed bed va snglephase flow experments; ) to compare the effectve dameter wth the mean dameters obtaned by Eqs.(5)-(8); and ) to examne the valdty of the effectve dameter n twophase flow modelng. The tem # s realzed by the measurement of pressure gradents of gas/water sngle-phase flow through the packed beds. The tem # s conducted to see f the effectve dameter can be represented by the number mean dameter; f not, whch one of the mean dameters s close n the frst place. Item # s performed to confrm the applcablty of the effectve partcle dameter to coolablty assessment. The experment s carred out on the POMECO-FL test faclty, as llustrated n Fg. 3, whch s bascally an adabatc water/ar sngle- and two-phase flow loop for porous meda. Major components of the test faclty are made of transparent Plexglas to facltate vsual observaton. The test secton accommodatng the partculate bed s made of a Plexglas ppe wth the nsde dameter of 90mm and the heght of 635mm. Four annular chambers for pressure tappng are desgned to surround the ppe at dfferent levels, wth radal holes (0.5mm n dameter) unformly dstrbuted as openng from the bed to the annular chambers. The chambers do not only provde an average pressure readng over the entre crcumference of each tappng pont, but also prevent gas and partcles from enterng the mpulse lnes of pressure transducers. At both the nlet and the outlet of the test secton, two peces of stanless steel wre mesh are appled between the flanges to support the bed from below and prevent the partcles from leavng the bed. Ar s suppled from the bottom and 9

12 flows up through the packed bed, but water can be suppled from ether the bottom or the top for a bottom-fed (co-current flow) or a top-floodng (counter-current flow) bed. Fg. 3: The schematc dagram of POMECO-FL faclty. Two Rosement-3051 dfferental pressure transmtters wth hgh accuracy are mounted on the test secton to measure entre and half pressures drops, respectvely, of sngle or twophase flow through the bed. Valve manfolds are used wth the dfferental pressure transmtters to perform the block, equalzng and vent requrements of the transmtters. The flowrates of gas and water flows are measured by seven OMEGA flowmeters wth dfferent measurng ranges. The pressure and temperature are montored by usng OMEGA pressure transducers and K-type thermocouples. The flowmeters and pressure transducers were calbrated pror to experment. A Data Acquston System (DAS) s realzed va Natonal Instruments data acquston products and a computer program wrtten n LabVew. The program collects the data from thermocouples, pressure transducers, flowmeters (va manual nput), and employs the ndcators to show the numercal data and ts graphcal representaton such as charts. Snce the models for coolablty analyss are senstve to the bed porosty as well as the partcle sze, yeldng results from coolable to non-coolable stuaton wth a relatvely small change n the two parameters [11], a great care s taken here n determnng the bed porosty. Ths s acheved by accurate measurement of the materal densty (double check and verfcaton of factory data) and the partcle mass (free of mosture) loaded nto the bed. The porosty s then determned by M j ρ j ε = 1 (9) V 0 10

13 where ε s the porosty of the bed, M j s the mass of the partcles made of materal j and V o s the total volume the bed occupes (ncludng vod). The partcles are well mxed pror to fllng n the test secton so that a unform packed bed can be obtaned. In addton to the calbraton of nstrumentaton, the test faclty and ts measurement system were also qualfed by measurements of sngle-phase flow through three beds packed wth sngle-sze glass spheres of dameter 1.5mm, 3mm and 6mm, respectvely. Water and ar were employed as workng flud, respectvely. The measured pressure gradents were then compared wth those predcted by the Ergun s equaton, whose predctons are generally accepted for packed beds of spheres wth satsfactory accuracy. Fg. 4 shows the comparson between the measured data the analytcal results of Ergun s equaton, where trangle symbols represent expermental data, and the sold curves are analytcal results. The qualty of expermentaton and nstrumentaton can be ensured by the good agreement. (a) water flow (b) ar flow Fg. 4: Pressure gradents of flud flow through packed beds wth sngle-sze spheres. 11

14 The mult-sze partcles used n the beds are glass spheres (cf. Fg. 5), wth the dameter rangng from 0.7mm to 10mm. The partculate beds chosen n the present study are as shown n Table 3, n whch d n, d l, d a and d m are mean dameters of partcles based on ther dstrbuton n number, length, surface area and mass (see Eqs.5-8), respectvely. The effectve partcle dameter d e s derved from the expermental data. Reynolds number (Re) s defned as ρ j d Re = sd (10) μ(1 ε ) where j and d sd are the superfcal velocty of flud and Sauter mean dameter of the partcles, respectvely. Bed-1, Bed-2 and Bed-3 are packed wth two-dameter spheres, whle Bed-4 s composed of mult-dameter spheres whose sze dstrbuton s as shown n Fg. 6 and Table 4. Bed Dameters (mm) Table 3. Test beds packed wth mxture of mult-dameter spheres. Mass rato / Fracton Porosty ε d n (mm) d l (mm) d a (mm) d m (mm) : : : See Fg. 6 and Table Re d e (mm) < > < > < > < > Fg. 5: Spheres used n the packed beds. 12

15 Fg. 6: Partcle sze dstrbutons n FCI test wth real corum [23] and n Bed-4. Table 4. Partcle sze dstrbuton of Bed-4. Partcle sze (mm) Accumulatve fracton Fg. 7a shows the pressure gradents of sngle-phase flow n Bed-1 ncrease wth ncreasng Reynolds number, where the trangle marks are the measured values, whle the sold curves are predcted by Ergun s equaton usng the mean partcle dameters mentoned above. Notably, at low Reynolds number (Re<7) the pressure gradent predcted by Ergun equaton usng area mean dameter of the partcles s comparable wth the expermental data. Ths means that the area mean dameter can be employed as the effectve partcle dameter for the bed. However, as the Reynolds number ncreases to a threshold value (Re>7), the length mean dameter s more representatve of the effectve partcle dameter. As shown n Table 1, the same concluson s applcable to Bed-2 and Bed-3 that are all packed wth twodameter spheres as well. For Bed-4 packed wth mult-dameter spheres, the effectve partcle dameter s also vared wth ncreasng Reynolds number, as depcted n Fg. 7b. When the Reynolds number s small (Re<7), the effectve partcle dameter of the spheres s not equal but relatvely close to area mean dameter. For hgh Reynolds number (Re>7), the effectve partcle dameter s almost the same as the length mean dameter. 13

16 (a) Bed-1 (b) Bed-4 Fg. 7: Measured pressure gradents sngle-phase flow n the partcle beds and predcted values by Ergun s equaton usng varous mean partcle dameters of the bed. For understandng of the frcton law of two-phase flow through a partculate bed packed wth mult-sze partcles, an nvestgaton on pressure drop of ar-water two-phase flow through Bed-4 was carred out. Fg. 8 shows the expermental data are comparable wth the predctons of Reed s model, gven the effectve partcle dameter obtaned from above. In summary, the effectve partcle dameters can be represented by the area mean dameters of the partcles n the beds at low flowrate (Re<7), whle at hgh velocty (Re>7) the effectve partcle dameters are closer to the length mean dameters. The measured pressure drops of two-phase flow through the partculate beds have a good agreement wth predctons of Reed s model. 14

17 Fg. 8: Measured pressure gradents of two-phase flow n Bed-4 and predcted values by Ergun s equaton usng varous mean partcle dameters of the bed Steam exploson energetcs The objectve of the steam exploson study at KTH s to pursue new evdences of corum low explosvty by performng MISTEE experments. The new focus durng 2009 s to perform MISTEE experment wth eutectc and non-eutectc sngle oxdc molten droplet. Prevously, sngle drop steam exploson experments were conducted wth a metallc melt (tn) on the MISTEE test faclty [24-26], to pursue a basc understandng of mcronteractons n steam exploson. The anatomy of the exploson was realzed by a dagnostc system named SHARP [24] whch enables synchronzed vsualzaton of both bubble dynamcs and melt evoluton durng the exploson perod, grantng frst-of-a-knd data on mcro-nteractons n droplet exploson. The expermental results show that the vapor flm dynamcs experences three dstnct cycles of bubble expanson and collapse, and melt precondtonng (deformaton/pre-fragmentaton of a molten drop mmedately followng the pressure trgger) s nstrumental to the subsequent coolant entranment and resultng energetcs of the so-trggered steam exploson [25]. To reduce the propertes gap between the corum and ts smulant, the MISTEE test faclty was upgraded to be able to work wth oxdc materals at hgh meltng temperatures. The modfcatons nclude development and procurement of hgh-temperature crucble and delvery system, new hgh-speed camera, new trggerng system for hgh pressure shock hgh-temperature thermocouples and pyrometer. The frst step of experment [27] was to fnd a ceramc type bnary oxde wth lqudus temperature not hgher than 1400 C because of some lmtatons of the faclty. After testng of dfferent oxdc materals (e.g., MoO 3 - B 2 O 3, MnO-TO 2, MoO 3 -ZrO 2, MnO-MoO 3, MoO 3 -TO 2 ), the bnary mxture of WO 3 - B 2 O 3 (whose phase dagram s shown n Fg. 9) was selected to perform sngle droplet steam exploson experment. 12 testss were performed wth ths mxture, n whch 5 were n 15

18 a eutectc composton (27:73 mol%, T lqudus =870 C) and 7 n a non-eutectc composton (27:73 mol%, T lqudus =890 C), wth water temperatures varyng from 20 C to 80 C. Unfortunately, ths partcular materal had very pecular characterstcs (cf. Fgs. 10 & 11): very fne fragments, smlar to a thn powder, were left behnd durng the droplet plungng nto the water, makng t mpossble resolve the bubble dynamcs. Moreover, the probable droplet dsntegraton would nfluence the melt precondtonng and thus the energetcs of the nteracton. The fact that most of the experments rendered energetc steam explosons even under low subcooled condtons, strengthen such possblty. For these reasons, the WO 3 -B 2 O 3 was not consdered as a sutable smulant for the current study. Fg. 9: Phase dagram of WO 3 -B 2 O 3. Fg. 10: WO 3 -B 2 O 3 eutectc mxture undergong an energetc steam exploson and fne fragmentaton of the molten materal. Fg. 11: WO 3 -B 2 O 3 non-eutectc mxture undergong a mld steam exploson and coarse fragmentaton of the molten droplet. 16

19 Fnally, the bnary oxde mxture of WO 3 -CaO has been chosen as the corum smulant. Its phase dagram s as shown n Fg. 12. Total 37 experments usng WO 3 -CaO were performed on the MISTEE test faclty, beng 32 of an eutectc composton (75:25 mol%, T lqudus =1135 C, T superheat ~ C) and 5 experments wth a non-eutectc composton (72:27 mol%, T lqudus =1232 C, T soldus =1135 C T superheat ~200 C), under hgh subcooled condtons. However, only 12 eutectc tests and 3 non-eutectc tests (see Table 5) were chosen for the analyss due to ther data completeness, e.g. smultaneous record of bubble and melt dynamcs, and the absence of non-condensable gases. Fg. 12: Phase dagram of WO 3 -CaO. As establshed by the prevous experments wth tn, the steam exploson mcro-nteractons are depcted by the vapor flm and melt dynamcs, n whch melt precondtonng and converson rato wll be evaluated. Materal (WO3-CaO) Eutectc Non-eutectc Table 5. Test matrx. Experment no. T melt T coolant 17

20 Bubble and melt dynamcs Stll photographc pctures, wth a temporal resoluton of 0.05 ms per frame, are presented n the top of Fgs ; and the x-ray radography pctures, wth a temporal resoluton of 0.05 ms per frame 0.2 ms, are presented n the bottom of Fgs The mages reveal the vapor flm and melt progresson durng the steam exploson of ~1 g of eutectc and noneutectc molten WO 3 -CaO under hgh water subcoolng. Smlar to the sngle droplet experments performed wth a metallc melt (tn), the vapor flm dynamcs produces three defned cycles of bubble expanson and collapse. Due to the melt s hgh temperature, a vapor flm wth a small dome on the rear sde s mmedately formed at the tme when the molten WO 3 -CaO droplet enters the water, and endures as t descends nto the water. The nteracton ntates when the external pressure wave destablzes the vapor flm, trggerng the parallel oscllatory behavor of the rear and cyclc jet formaton underneath of the man bubble. The near lqud-lqud contact occurs and nucleaton takes place expandng the bubble and rear. To ths pont, the vapor flm dynamcs creates complex nternal flows whch lead to the deformaton/ prefragmentaton of the melt droplet. That can be clearly seen n the radographc mages, n whch the ntal ellptcal droplet, Fgs. 13a-16a (bottom), evolves nto a convoluted droplet wth fne fragments present n the droplet s perphery, Fgs. 13b-16b (bottom). The overgrown bubble/rear reaches ts maxmum and starts to collapse towards the molten droplet. The acceleratng nterface hts the molten droplet, addng the coolant nto the nteracton zone; at ths pont (t=0 ms), the actual drect melt-coolant contact takes place. Explosve bubble expanson and fne fragmentaton of the melt droplet s then accomplshed n the subsequent two cycles ms ms ms -1.1 ms ms 0 ms 1.35 ms 2.0ms 2.40 ms 3.30 ms 3.70ms 3.90 ms 18

21 a b c d -4ms -0.2ms 0ms e f 1.4ms 2.6ms 3.25ms Fg. 13: Vapor flm and melt dynamcs of a g of eutectc WO 3 -CaO (Intally at 1350 C n water at 20.4 C undergong steam exploson) ms ms ms ms ms 0 ms 1.20 ms 1.90ms 2.45 ms 3.25 ms 3.75 ms 4.30 ms 19

22 a -4ms b -0.25ms c 0ms d 1.25ms e 2.5ms f 3.25ms Fg. 14: Vapor flm and melt dynamcs of a g of non-eutectc WO 3 -CaO (Intally at 1480 C n water at 20.1 C undergong steam exploson) ms ms ms ms ms 0 ms 1.20 ms 1.65ms 2.00 ms 3.20 ms 3.90ms 4.55 ms 20

23 a b c -4ms -0.2ms 0ms d e f 1.2ms 2ms 3.2ms Fg. 15: Vapor flm and melt dynamcs of a g of eutectc WO 3 -CaO (Intally at 1350 C n water at 20.2 C undergong steam sxploson) ms ms ms ms ms 0 ms 1.25 ms 1.85 ms 2.05 ms 3.2 ms 4.25 ms 5.7 ms 21

24 a b c -4ms -0.25ms 0ms d e f 1.25ms 1.75ms 3ms Fg. 16: Vapor flm and melt dynamcs of a g of non-eutectc WO 3 -CaO (Intally at 1480 C n water at 24.1 C undergong steam exploson). What was not observed n the tn experments s that whenever the frst cycle expanson s underdeveloped, the second cycle expanson s also suppressed consequently, but t s then compensated by generatng an outszed thrd expanson; see Fgs One could ratonalze such sngularty by understandng the dynamcs of the frst cycle snce t establshes the ntal condtons for the energetc second cycle. The frst cycle s manly affected by two major aspects: coolant/melt temperature (vapor flm stablty) and vapor flm morphology (asymmetry); whch n turn affect the melt precondtonng. Snce all the experments wth oxdc materal were performed under smlar coolant/melt temperature, we turn our attenton to the vapor flm morphology. Durng the hot melt droplet trajectory n the water, the vapor flm forms a small dome on the rear of the droplet. Its presence plays an mportant role when dsturbed by the external pressure wave (smlar to the nteracton when non-condensable gases are present). Such asymmetry leads to complex pressure dynamcs governng the bubble s nternal flows and jet formaton on ts nterface. The resultng forces and nteractons are suffcent to dsturb the droplet surface, facltatng ts precondtonng. The ncreased precondtoned droplet adds heat to the growng bubble, whch s translated to a larger 1 st cycle expanson, Fg. 18a. At ths pont, the overgrown bubble wll then collapse, reachng a hgh mpact velocty; ths settng added to the hghly precondtoned droplet facltates the melt-coolant mxng, generatng an energetc second cycle, Fg. 18b. The opposte s also true, f a droplet melt presents a dmnutve vapor rear, the precondtonng wll be lesser, the frst cycle wll be less promnent, whch wll then lead to a less energetc second cycle. The later mples an neffectve melt fragmentaton, leadng to a larger resdual melt, as shown n Fgs. 15e-16e (bottom), whch s responsble for the thrd extensve expanson. 22

25 Gven the expermental work on the two dfferent eutectc and non-eutectc materals, no evdent dssmlarty n the vapor flm, Fg. 17, or melt hstory was found for the actual melt droplet superheat (200 C) ne-3 (20.1 o C) ne-4 (24.1 o C) ne-2 (23.4 o C) e-23 (20.0 o C) e-25 (19.2 o C) e-26 (20.4 o C) e-30 (21.8 o C) Deq/Deq tme (ms) Fg. 17: Radal hstory of eutectc and non-eutectc WO 3 -CaO sngle droplet. 3.0 a 2.7 Deq/Deq 0 max (1st Cycle) Aspect Rato 6 b Deq/Deq 0 max (2nd Cycle) Deq/Deq 0 max (1st Cycle) Fg. 18: (a) Vapor flm aspect rato n respect to the 1 st cycle maxmum radal; (b) Expanson 1 st and 2 nd cycle maxmum radal expanson. 23

26 Steam exploson energetcs From the bubble radal hstory, one can estmate the work done by the expandng bubble and thus the steam exploson converson rato. One should keep n mnd that t s not possble to estmate the thrd cycle converson rato due to the fragments dspersed n the coolant, whch dsables the possblty to dentfy of the vapor flm volume. That s to say, n the cases n whch the thrd cycle s the more energetc, the contrbuton to the total energetcs wll not be farly represented. However, the correlaton wth the precondtonng wll be conserved,.e. the hgher the precondtonng the hgher the second cycle converson rato, as seen n Fg. 19. Yet, no apparent dfferences between the eutectc and no-eutectc materal, n terms of steam exploson energetcs and precondtonng, can be dscerned. Ths may be due to the hgh superheat employed n the experments. The phase change of a bnary oxde melt droplet, e.g. soldfcaton and/or formaton of a mushy phase, typcally occurs over a specfc temperature range, whch s determned by the lqudus and soldus lne. Accordngly, f the droplet superheat s hgh enough, the materal phase wll be kept far above the lqudus lne,.e. n lqud form, untl the ntaton of the nteracton 1. Thus, the actual experments are probably away from the regon n whch soldfcaton behavors would play a role on the steam exploson energetcs, see Fg. 20. Melt Droplet Early Phase End of 1 st Cycle Non-eutectc WO 3 -CaO Precondtonng Eutectc WO 3 -CaO nd Cycle Cumulatve Converson Rato (%) Fg. 19: Melt droplet precondtonng and 2 nd cycle cumulatve converson rato. A faled experment (ne-5), n whch the melt was delvered earler than ntended due to a leak n the crucble, s shown n Fg. 21. Although the exact melt temperature s not known, 1 The TROI experments had a hgh ntal superheat, yet the UO 2-ZrO 2 melt mxture could be transformed quckly nto a non-lqud form due to the hgh radatve heat flux, whch leads to fast crust and/or of mushy zone formaton. 24

27 t certanly has a lower superheat than the tests above presented and analyzed. In ths partcular case a crust formaton s vsble durng fragmentaton (1.75 ms). It was not possble to estmate the.energetcs, but t s clear that the nteracton was mlder than the experments presented n Fgs , gven that the fne fragmentaton s dmnutve. The presence of the crust and suppresson of a thrd cycle mples the presence of less materal avalable for a steam exploson. In order to dentfy the materal effect threshold,.e. when mushy phase would start playng a role, one would need to perform experments wth lower superheat to develop a soldfcaton behavor map for non-eutectc melts and ts nfluence on the steam exploson energetcs. Actual MISTEE tests Energetcs - Precondtonng 0.0 NE-5 Mushy zone o x Crust + Mushy zone 0 Melt Superheat Fg. 20: Melt droplet energetcs/ precondtonng n respect to melt superheat and possble soldfcaton behavor. crust -5.5 ms -4.9 ms ms 0 ms 1.15 ms 1.75 ms Fg. 21: Vapor flm dynamcs of a 1.2g of non-eutectc WO 3 -CaO, n water at 24.6 C undergong a mld steam exploson (crust presence): NE Metallc (tn) versus Oxdc (WO 3 -CaO) materals Although one should not quanttatvely compare the bubble and melt dynamcs of the oxdc (WO 3 -CaO) experments wth the metallc (Tn) experments due to dfference n the water subcoolng (the hghest subcoolng for the tn experments s around 55 C, whle for the 25

28 oxde tests the subcoolng was around 70 C), Table 6 shows no major dfference between the tests. Moreover, features nherent from the metallc test, e.g. spontanety of the steam exploson, can be justfed by the lower temperature of the droplet melt whch n turn wll produce a less stable vapor flm susceptble to external dsturbances. The above mentoned arguments renforce the dea the oxdc melt droplets were n a complete lqud state by the tme that the steam exploson s ntated. Table 6. Some features from the Tn and WO 3 -CaO experments Vapor Flm Morphology 1 st Cycle Deq max /D o Precondtonng A fnal /A ntal 2 nd Cycle Deq max /D o Energetcs Up to the 2 nd Cycle 2 nd Cycle Deq max /D o Tn Experments Dmnutve Vapor Dome ( ) WO 3 -CaO Experments Pronounced Vapor Dome ( ) Concluded Remarks Sgnfcant progress was made and mportant fndngs were obtaned n the INCOSE project durng year Substantal advances n process modelng and new nsghts nto related mechansms were ganed from studes on ) corum coolablty n the BWR lower head by ntegral and coupled assessment for takng nto account the CRGT coolng as a potental SAM measure; ) debrs bed coolablty wth focus on characterzaton of the effectve partcle dameter of mult-sze partcles as found n debrs beds; and ) mcro dynamcs (anatomy) of a sngle oxdc droplet steam exploson. Specfcally, method for couplng of ECM/PECM models wth RELAP5 was developed to enhance predctve fdelty for melt pool heat transfer. MELCOR was employed to examne the CRGT coolng effcency by consderng an entre accdent scenaro. For a partculate bed packed wth mult-sze partcles, the effectve partcle dameter can be represented by the area mean dameter of the partcles, whle at hgh velocty (Re>7) the effectve partcle dameter s closer to the length mean dameter. The pressure drop of two-phase flow through the partculate bed can be predcted by Reed s model. The steam exploson experments usng oxdc mxture of WO 3 -CaO show that there s no apparent dfference n steam 26

29 exploson energetcs and precondtonng between the eutectc and no-eutectc materals, probably due to hgh superheat (200 o C) of the melt appled n the tests. More achevements and detaled descrptons can be found n the related publcatons lsted n References. Overall, the research of the INCOSE project n 2009 has advanced the knowledge of steam exploson and n-vessel coolablty n BWRs. As we enter 2010, the project contnues data generaton and methodology development, n order to reduce uncertanty n quantfcaton of corum melt rsk n a hypothetcal reactor severe accdent of LWRs. Specfcally, n the n-vessel coolablty topc we wll contnue ECM/PECM applcaton for melt pool heat transfer and develop a coupled analyss method of CFD-structural mechancs (melt pool convecton-vessel behavor), n order to reduce uncertanty n predcton of penetraton (IGT) falure on the vessel.. We wll contnue POMECO-FL tests for frcton laws of partcle beds wth prototypcal debrs characterstcs, and construct POMECO-HT test faclty for coolablty qualfcaton of such beds, and development and valdaton of related smulaton tools. In steam exploson topc, MISTEE experments wth the oxdc bnary mxture (WO 3 -CaO) wll be performed at lower superheat, snce the materal effect mght be perceved at lower superheat, where mechansms lke the mushy zone wll start playng a role. Methodology wll be developed for steam exploson rsk assessment under reactor applcaton. References [1] C.T. Tran and T.N. Dnh, The Effectve Convectvty Model for smulaton of melt pool heat transfer n a lght water reactor pressure vessel lower head. Part I: Physcal processes, modelng and model mplementaton, Progress n Nuclear Energy, 51 (8), pp , [2] C.T. Tran and T.N. Dnh, The Effectve Convectvty Model for smulaton of melt pool heat transfer n a lght water reactor pressure vessel lower head. Part II: Model assessment and applcaton, Progress n Nuclear Energy, 51 (8), pp , [3] C.T. Tran and T.N. Dnh, Smulaton of core melt pool formaton n a reactor pressure vessel lower head usng Effectve Convectvty Model, Nuclear Engneerng and Technology, 41 (7), pp [4] C.T. Tran, P. Kudnov and T.N. Dnh, An approach to numercal smulaton and analyss of molten corum coolablty n a BWR lower head, Nuclear Engneerng and Desgn, Nuclear Engneerng and Desgn, Artcle n press, do: /j.nucengdes [5] C.T. Tran, The Effectve Convectvty Model for smulaton and analyss of melt pool heat transfer n a lght water reactor pressure vessel lower head, Doctoral Thess n Energy Technology, Royal Insttute of Technology, Stockholm, Sweden [6] C.T. Tran and P. Kudnov, The effectve convectvty model for smulaton of molten metal layer heat transfer n a bolng water reactor lower head, Proc. of ICAPP'09, Shnjuku Tokyo, Japan, May 10-14, [7] F. Cadnu, T.C. Thanh and P. Kudnov, Analyss of n-vessel coolablty and retenton wth control rod gude tube coolng n bolng water reactors, NEA/SARNET2 Workshop - In-Vessel Coolablty, NEA Headquarters, Issy-les-Moulneaux, France, October 12-14, [8] W.M. Ma and C.T. Tran, On the Effectveness of CRGT coolng as a severe accdent management measure for BWRs, Proc. of OECD/NEA Workshop on Implementaton 27

30 of Severe Accdent Management Measures, Vllgen, Swtzerland, October 26-28, [9] G. Hofmann, On the locaton and mechansms of dryout n top-fed and bottom-fed partculate beds, Nuclear Technology, 65: 36 45, [10] A. W. Reed, E. D. Bergeron et al, Coolablty of UO2 debrs bed n pressurzed water pools: DCC-1 and DCC-2 experment results, Nuclear Engneerng and Desgn, 97: 81-88, [11] I. Lndholm, S. Holmström, J. Mettnen, V. Lestnen, J. Hyvärnen, P. Pankakosk, H. Sjövall, Dryout heat flux experments wth deep heterogeneous partcle bed, Nuclear Engneer and Desgn, 236, , [12] M. J. Konovalkhn, Investgatons on melt spreadng and coolablty n a LWR severe accdent, Ph. D thess of Royal Insttute of Technology, Stockholm, November [13] K. Hu and T.G. Theofanous, On the measurement and mechansm of dryout n volumetrcally heated coarse partcle beds, Int. J. Multphase Flow, 17 (4): , [14] W. Schmdt, Influence of multdmensonalty and nterfacal frcton on coolablty of fragmented corum, PhD thess of Unverstät Stuttgart, German, [15] W.M. Ma and T.N. Dnh, The effects of debrs bed s prototypcal characterstcs on corum coolablty n a LWR severe accdent, Nuclear Engneerng and Desgn, 240 (3), pp , [16] S. Ergun, Flud flow through packed columns, Chemcal Engneerng Progress, 48 (2): 89-94, [17] R.J. Lpnsk, A one dmensonal partcle bed dryout model. ANS Transactons, 38, , [18] A.W. Reed, The effect of channelng on the dryout of heated partculate beds mmersed n a lqud pool. PhD thess, Massachusetts Insttute of Technology, Cambrdge, [19] T. Schulenberg and U. Műller, An mproved model for two-phase flow through beds of course partcles, Int. J. Multphase flow, 13 (1): 87-97, [20] S. L. Soo, Multphase Flud Dynamcs, New York: Scence press Gower Techncal, [21] V.K. Dhr, Bolng and two-phase flow n porous meda, Annual Revew of Heat Transfer, 5: , CRC Press, Boca Raton, [22] A. Zesberger, F. Maynger, Heat transport and vod fracton n granulated debrs, Nuclear Engneerng and Desgn, 236: , [23] I. Lndholm, A revew of Dryout heat fluxes and coolablty of partcle beds, SKI report 02:17, [24] R.C. Hansson, H.S. Park, T.N. Dnh, Smultaneous hgh speed dgtal cnematographc and X-ray radographc magng of an ntense mult-flud nteracton wth rapd phase changes, Expermental Thermal and Flud Scence, 33, pp , [25] R.C. Hansson, H.S. Park, T.N. Dnh, Dynamcs and precondtonng n a sngle droplet vapor exploson, Nuclear Technology, 167, pp , [26] R.C. Hansson, Trggerng and energetcs of a sngle drop vapor exploson: the role of entrapped non-condensable gases, Nuclear Engneerng and Technology, 41 (9), pp , [27] Proc. of the 28 th Revew Meetng for Project Melt-Structure-Water Interactons n a Severe Accdent (MSWI-28), KTH, Stockholm, Sweden, June 10, 2009, 84p. [28] Proc. of the 29 th Revew Meetng for Project Melt-Structure-Water Interactons n a Severe Accdent (MSWI-29), KTH, Stockholm, Sweden, December 9, 2009, 90p. 28

31 Bblographc Data Sheet NKS-219 Ttle Author(s) Afflaton(s) In-vessel Coolablty and Steam Exploson n Nordc BWRs Wemn Ma, Roberta Hansson, Langxng L, Pavel Kudnov, Francesco Cadnu, Ch-Thanh Tran Royal Insttute of Technology (KTH), Sweden ISBN Date May 2010 Project NKS-R / INCOSE No. of pages 28 No. of tables 6 No. of llustratons 21 No. of references 28 Abstract Key words The INCOSE project s to reduce the uncertanty n quantfcaton of steam exploson rsk and n-vessel coolablty n Nordc BWR plants wth the cavty floodng as a severe accdent management (SAM) measure. Durng 2009 substantal advances and new nsghts nto physcal mechansms were ganed for studes of: () n-vessel corum coolablty development of the methodologes to assess the effcency of the control rod gude tube (CRGT) coolng as a potental SAM measure; () debrs bed coolablty characterzaton of the effectve partcle dameter of mult-sze partcles and qualfcaton of frcton law for two-phase flow n the beds packed wth mult-sze partcles; and () steam exploson nvestgaton of the effect of bnary oxdes mxture s propertes on steam exploson. An approach for couplng of ECM/PECM models wth RELAP5 was developed to enhance predctve fdelty for melt pool heat transfer. MELCOR was employed to examne the CRGT coolng effcency by consderng an entre accdent scenaro, and the smulaton results show that the nomnal flowrate (~10kg/s) of CRGT coolng s suffcent to mantan the ntegrty of the vessel n a BWR of 3900 MWth, f the water njecton s actvated no later than 1 hour after scram. The POMECO-FL expermental data suggest that for a partculate bed packed wth mult-sze partcles, the effectve partcle dameter can be represented by the area mean dameter of the partcles, whle at hgh velocty (Re>7) the effectve partcle dameter s closer to the length mean dameter. The pressure drop of two-phase flow through the partculate bed can be predcted by Reed s model. The steam exploson experments performed at hgh melt superheat (>200oC) usng oxdc mxture of WO3-CaO ddn t detect an apparent dfference n steam exploson energetcs and precondtonng between the eutectc and noneutectc melts. Ths ponts out that the next step of MISTEE experment wll be conducted at lower superheat. severe accdent, debrs coolablty, steam exploson Avalable on request from the NKS Secretarat, P.O.Box 49, DK-4000 Rosklde, Denmark. Phone (+45) , fax (+45) , e-mal nks@nks.org,