ML20138C464
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| Issue date: | 11/20/1996 |
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/Karlsruhe,20 November 1996
! - H. Jacobs s
l Review of Report DOEllD-10541: Lower Head integrity Under in vessel Steam Explosion Loads by T. G. Theofanous et al.
4
- 1. Introductory remark 4
' In order to put my comments to follow into the right perspective, I must state first i of all that I fully agree with the general approach to the problem taken by the au-thors,i.e. the ROAAM. To what extent probabilities are used within this approac may depend on the purpose and the problem of the study.' However, dividing problem into its physical aspects, treating them in separate parts of the stud j can be scrutinized by other experts and linking them in a well defined and verifi-able way defines a clear path towards the resolution of the full problem.
Similarly I fully support.the basic pproach taken to treat the steam explosion f
- f. problem. The material presented is band on and incorporates a lot of pione and exemplary work in this field. I do not wet to shed any doubt on that. The
' only question I'm discussing is: is the state of development sufficient to fi .
answer the question under discussion. This forces me to elaborate on potentia/
weak points in the argumentation. If a technical field isn't developed sufficien
' even a ' peer review' cannot finally ensure the correctness of an evaluation.
' Quite obviously, steam explosions are not phenomena that are well unders; the scientific sense, especially if we are concerned J events lie far outside the parameter range that can easily be studied exper-imentally. This is true of the initial temperature and the composition of the as well as the masses involved (as mentioned above). This d
~
largely rely on codes for extrapolating from the accessible parameter ran
- . that of the envisaged accident situations. Ideally this extrapolation requires j
j- knowledge and appropriate modeling of all relevan of ' fitness for purpose' may be helpful in areas in which the consequences o glecting something can be estimated. But how about problems In the which h been identifled or the importance of which has not yet been perceived?
present state of knowledge bad surprises cannot b (take a conservative approach) and to require a large safety factor. To som tent, this principle is followed in the study discussed here. But in my judg not to a sufficient extent.
From the point of view of quality assurance, a peer review like this one can come fully effective only if at least the background material was published s l
quite some time so that a thorough discussion o ;
delivered very late during the review process. This reduces the relevance o ,
J present review process. '
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- 2. Scope of this review The !
l This review is concerned with the steam-explosion aspects of the study. l contribution of this part of the study to the positive final conclusion,i.e. Interacting i
- masses that are insignificant from an overall energetic standpoint and even local l
' loads that lead to elastic strain only, can be attributed to small pouring rates, a strong volding of the premixing zone and early explosions. The first of these are to some extent a consequence of the melt-water mixing scenarios chosen and 4
though core melt down is not my proper field of experience i must make a fe comments on this because the way in which melt and water are brought into con-tact is basic for the subsequent events. The possibility The second point,of i.e.a small steam explosl the pro-l inctucing a larger one is neglected altogether. posed strong vo i
prior to an explosion (the mixing zone or premixture),is !
same time, this voiding seems to be one reason for the dying away of the ener-getics of explosions with increasing time of triggering which is is the most c vincing argument for considering early explosions. Of course, this finding a depends on the third point,i.e. the way in which the steam explosion pro ,
modeled. The above three aspects, i.e. scenarios and modeling of premixingl explosion are discusses one after the other below. l 4
- 3. Technical evaluation i
3.1 Melt relocation scenarios By the scenarios it is defined how the melt relocates into the lower p this gives the rates at which corium is fed into the lower plenum. Th l
is an important aspect that must be scrutinized during the review step o I am not really an expert in this field, myself, but i must raise the question w it is really possible to exclude with sufficient certainty a downward reloca could lead to much higher corfum flow rates depending on the number o the core support plate through which corium flows into the lower plenum doubts in this respect come from the agreement of the experts in this fiel late phase of core melt down,i.e. the melt relocation phase,is not we and from the virtual absence of mechanistic models for growth and espec i
dial expansion of molten pools. The study that is under discussion here bridge this gap using simple and clear estimates of conditions influen l' thermal stability of a metallic crust. But in these estimates, e.g. no cons is given to the possible formation of outectics which might drastical melting temperature and thus crust stability. One might also speculate hot material could drop into the water remaining below the corium poo creasing the time untilit is evaporated and thus i i would take to melt through the reflector and core barrel.' Of course, there s
' addition the thermal inertia of the core support plate. But as soon as it dry, its surface temperature will increase and thus reduce the effe heat transfer.
Another possible uncertainty is the stability (leak-tightness) of a sidew ly) advancing crust. This process might induce transverse forces ing stubs of fuel pins which these cannot withstand in their damag i
So the crust could fait and the oxydic melt could flow freely towards the core l 2
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2 suppod plate and possibly through it. (Table 4.1 indicates that the ' cold trap' is not likely to stop flowing oxydic corium.) Here one may recall that processes of this' nature occurred during the TMI 2 accident [1] although, in that case, the )
j whole melt pool was submerged. As witnessed by several tonnes of corium that !
1 solidified within the core support assembly, a large amount of corium has flown
' down through about 4 peripheral fuel elements around The latter core position may have beenR6. Another downward relocation occurred at core position K8.
4 j brought to a stop above the core support assembly. But we do not know ho l
by what margin.
Finally, the possibility of a large coherent steam explo might proceed in different ways. The common startin directly by the action of the pressure of the first steam explosion or indirectly b the pressure of another melt-coolant interaction The induced due steamto theexplosion addittion of some into the upper zone or on top of the melt pool.
i would then occur either within the core volume (if there were stil the lower plenum after the melt released from the broken melt pool has drain ,
j through the still open holes in the lower grid plate. It is sometimes argued th j such melt couldn't encounter water in the lower plenum because that would havel l
i been driven away by the initial steam explosion. However, the first (weak) explo
- sion might have caused essentially a slashing move l i
one should also keep in mind that with a large molten corium mass availablj l melt-water interactions occurring, large amounts of mechanical energy may b i
t come available. So it is often hard to argue that some process was unlikely.
3.2 Modell'ng of premixing f
i^ ?rt, nixing is the process that is thought to be required to set the stage for any 1
- large scale coherent steam explosion, it is, at the same time, expected to; l ently limit the masses participating in an explosion by the ' water depleti 1.e. removal of liquid water from .the premixture by large amounts of steaml l
' are created due to fast heat transfer. As these processes are difficult to sim directly in experiments, recourse is taken to numerical modeling with the ,
, PM ALPHA.30. For the scenarios considered, this code predicts strong voidl i
of the volumes accessed by melt. In combination with a cutoff of propagat !
is effective at high voiding this gives a strong limitation of the melt masses can interact. And this is the second pillar on which the final result ~ '
of the s 1
, i i resting.
While there are good arguments for the concept of ' water depletion' and al some experimental observations that appear to support the idea in principlel remains the question whether the quantification given by PM ALPHA.30 is; ciently reliable. The program predicts 'the major l very steep', i.e. the void fraction increases from values around 20 % t; 80 % within a short distance. Such behavior, however, was not seen in the mixing experiments that are being conducted at
[4]. it is too early to draw final conclusions from these experiments
'. fractions in the surrounding of broken up ' fuel' appear to be smaller than ex-l 3
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pected. One may also draw attention to data reported of the KROTOS exper-iments [5]. In these tests molten alumina was poured through an orifice with 3 cm diameter into a 10 cm wide tube filled with water, it mixed with the water an strong steam explosions occurred either spontaneously or following an exter trigger. The melt temperature was high, typically 2600 K, but the water wa cooled which, of course, tends to reduce volding. InInthe bothKROTOS cases the tests #28 a 1
' #29 the water was subcooled by 10 K and 80 K, respectively.
steam volume fractions within the reaction tube were 4 % only. But as these are j
mean values over the whole tube which may contain some regions occupied water only, it may be more relevant to point out that the steam volume was o about half the melt volume. In test #30, subcooling was again 80 K but the m mass was larger and its breakup was more intensive. In this case the steam i
' ume fraction reached 23 % but this is again only 1.3 times the melt volume.
we foundedmust check which how imply well steam the above volume fractionscited that are calculational larger than the results melt volo fractions by well over an order of magnitude.
The original PM-ALPHA was one of the two pioneering codes that used three
- locity fields for describing the separate motions of m 1
two-field description of two-phase flow. But this is the only way in which one ca hope to develop a reasonable description of the phenomena during a steam i plosion. The fairly standard multiphase equations used provide compl the conservation equations only. All the controlling and very complicated p i
in the three-pnase (and at least) three-component mixture must be desc
! constitutive relations. Here the difficulty arises that one of the main pur
- such codes is to extrapolate from the experiments that are possible in pr j the envisaged accident situation. This implies extrapolation from simulation terials (sometimes even solid spheres) to the expected (but still quite molten corium, from often quite low ' melt' temperatures to temperature 3000 K, and from the mostly very small scale of e tions is not as bad as indicated here but as the experimental difficulties grow l
' enormously as the expected accident conditions are approached, the expe imental information on the initial conditions and details of the processes l poor in these cases so that a successful comparison of calculation i
integral experimental results doesn't necessarily indicate correctness oretical model. Indeed, one can expect a code to perform the required extr i
lations sufilcient accuracy.
only, if all relevant mechanisms are modeled m
' often heuristic, sometimes parametrical. The latter is described in the the melt breakup model but is true as well for one formulation of the ev i
rate. The other formulation looks more physical but still does not allow possibility that evaporation and condensation occur concurrently tegration volume (calculational mesh) due to lim drops are covered by a thin vapor film only, as e.g. on those parts of faces that are oriented towards the direction of motion. So any extrapo accident conditions must be afflicted with large uncertainties.
Validation of the original PM-ALPHA code by co special verification report [7). An appeal of the general agreement re
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l be obtained from the data on the leading edge advancement. With cold spheres
' this agreement is mostly reasonable. With sphere temperatures of about 1 the data are reproduced within about a factor 2. In the ' production runs' of th j
' present study the initial temperature of the melt will have been beyond that the uncertaintles will certainly have increased quite considerably.
' Here we are mainly interested in the high vold fractions that have been m and predicted during the verification process. The data given in [6] have b i obtained with the MAGICO experiment and have been described as highl l vant ('the measurement not only provides insight into premixing, but rep i
probably the most important test for computer codes'). Hence our ex j find high local void fractions in our own experiments. However, the loca i
data preuented in [6] have been measured in a position or better line region' (of unknown size) 15 cm below How theitsinitial w i
uring volume was centered with respect to the particle jet (the pour).
width compares to the width of the pour is not known. The measurement wa c
performed at 0.35 sec, i.e. Just after the end of (or behind) the pou
, order to avoid the presence of many spheres at the level of the measurem i
These circumstances appear to have produced the observed high void fra ;
]
l possibly without too much contribution of steamin d 1 l pool in a similar way, that the particle cloud is always followed by a l
- chimney - with cold spheres as well as with hot spheres. This is largely a c i quence of the momentum transfer between the particles and the wa l mal effects are of secondary importance - they essentially influence th which the gas chimney is closed again. That this is also true in the MA l periments i .s clearly shown by Figures 14 and 15 in Reference [6
- a ' cold' run. This means that the reported high vold fractions h have li the so-called ' water depleflon' effect and there is no experimental sup high void fractions calculated in the ' production' runs at positions f the melt entrance. Onew might add that corresponding to our observa i QUEOS experiments, thermal effects just start to be detectable in an o (beyond local effe
- :ts around each individual sphere) at sphere temp l low as 1600 K. Even at the much higher temperatures beyond 2300 K 4
been reached in QUEOS, no high void fraction momentum transfer).
In the mair!
body of the verification report [7] global estimates of the water con l tent within the mixing zone in QUEOS are used for further checking Unfortunately this type of data is hardly suited for a q code calculations.
choice of the outer radius of this zone because, due to the weighing w dius squared, it is this region that dominates the integration over the in the experiment this difficulty can be overcome to some extent by 4
termining the shape of the mixing zone from high-quality photogra the extent that a qualitative result can be obtained. However,in cod the calculational mesh is not able to sufficiently resolve this outer b j what is given in [7]is the 'pM-ALPHA result for the central region of th L
containing the main portion of the particle cloud.' As a consequenc
- lated value is somewhat ambiguous and Figure 13 in Chapter 2 unavoidably compares quantitles with different definitions.
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- It remains that the code in this case predicts low voiding (in contrast with the j 4
production runs). But here the code appears to have gone to the other ext ;
2 due to its inability to describe evaporation in the presence of subcooled water l which even leads to the reported underestimation of evaporation (steam flow)l
/
4 and pressure rise. To explain these discrepancies by ,
free surfaces., ,
Another uncertainty of the calculational results is due to modeling the cor breakup The surface of a certain amount of material varies linearly with t verse of the) particle radius. Therefore modeling the corium as individual dro l lets with 2 cm diameter from the very beginning gives it already a quarter surface that it would have with drop diameters of 0.5 cm which can certain considered as well prefragmented (broken up). In the calculations presen initial diameter is combined with an entrance volume fraction of 25 % on there is an intensive thermal interaction from the very beginning. Howev PREMIX experiments being performed at Forsch water (e.g. 0.5 m for a jet diameter of about f 4 cm
, alumina corium.
at about 2600 K the density of which is only a l with corium. Such dynamic breakup process with virtually no breakup in ginning that allow the melt to penetrate deeply into the water followe rapid fragmentation that breaks the melt into medium-sized i drops be the most dangerous configuration) cannot be bounded t by t l breakup model that was employed.
well the entrance of coherent melt (melt being the continuous phase) t premixed with water artificially (by assumption) from the very b ,
context it is also important to note that breaking the melt into very smf
, (e.g. 0.2 cm) may be very optimistic because these small dropl 1 l vapor,i.e. high voiding, and may already start to freeze so that ttwy c participate in an explosive interaction. The importance of freezinl l I
explosion results reported is not discussed.
3.3 Modeling of explosions The most important finding of the calculations in this area is the cutoff t at higher vold fractions. However, the model used to describe expl l actions - the microinteraction model - has been developed on the ba i imental observations in a situation with virtually zero volding. The the model have been fixed using these experiments and it has bee ,
the model can be made to give results looking reasonable (by propel choices) by simulating a KROTOS experiment in which the local vold l assumed to be between 25 and 40 %. It has been the declared purp l microinteraction model to explain the occurrence of strong pressur the presence of large amounts of water (Iow fuel to water mass r ,
, such it is highly interesting from a scientific point of view and may bl vant - in this special situation. But one cannot expect this same model l same parameter settings) to work properly in a completely differel which there is very little water present. The failure of this special inte !
del to predict strong steam explosions unde 6
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12/06/96 FRI 14: 35 FAI 1 630 252 4rGO .e, L . g +'
- S . 5dKMLL 4009 ,
j sions in situations as suggested by the premixing calculations should these ever i
occur. Especially in the case of larger melt masses (and possibly smaller overall void fractions) the lower plenum of a pressurized water reactor might provide enough external confinement for completely different interaction mechanisms to
- become effective. These mechanisms may need more time for their development
' but might in the end arrive at similarly effective interactions. An important ex-ample of mechanisms that may contribute to such alternate types of interact are the thermal fragmentation mechanisms that may not need much water and a 4
completely left aside in the present study. This migh efficient interactions at later times in all cases with a finite breakup parameter.
The picture is less clear in the cases in which additional breakup was assu l
j not to occur. As outlined in the previous section these might be the most inter-2 sting cases in this study. Here no clear maximum of explosivity has been among the cases considered and it is argued that 'slightly broken up premix 4
remain very benign.' However, Table 6.1 shows that in the case C2-nb the i
mum peak local impulse is 30 kPa.s which may already be viewed as a lo termediate value and that it occurs at the last trigger time considered,i.e.1.0 sec j
Nothing in the results presented supports the idea that the value might not larger (and .aaybe important) at even larger triggering times. '
j There is a further and independent argument for early triggering. It states t early triggering is due to the interaction of melt (Jets) with structures.
l used contention, however, does not agree with the observations from th l experiments at Forschungszentrum Karlsruhe. We have now perform l
tests on horizontal and in 4 of plate - inthese one casa the melt even was with equipped forced to interact compartments). Only wi 4
of these tests (the last one performed on 2t' August 1996) lead to a v interaction (a weak steam explosion) about 0.8 s
, which the otherwise very explosive alumina melt settled at the bottom of the f
action vessel copying its shape when solidifying, i does not necessarily provide early tiggering.
- 4. Summary
! low corium-The affirmative final result of the study follows from three findings:
water mixing rates, very high void fractions in the premixture, and, pa pending on that, effective explosions being pos With respect to the melt relocation scenarios I doubt that the prese knowledge allows to definitely exclude downw 1
' leave the judgement to those experts, provided they can positively defea guments in addition, processes that are induced by a first (wea 1
sion might lead to a more effective melt water the code PM-ALPHA even outside the gas channel that immediately foll mass plunging into water don't seem to be supported by experimen t The code itself is not provided with sufficiently mechanistic models an sufficiently validated to support the high void fractions by itself. With 7
inou se ext ic .u e u i g m ,,,, ,
~a. s - .- __
l \
the explosions, the failure of the code ESPROSE.m, >
volded premixtures, doesn't prove that such explosio cur.
! Literature J.' M. Broughton, P'ul Kuan, D. A. Petti, and E. L. Tolman, A scenario o
- [1] Three Mile Island Unit 2 accident, Nuclear Technology 87 (1989) 34 - 53 j-F. Huber, A. Kaiser, M. Steinbr0ck, and H. Will, PREMIX, Documenta
[2] the Results of Experiments PM01 to PM06, Forschungszentrum Karlsr T i
Report, FZKA 5756 (March 1996)
' L. Meyer and G. Schumacher, QUEOS, a Simulation-Experiment of t
[3] mixing Phase of a Steam Explosion with Hot Spheres in Water, Base Experiments, Forschungszentrum Karlsruhe Report, FZKA 5612
[4] L. Meyer, The interaction of a falling mass of h ,
3-6, 1996: ANS Proceedings, HTC-Vol. 9, pp.105-114 l H. Hohmann, D. Magallon, H. Schins and A Yerkess, FCI experim
[5] aluminum oxide / water system, Proc. CSNI Specialist Meeting on F lant interactions, Santa Barbara, CA, January 5-8,1993, U.S. Nucle latory Cor.nmission Report NUREG/CP-0127, NEA/CSNt/R(93 pp.193-201 f [6] S. Angelini, T.
Saratoga G. Theof/fnous, Springs. NY, September and 10-15, W. W. Y 1995, lics NURETH-7, 3 NUREG/CP-0142, Vol. 3, pp.1754 - 1778
[7] T.
sions:
G. Theofanous, W. W. Yuen, and S. Angelin 3
1996) l l H. Will, private communication (to be presented at the OECD/N l
- [8] Spec. Mtg on Fuel-Coolant Interactions, Tokal, Japan,19-21 M l
[ 4 .
Author's address: Dr. Helmut Jacobs
, FZK-INR Phone: +49 7247-82-2443
.#~
Postfach 3640 . Fax: + 49-7247-82-3824 D-76021 Karfsruhe E-mail: helmut.jacobs@inr.fzk.de l Germany 8
A.N L- RE ..a 3- su kM LL 4,vog
_ g/03/93 Till' 3 10 FA_I 1 830_232 4750 Fers:hungszentrum Kartruha Technik und Umwelt Institut f0r Neutronenphysik und Reaktortechnik L.it.c Prot Dr.-Ing. Dr. h.c. G. Koht
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Dr. L. W. Deitrich Datum:
- 20. September 1996 Reactor Engineering Division s..%,i.c Dr.H.Jacobs 2443 Argonne National Laboratory T. won 07247 t a2 T.;. fax 07247 i s2 3824 or 4874 9700 Scth Cass Avenue w . w uns:
- J Review of report DOEllD-10541
Dear Dr. Deitrich,
l J
please find enclosed a signed copy of my review of the report.
This is my last day in the office before 30 September and the verification report o PM-Alpha has not yet arrived. So my review is based on the information available until now, including the contnbution to NURETH.7.
l Sincerely RECEIVED M- 4 REACTOR ENGINEER:NG ON!G:CH
-D! RECTOR'3 OFF:C2-Dr.H.Jacobs -
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