ML20138C419
| ML20138C419 | |
| Person / Time | |
|---|---|
| Site: | 05200003 |
| Issue date: | 08/23/1996 |
| From: | Bankoff S NORTHWESTERN UNIV., EVANSTON, IL |
| To: | |
| Shared Package | |
| ML20138C152 | List: |
| References | |
| NUDOCS 9704300111 | |
| Download: ML20138C419 (6) | |
Text
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ogl2/96 IgT* 09 :A7 FA.I L 630 2s3 4750 ANL-RE
- s. s0RRELL 4 o03 REVIEW OF DOE /ID-10541 AND SUPPORTING DOCUMEN S. G. Bankoff
GENERAL COMMENT
S The principal documents which were read by the author were DO i
1996), DOE-10460 Vols.1 and 2 (July 1995), and DOE-10849 (Jan.1 papers published and/or presented by Prof. Theofanous and o My general impression is that this is a massive piece of work d
tes steam explosion problem in the Westinghouse AP-600 reactor, and co ll onable. If no that failure of the vessel, to say nothing of the containment,is physica y unreas l
failure occurs in the reactor vessel, essentially no release can occu d In containment building, and hence the threat to the public health and safe i
ts development of new the ' process of developing the evidence in terms of focused exper men,
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fr and improved codes, tying in work done around the world, and d d
t of tools assessing the safety goals and margins for rare, but high-consequence has been developed which represents a huge step forward in e new types of advanced nuclear reactors and in existing nuclear reactors.
i In other words, in execution, scope and potential consequen represents a very important achievement.
SPECIFIC COMMENTS l
I think that the ROAMM approach makes very good sense for r
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consequence, events. I believe that a similar approach has bee l
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explicitly and clearly stated. In particular, the recognition that there are " int will never be known in advance, conservatively bounding them at each stage, and th 2
enveloping the pdf passed on to the next stage, makes the uncertainties clear.
There is no estimate of the conservatisms induced by ignoring fluid-structure 2.
interactions, assuming plastic flow with no strain hardening, and using a 3
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stress (330 MPA) compared to (be measured yield stress (450 MPA) j be helpful.
All the codes (ESPROSE.m ABAQUS and PM-ALPHA) appear to be 3.
against available data, and have been conservatively adjusted.
4 I would thirdt the result that the greater the load localization, the sm 4.
impulse, has practical limits. For a perfectly plastic material, a delt l
d source) will always penetrate continuously. The introduction of bending f d h use of the the load, of course, represents the real situation. However, how ar oes t e effective impulse go for failure criterion with a very concentrated load?
i The net result, from the axisymmetric and non-axisymmetric calcula l'
5.
details of the loading pattem are not particularly important is, in itse i
result would seem to hold not only for the AP-600 reactor, but for a 1
reduces the probability of local failure.
i All the prei@e work that I am famdiar with (including my own) a 6.
corium jet falls into the lower pool by melt-through of the LSP (low great deal of work has been done (and is contemplated for th d
his various materials, jet diameters, temperatures and velocities, that is predicat h
h the assumption. The unexpected result that for the AP 600 melt through w 4
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j side, rather than the bottom, of the melt pool, is therefore of first importance.
l happened at TML but in AP 600 there is a thick stainless-steel reflector code capability allows calculation of the subsequent premixing and melt / w l
There can be no large melt jet in transit; stratification appears very likely the whole scenario of damaging muldple explosions disappears.
The simple models for blockage formation and blockage coolability, leadi 7.
dible availability of downwards relocation paths, and transidon to a molten pooI l
h AP-600 design. This lumped
. because of the relatively flat radial power distribution in t e approximation would have to be re examined for other reactor designs.
l The calculations of melt length scales and local void fractions lead to qua S.
id results which are more realistic and detailed than previously available. >
di f such water is rapidly depleted from regions of high fuel concentration, and the bo k
regions can be quite sharp. Board Hall thermodynamma theory for s i
previous multiphase calculations of the flow fields behind the sh Bankoff) agree that regions of large void content cannot sustain shock pro supercritical pressures. This is the principal reason that the SERG-2 h
mode failure was not physically reasonable. Precisely the same results PM Alpha and ESPROSE.m calculations.
The residual uncertainties proposed in NUREG-1524 were jet bre 9.
jor vs. 3D codes, and chemical augmentation. For the AP-600, jet breakup is n concern, as discussed above. 'ne 2D vs. 3D controversy is no longer r i h the real corium produced validated 3D codes are now available. Chemical augmentation w t i h will in the reactor will have no important effects. Triggering is the sole intangi i
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- 3. 5v&AILL gaud 09/,$pS6 TEU_09:As FAI A 630 as2 67s0 a,sL-RE never be known for a real accident. However, it is irrelevant if it is assumed that triggering 3
always occurs at the worst time and place, and the resuit is evaluated by energetics, to say the validated codes. The approach taken of triggering by setting one mesh t initial pressure seems to me to be a perfectly valid procedure.
SUMMARY
Convincing arguments have been addressed, backed up by a huge volume of h quality experimental, analytical and computational work, that the AP-600 in the course of a severe accident. This implies that all later scenarios of contamment-building pressurization and heat-up are no longer necessary. In my opinion, this 3
severe accident scenario for the AP-600, and leads to consideration for licensing.
consideration of other reactor types, on the other hand, does not appear to be so straightforward, and further work needs to be done.
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PAUL SHEWMON 2477 Lytham Rd, Columbus, OH 43220 Tel. 614-457-4378, Fax 614-457-2995 August 6,1996 Re: Evaluation of report by T.G. Theofanous, et al., " Lower Head Integrity Under in Vessel Explosion Loads", DOE /ID-10541 The analysis of head failure sets up a model of the lower head using ABAQUS (a well established finite element code) to relate the stress pulse from steam explosion to local strain. The vessel material (ferritic SA508 steel) will undergo large amounts of strain (elongations of 50 to 100%) before fracture occurs. Whether or not the vessel undergoes any plastic strain depends on the yield stress of the metal and the impulse from the steam.
For reason never explained or discussed, the authors chose 330 MPa for the yield stress of the vessel. They state that the conservative ' Code Allowable' is 345 MPa, and the actual value (found in a conventional tensile te.t) is 450 MPa. The choice of 330 MPa introduces a large conservatism (safety margin) since a best estimate should use 450 MPa.
The impulse applied to the steelin the lower head would have a rise time of a few milliseconds. When ferritic steels are loaded this quickly their yield stress is substantially greater than that observed in a normal tensile test. The authors quote references that show the yield stress at this strain rate is about 40% greater than that found in a tensile test. They take full credit for this strain rate increment, which is justified and appropriate.
In summary, the analysis of head failure seems to be competently and conservatively done, and the conclusions drawn are appropriate. I have also looked at the discussion of loads and loading. I am less of a specialist in this area, but it also seems to be well done.
Though no mention of radiation effects is made in the report, the analysis should be made for the vessel at end of life (40 years?). The temperature of the head during the accident considered would be less than 212 F. This is beneath the RNDT for the beltline of some of the vessels now in service, i.e. such material might well behave in a brittle manner during an accident of the type considered here. I considered this, but feel such radiation effects are not germane in the case of the AP600 for at least two reasons:
- 1) The fast neutron and hard gamma flux in the lower head will be at least a couple of orders of magnitude less than that in the beltline region of the vessel, so radiation j
effects should be negligible.
- 2) The steel to be used in the AP600 vessel should be appreciably lower in the elements than have lead to radiation embrittlement (copper, and phosphorous) in the older vessels now of concern in plants in the U.S.A.
With this in mind, I believe there is every reason to believe that the materialin the lower head would behave in a ductile manner and that the analysis given in the report is appropriate for (would apply to) a vesselin the AP600 after 40 years of service.
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Paul Shewmon
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Dear Dr. Deitrich,
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