ML20138C479
| ML20138C479 | |
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| Site: | 05200003 |
| Issue date: | 09/20/1996 |
| From: | Jacobs H GERMANY, FEDERAL REPUBLIC OF |
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20 Septembe'r 1996
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H.Jacobs FZK-lNR 1
i i Review of Report DOEIID-10541: Lower Head Integrity Under in-vessel Steam Explosion Loads by T. G. Theofanous et al.
' 1. Introductory remark In order to put my comments to follow into the right perspective, I must state first l of all that i fully agree with the general approach to the problem taken by the au-1 i thors, i.e. the ROAAM. To what extent probabillties are used within this approach may depend on the purpose and the problem of the study. However, dividing the ;
problem into its physical aspects, treating them in separate parts of the study that l 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.
i Similarly I fully support the basic approach taken to treat the steam explosion problem. The material presented is based on and Incorporates a lot of pioneering and exemplary work in this field. I do not want to shed any doubt on that. The only question I'm discussing is: la the state of development sufficient to finally 1
4 answer the question under discussion. This forces me to elaborate on potential weak points in the argumentation. if a technical field isn't developed sufficiently, i
' even a ' peer review' cannot finally ensure the correctness of an evaluation, Quite obviously, steam explosions are not phenomena that are well understood in the scientific sense, especially if we are concerned with such large scale events l
as are discussed in connection with reactor safety analysis. Unfortunately, such
' events lie far outsides the parameter range that can easily be studied exper-imentally. This is true of the initial temperature and the composition of the melt l
as well as the masses involved (as mentioned above). This dilemma f l largely rely on codes for extrapolating from the accessible parameter range to that of the envisaged accident situations. Ideally this extrapolation requires full
- knowledge and appropriate modeling of all relevant phenomena. Here again we 4'
are confronted with gaps, the relevance of which is difficult to judge. The concept 1 of ' fitness for purpose' may be helpful in areas in which the consequences of ne- I glecting something can be estimated. But how about problems In which the have no been identified or the importance of which has not yet been perceived?
present state of knowledge bad surprises cannot be excluded. The (only?) wa deal with this difficulty is to account for all (known) possible traps in the analysis i
(take a conservative approach) and to require a large safety factor. To some ex-tent, this principle is followed in the study discussed here. But in my judgement j not to a sufficient extent.
From a technical point of view, a peer review like this one can become fully ef-factive only if at least the background material was published since quite some time so that a thorough discussion of it has been possible among the experts. In the present case an important part of the background material was delivered la during the review process and some i.e. the PM-ALPHA verification report, even hasn't reached me in time so that my review, in this aspect, is based on the available previous papers only.
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10/03/96 TUr 13: 11 Fu 1630 252 4780 D L-RE ~ S. SORRELL goo 4
. 2. Scope of this review This review is concerned with the steam-explosion aspects of the study. The contribution of this part of the study to the positive final conclusion,i.e. Interacting masses that are insignificant from an overall energetic standpoint and even local loads that lead to elastic strain only, can be attributed to small pouring rates, a strong voiding of the premixing zone and early explosions. The first of these are a consequence of the scenarios chosen and although core melt down is not my proper field of experience I must make a few comments on this because the way in which the melt is relocated into the lower plenum is basic for the consequent events. A second reason for the small interacting masses is the proposed strong voiding of the zone in which corium melt and water are intermixed prior to an explosion (the mixing zone or premixture). At the same time this voiding is a reason for considering early explosions since the demonstration that the ener-getics of expInsions dies away with increasing time of triggering (which is appar-ently due to increasing voiding) is the most convincing argument for considering early explosions. Of course, this finding also depends on the way in which the steam explosion proper is modeled. The above three aspects, i.e. scenarios and l modeling of premixing and explosion are discusses one after the other below.
- 3. Technical evaluation, 3.1 Melt relocation scenarios By the scenarios it is defined how the melt relocates into the lower plenum and this gives the rates at which corium is fed into the lower plenum. Therefore this
.is an important aspect that must be scrutinized during the review step of ROAAM.
I am not really an expert in this field, myself, but I must raise the question whether it is really possible to exclude with sufficient certainty a downward relocation that could lead to much higher corium flow rates depending on the number of holesMy in 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 field that the j
! late phase of core melt down,i.e. the melt relocation phase,is not well understood and from the virtual absence of mechanistic models for growth and especially ra-l dial expansion of molten pools. The study that is under discussion here tries to bridge this gap using simple and clear estimates of conditions influencing the thermal stability of a metallic crust. But in these estimates, e.g. no consideration is given to the possible formation of eutectics which might drastically reduce the melting temperature and thus crust stability. One might also speculate that some i
hot material could drop into the water remaining below the corium pool, thus de; creasing the time until it is evaporated and thus the time of crust stability. In the present study the evaporation time 'happens to be lust about equal the time it would take to melt through the reflector and core barrel.' Of course, there is in addition the thermal inertia of the core support plate. But as soon as its top falls dry, its surface temperature will increase an thus reduce the effect of radiative heat transfer.
Another possible uncertainty is the stability (leak-tightness) of a sideways (radial-ly) advancing crust. This process might induce transverse forces on the suppor ing stubs of fuel pins which these cannot withstand in their damaged condition.
So the crust cculd fall and the oxydic melt could flow freely towards the core j
support 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
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this nature occurred during the TMI-2 accident [1]although, in that' case, the whole melt pool was submerged. As witnessed by several tonnes of corium that a
solidified within the core support assembly, a large amount of corium has flown
' down through about 4 peripheral fuel elements around core The latter position may haveR6. Another been downward relocation occurred at core position K8.
brought to a stop above the core support assembly. But we do not know how and l
l by what margin.
3.2 Modeling of premixing Premixing is the process that is thought to be required to set the stage for any large scale coherent steam explosion. It is, at the same time, expected to inher-ently limit the masses participating in an explosion by the ' water depletion' effect, i.e. removal of liquid water from the premixture by large amounts of steam that are created due to fast heat transfer. As these processes are difficult to simulate directly in experiments, recourse is taken to numerical modeling with the code PM-ALPHA 30. For the scenarios considered, this code predicts strong volding of the volumes accessed by melt. In combination with a cut-off of propagation ihat l is effective at high voiding this gives a strong limitation of the melt masses that 4
can Interact. And this is the second pillar on which the final result of the study is
- resting.
While there are good arguments for the concept of ' water depletion' and also l some experimental observations that appear to support the idea in principle, there l remains the quastion whether the quantification given by PM-ALPHA.30 la sufft-
- ciently reliable. The program predicts 'the major portion of it [i.e. the fuel] being l in a highly volded region (s > 80%)' and also that the void fraction ' gradient is '
' very steep', i.e. the void fraction increases from values around 20 % to more than 80 % within a short distance. Such behavior, however, was not seen in the pre .
' mixing experiments that are being conducted at Forschungszentrum Karlsruhe in 4
order to study the phenomenon and to collect data for code validation [2], [3],
[4]. It is too early to draw final conclusions from these experiments, but the v fractions in the surrounding of broken up ' fuel' appear to be smaller than ex-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 and strong steam explosions occurred either spontaneously or following an extern trigger. The melt temperature was high, typically 2600 K, but the water was su l cooled which, of course, tends to reduce voiding, in the KROTOS tests #28 and
- 29 the water was subcooled by 10 K and 80 K, respectively. In both cases the steam volume fractions within the reaction tube were 4 % only. But as these are mean values over the whole tube which may contain some regions occupied by water only,it may be more relevant to point out that the steam volume was only about half the melt volume. In test #30, subcooling was again 80 % but the melt mass was larger and its breakup was more intensive, in this case the steam vol-uma fraction reached 23 % but this is again only 1.3 times the melt volume. So
- we must check how well the above cited calculational results of PM founded which imply steam volume fractions that are larger than the melt volume fractions by well over an order of magnitude.
The original PM-ALPHA was one of the two pioneering codes that used three ve-locity fields for describing the separate motions of melt, liquid water and steam at the cost of adding considerable complexity to the already quite complicated 3
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two-field description of two-phase flow. But this is the only way in which one can f hope to develop a reasonable description of the phenomena during a steam ex-i ith l
- plosion. The fairly standard multiphase equations used provide compliance w the conservation equations only. All the controlling and very complicated physics in the three-phase (and at least) three component mixture must be described by constitutive relations. Here the difficulty arises that one of the main purposes of ,
such codes is to extrapolate from the experiments that are possible in practice to the envisaged accident situation. This implies extrapolation from simulation ma-terials (sometimes even solid spheres) to the expected (but still quite uncertain) molten corium, from often quite low ' melt' temperatures to temperatures around ;
3000 K, and from the mostly very small scale of experiments to the reactor size.
There are a few experiments in which one or the other of the above initial condi-tions is not as bad as indicated here but as the experimental difficulties grow enormously as the expected accident conditions are approached, the exper-i Imental information on the initial conditions and details of the processes is often l
poor in these cases so that a successful comparison of calculational results with integral experimental results doesn't necessarily indicate correctness of the the-l oretical model. Indeed, one can expect a code to perform the required extrapo- -
j lations only, if all relevant mechanisms are modeled mechanistically and with sufficient accuracy. However, the constitutive relations used in PM-AL."HA are i
mostly heuristic, sometimes parametrical. The latter is described in the report for l the melt breakup model but is true as weil for the evaporation rate. The equally important condensation of steam (e.g. due to subcooling caused by pressure in-crease or on water in the surroundings that has remained cold) does not seem to i be modeled at all. So any extrapolation to accident conditions must be afflicted with large uncertainties.
l Validation of the original PM-ALPHA code by comparison with experiments is de-scribed in Reference [6]. An appeal of the general agreement reached may be i
obtained from the data on the leading edge advancement. With cold spheres this i agreement is mostly reasonable. With sphere temperatures of about 1600 K the j data are reproduced within about a factor 2. In the ' production runs' of the pres- !
ent study the initial temperature of the melt will have been beyond 2900 K so that !
the uncertainties will certainly have increased quite considerably.
j Here we are mainly interested in the high void fractions that have been measured I and predicted during the verification process. The data given in [6] have been j j measured in a position or better line or 'small region' (of unknown size) 15 cm i below the initial water level. This depth is only two thirds of the equivalent diam- '
eter of the pour. We may guess that the measuring volume was centered with' 1
respect to the particle jet (the pour). How its width compares to the width of the pour is not known. The measurement was performed at 0.35 sec, i.e. Just after
- end of the pour, probably in order to avoid the presence of many spheres at the These circumstances appear to have produced the level of the measurement.
observed high void fractions. It is our observation from the QUEOS experiments
[3], [4] in which streams of spheres are poured into a water pool in a similar wa that the particle cloud is always followed by a gas filled chimney - with cold j j spheres as well as with hot spheres. This is essentially a consequence of the i momentum transfer between the particles and the water while thermal effects are of secondary importance. That this is also true in the MAGICO experiments is l l
' clearly shown by Figures 14 and 15 in Reference [6]. This means that the re-c ported high void fractions have little to do with the so-called ' water depletion feet and there is no experimental support for the high void fractions calculated in i i
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i the ' production' runs at positions far away from the melt entrance. One might add that corresponding to our observations in the QUEOS experiments, thermal effects
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Just start to be detectable in an overall sense (beyond local effects around each j individual sphere) at sphere temperatures as low as 1600 K. Even at the much j
higher temperatures beyond 2300 K that have been reached in QUEOS, no high void fractions could be observed outsides the initial gas chimney produced by the entering clouds of spheres (essentially by momentum transfer).
l i
Another uncertainty connected with the calculational results is due to modeling l the corium breakup. The surface of a certain amount of material varies linearly j
with the (inverse of the) particle radius. Therefore modeling the corium as indi-vidual droplets with 2 cm diameter from the very beginning gives it already a J
quarter of the surface that it would have with drop diameters of 0.5 cm which can certainly be considered as well prefragmented (broken up). In the calculations 4
presented this initial diameter is combined with an entrance volume fraction of 25 l % only so that there is an intensive thermal interaction from the very beginning.
l However, in the PREMIX experiments being performed at Forschungszentrum i
i Karlsruhe [2], we have observed that a melt jet can penetrate to quite some depth into saturated water (e.g. 0.5 m for a diameter of about 4 cm) before it starts to break up and to interact more violently (still not explosively). In these cases the melt is molten alumina at about 2600 K the density of which is only about one third of that of corium. So this behavior is even more probable (should be more pro-i nounced) with corium. Such dynamic breakup process with virtually no breakup
! in the beginning that allow the melt to penetrate deeply into the water followed
~
by more rapid fragmentation that breaks the melt into medium-sized drops (which might be the most dangerous configuration) cannot be bounded by the parametric In this context it is important to note that j breakup model that was employed.
breaking the melt into very small droplets (e.g. 0.2 cm) may be very optimistic l because these small drops produce a lot of vapor, l.a. high volding, and may al-l ready start to freeze so that they can no longer participate in an explosive inter-action. The importance of freezing for the benign explosion results reported is not discussed.
I 3.3 Modeling of explosions f
The most important finding of the calculations in this area is tne cutoff that occurs at higher void fractions. However, the model used to describe explosive inter-actions - the microinteraction model - has been developed on the h=Is of exper-l Imental observations in a situation with virtually zero volding. The parameters of the model have been fixed using these experiments and it has been shown that' the model can be made to give results looking reasonable (by proper parameter choices) by simulating a KROTOS experiment in which the local void fraction was assumed to be between 25 and 40 %. It has been the declared purpose of the microinteraction model to explain the occurrence of strong pressure increases in
' the presence of large amounts of water (Iow fuel to water mass ratio). And a such it is highly interesting from a scientific point of view and may be very rele-vant - in this special situation. But one cannot expect this same model (with the same parameter settings) to work property in a completely different situation in l which there is very little water present. The failure of this special interaction mo-1 del to predict strong steam explosions under conditions for which it wasn't de-signed does not necessarily say anything about the occurrence of steam e sions in situations as suggested by the premixing calculations should these ever occur. Especially in the case of larger melt masses (and possibly smaller over l 5
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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 c. mechanisms that may contribute to such alternate types of interactions are the thermal fragmentation mechanisms that may not need much water and are i
completely left aside in the present study. This might explain why the most effi-cient explosions are obtained very early (prior to 0.12 sec) followed by much les e
efficient interactions at later times in all cases with a finite breakup parameter.
4 The picture is less clear in the cases in which additional breakup was assum l
" not to occur. As outlined in the previous section these might be the most inter-sting cases in this study. Here no clear maximum of explosivity has been fou among the cases considered and it is argued that 'slightly broken up premixtures remain very benign.' However, Table 6.1 shows that in the case C2-nb the maxi-mum peak local impulse is 30 kPa.s which may already be viewed as a low to in termediate value and that it occurs at the last trigger time considered,i.e.1.0 sec.
Nothing in the results presented supports the idea that the value might not be larger (and maybe impor1 ant) at even larger triggering times.
1 There is a further and independent argument for early triggering. It states that I
early triggering is due to the interaction of melt (jets experiments at Forschungszentrum Kartsruhe. We have now performed 11 i tests and in 4 of these the melt was forced to interact with structures (vertical i
! on horizontal plate - in one case even equipped with compartments). Only one '
l of these tests (the last one performed on 2t August 1996) lead to aafter violent therm
! interaction (a weak steam explosion) about 0.8 sec (almost a full secondl) melt-structure contact [7]. One may also make reference to the KROTOS tests,in which the otherwise very explosive alumina melt settled at the bottom of the re-j action vessel copying its shape when solidifying, in cases in which the water was l
saturated and no external trigger was applied [5].
l 4. Summary low corium The affirmative final result of the study follows from three findings:
relocation rates, very high void fractions in the premixture, and, partly depend on that, effective explosions being possible only during a subsecond period beginning of premixing. I have serious doubts about all three of these. Wi!
spect to the relocation scenarios i doubt that the present state of know lows to definitely exclude downward relocation paths that could lead to muc l Not really being an expert in this field I must leave the ger relocation rates.
judgement to those experts, provided they can positively defeat my With respect to premixing the very high void fractions predicted by the co PM-ALPHA even outsides the gas channel that immediately follows a mass
- plunging into water don't seem to be supported b l ciently validated to support the high void fractions by itself. With respec l' explosions, the failure of the code ESPROSE.m,i.e. the peculiar inter in it (the microinteraction model), to predict efficient explosions in hi premixtures, doesn't prove that such explosions were not possibl diMerent interaction mechanisms, even if highly volded states would occur. l l
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- 5. Literature l
J. M. Broughton, Pui Kuan, D. A. Petti, and E. L. Tolman, A scenario of the
[1] Three Mlle Island Unit 2 accident, Nuclear Technology 87 (1989) 34 - 53 F. Huber, A. Kaiser, M. Steinbr0ck, and H. Will, PREMIX, Documentation of
[2] ' the Results of Experiments PM01 to PM06, Forschungszentrum Karlsruhe Report, FZKA 5756 (March 1996)
L. Meyer and G. Schumacher, QUEOS, a Simulation-Experiment of the Pre-
[3] mixing Phase of a Steam Explosion with Hot Spheres in Water, Base Case Experiments, Forschungszentrum Karlsruhe Report, FZKA 5612 (April 1996)
L. Meyer, The interaction of a falling mass of hot spheres with water,1996
[4] ASME/AIChe/ANS National Heat Transfer Conference, Houston, TX, August
' 3-6, 1996 H. Hohmann, D. Magallon, H. Schins and A Yerkess, FCI experiments in the
[5] aluminum oxide / water system, Proc. CSNI Specialist Meeting on Fuel-Coo-j lant Interactions, Santa Barbara,' CA, January S-8,1993, U.S. Nuclear Regu-
.latory Commission Report NUREG/CP-0127, NEA/CSNI/R(93)8 (March 1994) l pp.193-201 l
[6]. S. Angelini, T. G. Theofanous, and W. W. Yuen, The mixing of particle 4
' plunging into water,. Proc. 7th Int. Mtg on Nuclear Reactor Thermal Hydrau-September 10-15, 1995, 4
NY, lics NURETH-7, Saratoga Springs, NUREG/CP-0142, Vol. 3, pp.1754 - 1778 H. Will, private communication (to be presented at the OECD/NEA/CSNI
, [7] Spec. Mig on Fuel-Coolant Interactions, Tokal, Japan,19-21 May 1997) 7
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