ML20138C379
| ML20138C379 | |
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|---|---|
| Site: | 05200003 |
| Issue date: | 10/30/1996 |
| From: | Turland B UNITED KINGDOM |
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| ML20138C152 | List: |
| References | |
| NUDOCS 9704300101 | |
| Download: ML20138C379 (9) | |
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REVIEW OF DOF/ID-10541:
i LOWER HEAD INTEGRITY UNDER IN-VESSEL STEAM EXPLOSION LOADS by T G Theofanous, W W Yuen, S Angelini, J J Sienicki, K Freeman, X Chen and T Salmassi.
REVIEWER: B D Turland (AEA Technology plc, UK)
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1 I
OVERALL COMMENTS l
This report and its associated documents represent the culmination of several years work by Prof. Theofanous and his colleagues. They have now demonstrated that the basic framework for a steam explosion assessment in realistic geometry is in place. This is a major achievement.
The reliance on detailed modelling codes makes the reviewer's task difficult - in the end one can look at the validation offered and consider whether the results presented look reasonable. In the supporting documents the authors make good use of the available experimental data to benchmark their calculational models. However, it is accepted that some of the constitutive physics used in the premixing and propagation codes is uncertain, as are, to some extent, the melt pour characteristics.
A review, such as this, can indicate that the codes appear ' fit for purpose' but cannot give l
a full endorsement for all the models they contain, without significantly greater effort.
The situation considered in the application presented, a modest pour of melt into L
saturated water at ambient pressure, is not conducive to Iarge steam explosion loadings, and this is demonstrated by the calculations presented. Sufficient parameter variations are t
investigated to indicate that this is likely to be a robust result for these conditions. As indicated below, my residual concems relate to the confidence in having low pour rates
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and the possibility of operator actions leading to some subcooling.
SPECIFIC COMMENTS Chapter 2: Problem Definition and Overall Approach Although the text makes clear that it was an intentional conservatism not to claim L
credit for lower head venting in the Sizewell B study, it is wrong to interpret this in the phrase lower headfalure cannot be dismissed as readily any longer. We found 9704300101 970415 PDR ADOCK 05200003 A
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the previous claims for lower head failure could not be substantiated as large explosions did not necessarily imply sustained high pressures.
2.
It is arguable whether the ' essential basis for the current work' is the progress made in modelling explosion propagation and the pre-mixing phase, or in the assessment of melt progression.
3.
The statement that between 3 to 5 tons offuel must participate to produce a ! GJ explosion, and consequentiy incipient lower headfailure, ratses the question oi whether larger explosions are possible that do not fail the lower head.
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l 4
I agree that the size of any breach is indeed a tough question. I consider it to be the key question unless the mixmg/ propagation analysis by itself can be shown to be sufficient. I do not believe that this has been shown to be sufficient (as yet?).
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'5 The higher explosivity of a premixture with reduced voiding appears to be a conjecture that is not fully supported by the experimental evidence. Reported explosions in the JAERI AI.PHA facility occurred with large volds in the mixture region.
l 6.'
I need to be convinced that we need only to be worried about thefirst relocation event. I l
think that it depends on the timing of any subsequent relocation events.
Chapter 3: Structural Failure Criteria In principle, the loading may have components both shorter and longer than the 1.
natural timescale of the vessel. Indeed the constrained expansions considered in the early studies do have a longer tunescale. One needs to refer ahead to the results of the propagation modelling to justify the assumption through early venting of the explosion region.
The boundary conditions on the ABAQUS model are not speihd - from later 2.
examples they appear to be symmetry conditions at the equator of a sphere. As potentia 1 explosions may occur near the join of the lower head to the cylindrical section, it is not clear that this provides a good choice (apart from validating the simpler model-which could have been done in t-D).
3.
From a non expert viewpoint, the analysis presented in this Chapter appears a reasonable approach. However, I did note that Figure 3.9 was not consistent with Figure 3.4. To support the mitigative factor for local loading, more highly IWi=1 ABAQUS calculations should have been p fviaed. To avoid eso falling to zero for finite values of I and do/D.,
I suggest assummg that energy dissipation is proportional to the magnitude of the effective impulse.
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Chapter 4: Quantification of Melt Relocation Characteristics i
1.
It would be useful to give an indication of the diameter of the cooling holes.
2.
I do not see that the heat sink associated with the core support plate plays a significant role, as water provides the major heat sink. In the absence of water, melt passage through the plate would depend on the diameter of the flow channels.
Melt appears to have passed through relativeit small diameter holes in the presence of water at TMI-2. If these holes are similar to that of the hole in a PWR 1ower core plate, then they probably offer little resistance to melt flow. The lower core plate would prevent large d2ameter pours penetrating the lower plenum, if downward relocation were to occur, 3.
At this stage (page 4.1) translating We expect this path to be blocked into ' Downward relocation is physically unreasonable' still appears a large step.
Reference to TMI-2. Looking again at the TMI-2 melt relocation event, I am struck l
4.
by how far melt managed to progress downwards through the core, given the water inventory that is generally believed to have been present (muumum of 0.5 m above the base of the core). For instance at position K9 near the centre of the core there was evidence of previously molten matenal between rods near the first spacer gnd and in the spaces around the lower end fitting [TMI-2 Core Bore Acquisition Summary Report, EGG-TM1-7385, rev 1, February 1987, page B 30]. This relocation l
l was physically reasonable, as it occurred, but I do not see how it differs substantially from the claim that in the APR-like core downward relocation is l
' physically unreasonable.' I am happy with the notion that relocation into the bypass (most PWRs) or downcomer in the APR 600 is most likely - it is the degree
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of certainty that Iquestion.
The low melting point control rod matenals are expected to escape early (page 4-4).
5.
This seems counter to other arguments about heat sinks. However,if they do form a blockage, this may be relatively weak, 6 ving the potential for a later downward 4
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relocation.
I 6.
While the results on blockage formation appear realistic, the thermal equilibrium assumption in equation 4.1 is inconsistent with the growth of thermal boundary layers in the solid represented by equation 4.2. This may lead to an underprediction of the plugging time, particularly for cooler structures.
Tabte 4.1 - what is the meaning and significance of Meltfreering capacity as multiple 7.
of thefuel rod volume? For companson (1 think) one needs the channel volume i
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~4 divided by the fuel rod volume to ensure that there is sufficient heat capacity to form a blockage.
8.
Page 4-6: The effective thermal conductivity is taken as the volume-weighted average. Here and elsewhere it would be useful to indicate what physical properties were used.
The use of a volume weighted average is probably reasonable for this application (but not generally so). Was any allowance made for the porosity of the blockage in this evaluation.
4,
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Page 4-7. While low melting point components of the core such as control rods are expected to relocate as they melt, this does not apply to the major metallic component l
- Zr. Best fits to expenmental data indicate that relocation following clad breach occurs at temperatures in the range 2400 - 2450 K. Relocation involves a significant fuel component.
1 10.
Nomenclature: Equations 4.8 and 4.9 refer to Cr etc while Figure 4.6 and 4.7 have.
Cu,4. tetc.
IL The radial heat up calculations (Section 4.2) are qualitatively in line with sinular
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calculations that we performed for Sizewell. Was a radial power deposition shape factor used? We found that somewhat different results were obtained when we did
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the same calculation using a 2-D, rather than cylindrically symmetric model, that took account of the proper core geometry and the power rating of individual assemblies. A difficulty with both your and our modelis the absence of relocation, which may invalidate the model once any melting of material occurs.
The assumption of a fully oxidised pool (Section 4.3) may be inappropriate for a l
12.
low pressure sequence. This raises issues on the interactions of the corium with a more metallic blockage (partially addressed in the MP tests). However, to conclude i
that something is ' physically unreasonable' all processes that may have an impact should be discussed.
J The proposed melt release conditions and mechanism appes reasonable. The 13.
dimensions and the pour rate are no more than educated guesses ( I would probably have made similar guesses). I note that to achieve the melt flow rate of Im/s, only a 5 cm driving head is assumed, although there is no quantification of l
how close to the top of the pool the breach might occur. It is desirable to analyse whether heat transfer from the melt stream through the breach may deepen the breach and lead to an increase in pour rate.
Chapter 5: Quantification of Premixtures f
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Is there any likelihood for this plant of subcooled water in the lower head (eg in an extended accident sequence with some injection)?
2.
The comment that the break up parameter p set to 10 produces very rapid break up in ~10 cm of water suggests that the modelling is somewhat more efficient at producing fragmentation than origmally desired (break up in a specified fall distance taken as the smaller of the actual fall distance or p.Dr ). This also depends greatly on the assigned value of Dr - here set to the initial particle size (20 mm). If the melt was assumed to fall as a thinning sheet (quite possible) then the initial penetration of the water may be more local than represented in the PM-ALPHA calculations. However, I am happy with the range chosen for p.
. Please note that in Figure 5.2 and Appendix B the void is represented by shading, 3.
the fuel by contours. Explain the contours that follow the domain boundary.
4.~
Specify the boundary conditions for the calculation. What pressunsationis predicted?
5.
The length scale increase referred to on page 5-5 is not evident in Figure 5.4. The area averaged over is not clear, it is obviously not the whole cross-section. Since writing this I found the 1% fuel volume fraction limit on tim region considered in the text - for clarity add to caption of Figure 5.3.
Middle of page 5-10: 'Only a very small fraction of the coolant is found to co-exist 6.
with the water'
- I know what you mean! It is clear though that here we have the key result anticipated for the mixing codes. This implies that the key region to seek validation of the code is in the production of the high void fraction.
In my view the THIRMAL calculations raise as many questions as they answer, 7.
because of the poor validation status of any jet break up model. However, I do not thmk this is a key part of the argument.
Chapter 6: Quantification of Explosion Loads 1.
Where is the trigger cell?
At what time was the effective area evaluated - that of peak pressure? If not, you 2.
obtain larger effective areas than ~0.1 m2 The question raised by the calculations is how far is one from the dan 8er zone?
i 3.
Could we get there by a modest increase in system pressure (what value was assumed?) and/or varying the value of p?
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Chapter 7: Inte5 ration and Assessment 1.
The conclusions reached are justified on the basis of the analysis presented. On the _
basis 'of current knowledge I am still not comfortable with the observation that downward relocation scenarios are ' physically unreasonable'.
r I agree that there is a greater threat from subcooled conditions. It is not obvious,
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though, that a ' highly subcooled pool' is necessary. Perhaps this might be illustrated by a calculation with modest subcooling (eg 10 degrees) to show there is no threshold effect.
4 Chapter 8: Consideration of Reflood FCIs i
This chapter has not been considered in any detail The arguments presented 1.
appear persuasive provided that are no other means of fast reflooding not L
considered by the authors and that crust formation proceeds in the way that they envisage.
j.
Chapter 9: Conclusions I have indicated above that my pnncipal reservations lie in the areas of the a
1.
downward blockage and in ensuring that there are no operator actions that may prejudice the assumptions made in the analysis. I agree with the authors that, consideration of additional pathways is unlikely to change the conclusion.
For this application, the supporting analysis ought to concentrate on the melt 2.
relocation scenario. This would include obtaining a better understanding of melt relocation in TMI-2 (eg why did it occur after reflooding the vessel?), to demonstrate that the processes are indeed understood.
It would have been useful to have an indication of the effects of uncertainty in the 3.
constitutive laws (eg microinteractions) to detennine where confirmatory studies are required.
Comments on DOF/ID-10503: Propagation of Steam Explosions: ESPROSE.m Verification Studies Only a limited time was available to review this supporting document.
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.m Much of the document is concerned with the ability of the ESPROSE.m code to represent the wave dynamics correctly for single and two phase regions in one and two dimensions.
l The information presented, along with the compansons with the SIGMA expenments J
with a voided expansion region, indicate that this part of the code is doing its job correctly, even when relatively coarse (~0.01 m) meshes are used. This does not surprise me.
Numerical studies we performed when extending CULDESAC from one to two dimensions indicated good capabilities to capture the wave dynamics with relatively simple numencal schemes (the numerics of propagation are simpler than those of pre-i nuxing). I am therefore satisfied with the code's capabilities in this area and would expect I
that the 3-D version of the code would also perform satisfactorily in this respect.
t While Chapter 2.1 uses a homogeneous model for the two-phase behaviour (by forcing large drag between the phases), it is unclear whether the calculations reported in Chapter 2
2.2 still use this model. If not, it would be interesting to compare how much better the full i
model performs against the expenmental data, compared with the homogeneous model.
The authors of ESPROSE.m have implemented an, at the time, novel approach to cover lack of thermal equilibrium in the coolant during the propagation. This approach is j
physically based and can be considered 4o be well-justified.
l I
The application of the ESPROSE.m code to steam explosions depends on the assumed i
constitutive physics. As Appendix D (particularly figures D8 and D9) illustrates, the j
assumed parameters of the microinteraction model can have a major impact on the prediction (eg changing the parameter for coolant entrainment can change the C-J i
pressures by two orders of magnitude). Appendix C provides results from a series expenments with one high temperature simulant, that has been used to modify a hydrodynamic fragmentation model to take account of thermal effects. This approa acceptable, but the range of uncertamty in the model parameters needs to be allow j
in any assessment.
The authors note that 'the main need identified is for constitutive laws for microinteractions with reactor matenals' (Abstract} - I agree. They also claim that
' reasonably conservative awaments are possible' - however the main report does not indicate what parameters were used to obtain a sufficiently conservative assessment.
I would have expected to see more discussion of the comparison with KRMOS experiments in the report as origmally supplied, rather than a reference back to Although there are some limitations on knowledge of the initial conditions and, in m the tests with explosions, some loss of data, these provide the greatest confidence in th application of any model to the steam explosion propagation phase. The calculatio l
KROTOS 38 provided as a supplement are useful. With current knowledge it is more s
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important to be able to demonstrate conservatism in the calculations rather than good agreement through parameter adjustment. Recently I saw calculations with TEXAS-IV for this test, where a very different melt distribution was calculated that led to very good agreement with the observed pressunsation following the trigger. Until there is a visual record of such tests it is not possible to detemune which simualtion is closer to reality.
P In reading the material, I noted a number of examples where detail was not clear to me.
These are listed here for convenience, but have no impact on my overall assessment of the methodology:
i l
1.
In chapter 2.1 what value is used for Pi? Figure 6 a implies 100 bar, but elsewhere i
i finite results are given when P21s only 10 bar.
l 2.
In figs 7 and 8 of Chapter 2.1 a is shown as varymg. I assume a is a void fraction -
of which region?
Chapter 2.1 presents results with and without phase change of the gas. It would 3.
have been instructive to see a direct comparison to illustrate the importance, or otherwise, of the phase change on wave propagation.
i 4.
I had difficulty understandmg the location of the pre-voided region discussed in Chapter 2.2. Note that Fig 7 is incosst!y referred to as Fig 8 in the text. If for Fig 3 the pre-voided region stretches to the base of the tube, I do not understand the respective difference in timings of (1) the time between the shock arriving at PI3 after Im and (2) the time between the shock arriving at PT3 and its reflection from the base arriving at FI3. Note that you have offset the pressures in the figures for ease of presentation.
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%.4L, rL iAoibir M 65i?%id M UELEE3Di?%%E WEE 5Y21119 Ef?A WS?NEiM From: Brian Turland Sent: Friday, November 29,1996 9:59 AM -
To:
Reactor Engineering
Subject:
Review of DOE /ID-10541
- rucmooc Walt Deitnch Reactor Engineenng Argonne National Laboratory
Dear Walt,
When i sent my formal comments on DOE /ID-10541, I promised to send i
some additional comments on the supporting document concemed with venfiaction of the pre-mixing code, PM. ALPHA. These additional set of comments are attached as a MS-WORD 6 file.
I had used my full allocation of' effort for the review in preparing the original set of comments, including a preliminary read-through of the PM ALPHA report. Re4eading the PM ALPHA report,some Investigative work, and preparing the wrtten comments has taken an additional 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br />. I am prepared to cover this in my own time, but it would clearly be preferable for myself and my orgnisation if I could be paid for the work. I believe from Stephen Sorrell that some additional fundign may be available.
The additional time spent has arisen in part from the volume of the material supplied for review, the f act that different reports came at different times, and the time I spent uncovering some inconsistencies in the structural analysis (communicated to UCSB) earlier.
Thank you, again, for asking me to perform this review, Best regards, Brian Turtand Phone 41305 203029 Fax +441305 202508 e mail: brian.turlande aeat.co.uk 9