ML20138C193

Review of DOE/ID-10541, Lower Head Integrity Under In-Vessel Steam Explosion Loads

ML20138C193
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Site:	05200003
Issue date:	01/08/1997
From:	Butler T LOS ALAMOS NATIONAL LABORATORY
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References
NUDOCS 9704300063
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LE'-17-1996 la:15 FRCf1 UC5B ENGR. REFEI'RCH CTRS. TO 672091912085266249 P.01 l Review Comments on Report DOE /ID-10541, " Lower Head Integrity Under In-Vessel Steam Explosion Loads" Tom Butler Group ESA-EA Los Alamos

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INTRODUCTION AND

SUMMARY

The comments contained in this review are restricted only to a review of Section 3 ,

of the subject report. The report's authors have done a goodjob of scoping the possibilities of failing the lower vessel head under the assumed loading conditions. Well established analytical approximations were used to establish the validity of the finite element model that was developed to study local failure of the head. A more detailed model needs to be developed to include transverse shear effects and to simulate failure of damaged elements dadng the course of the calculation. This lack of simulating progressive failure is the weakest point of the analysis. Appropriate simulation of progressive failure has to be included in order to obtain defensible results that can be included in probabilistic evaluations.

SPECIFIC COMMENTS Finite Element Modch Use of the shell elements in ABAQUS is acceptable for determining the distribution through the thickness of all components except forgansverse shear. In the AB AQUS thick shell elements transverse shear is approximated by constant shear through the section. This is notjudged to be adequate for evaluating the possibility of a shear type of failure. A better method for getting good approximations for all of the strain components would be to use several continuum elements through the thickness rather than the thick shell element.

Use of many more elements would make the runs longer, but use of the explicit version of ABAQUS would help in this regard (see below). In addition, the use of an axisymmetric finite element model would afford the opportunity to use a much more dense mesh in the i 9704300063 970415 PDR ADOCK 05200003 A PDR

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TEC-17-1996 10:15 FROM UCSB ENGR. RESEMCH CTRS. TO 672091912

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j analysis with run times that are still relatively short The structure and all of the loads that were considered are axisymmeuic.

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The mesh should be considerably more dense in order to resolve Sne details in the strain distribution, especially those details relating to strains other than in-plane strains.

Referring to Figures 3.5a-c, even the in-plane strains vary from their maximum levels to just half that level overjust one or two elements.

i Although not stated in the report,I assume that the implicit venion of the ABAQUS

code was used for these calculations. The implicit version is always stable but may not always be convergent. There is no indacation in the report as to wherber the time step was varied to ensure a convergent solution. A better alternative rnay be to use the explicit venion of ABAQUS for the short transient solutions that are required for the types of loads being considered here. The explicit version of ABAQUS also offers the opportunity to use 2- a failure roodel that would give more realistic failure predictions (see below).

The statement that the time duradon of the loads is less than the natural frequency of l the head may not be correct. A h=M solution of the L@cacy of a full sphere with the same dimensions as the hemispMa! head gives a natural period of 1.5 ms, very near

! to the 2 ms pulse duration used in this study. It is no wonder that, as stated, the impulse ,

e i time is "non-negligible."

i Load-Strain Behavior:

Use of the Bodner and Symonds approximation for the dynamic yield stress is a reasonable approximation. However, use of the values assumed for the constants D and p

! should be justined more thoroughly. De values used here are for mild steels, and may not be appropriate for the alloy steel that is used in the pressure vessel being ev@aM Obtaining a good approximation for this relation is particularly important because the maximum strain is very dependent upon it. f used an axisymmetric model with 15 continuum elements through the thickness to replicate some of the calculations in the report.

f The results showed that the maxunum stram went from 0.52 to 0.16 with addition of the rate model for the yield stress. Considering the magnitude of this difference, one should

cettainly be very carefbi in the selection of the rare parameten.

Dexter and Chan (1990) address the effects of stram rate and temperature on A533B stw!s. This alloy is close to A508 steel and may provide some usefbl information in i developing an appropriate dynamic yield stress model

UCSB ENGR. RESEff<CH CTRS. TO 672091912085266249 P.03 DEC-17-1996 10:16 FROM l Failure Criteria and Fragility:

i This is probably the most difficult aspect of modeling the response of the vessel i head. The failure criterion that is used in the report is probably realistic and conservative i except for one important aspect. The model. as sper d here does not remove the load carrying capability of elements that have exceeded the failure criterion. Maintaining the }

load canying capacity of damaged elements can give significant over-estimates of the l capacity of the structure. I used the explicit finite element model mentioned above to look j at this aspect of the problem and found that, depending on tbc parameters used for the j l i ABAQUS failure model, the head could fall for the loads that are spmbd. I strongly j suggest using some sort of failure criterion embedded in the computadonal model for future !

! calculations.

The subject report briefly mentions the effect of stress anisotropy on the failure '

strain. This is an important issue and needs to be more fully evaluated. The work referenced in the report by Pao and Gilat was pe.fesned on Charpy bars (roughly uniaxial l

j strain) and by Shockey et al. was pfessed in pure shear (no hydrostade component).

Therefore these data don't address the important effects coming from multi-dimensional stress fields. Data summarized by Ju and Butler (1984) show that A533 alloy steel when in equal biaxial tension fails at an equivalcat strain equal to about one third the strain for 4 uruaxral tension. Equal biaxial tension is the stress state at the " pole" of the lower head where failure would first be expected. The alloy content of A533 steel is similar to that of I the A508 steel considered in the subject report. Mirza, Bartos . and Church (1996) reported the effect of the stress field on failure strain and its effects in transMoning from ductile to brittle failure characteristics. Johnson and Cook (1985) also discuss the efracts of the stress field on fracture of ductile metals. Other references that may be of help include Jones and Shen (1993) and Ferron and Zeghloul (1993).

As previously mentioned, the head would have to be modeled with continuum elements to accurately predict transverse shear strains. In addition,4 failure criterion for l

transverse shear needs to be established. It is untiltaly that the failure criteria discussed in the above references are adequate. They may however give sorne guidance in establishing the appropriate criteria. It is possible that when the loading conditions are investigated

> mote closely, the load cases that lead to the highest shear. load (such as case 1+) can be eliminated obviating the need for this criterion.

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MC-17-1996 10816 FRCN UCSB ENGR. RESEARCH CTRS. TC 672091912085266249 P.04 4

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MisccHam:

1) The use of the higher yield stress 450 MPa is justified based on actual data from Server and Oldfleid (1978) where the average yield stress is approximately 440 MPa for A508 steel (very close to the Japan Steel Works Ltd, value of 450 MPa). This is one parameter with ample data to support the use of the actual, as-tested value.

2) The statement is made that A533B steel has a carbon content of 0.19 vs 0.16% for i A508. Information from Server and Oldfield (1978) and the ASME Code show that A533 has a carbon content of 0.25% maximum and A508, Class 3 has the same upper limit for o carbon conteat. Actual analyses show carbon content from 0.21 to 0.25% for both steels.

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3) Chapter 3 in the subject report does not mention radiation embrirtlamant effects. If they can be dismissed, the reasons should be given.

4) For SA508 the transition from ductile to brittle behavior starts at about room

! kWore. The report should give the approximmen marerial temperatures during the peut *d event to show that it is well above room temperamre.

5) The presence of flaws is not addressed. I assume that in service inspection will have identified any that are significant in affecting ductile fracture.

i Refesences Dexter, R. J. and K. S. Chan, "Viscoplastic Characterization of A533B Steel at High l Strain Rates,'" Joumal of Pressure Vessel Technology, Vol 112,(1990) 218-224.

1 i Ferron, G. and A Zeghloul, " Stain LWWadon and Fracture in Metal Sheets and Thin-l Walled Structures," in Structural Crashworthiness and Failure, N Jones and T. Wierzbicki, Eds. (Elsevier Applied Science, London and New York,1993). Chap. 4, pp.131-163.

f Johnson, O. R. and W. H. Cook, " Fracture Characteristics of Three Metals Subjected to Various Strams, Strain rates. Temperatures, and Pressures," Journal of Engineenng i

Fracture Mechanics, Vol. 21, No.1, (1985) 31-48.

Jones, N. and W. Q. Shen, "Criseria for the Inelastic Rupture of Ductile hual Beams i Subjected to Large Dynamic Loads," in Structural Crashworthiness and Failure, N Jones L and T. Wierzbicki, Eds. (Elsevie.t Applied Science, London and New York,1993), Chap.

. 3 pp.95-130.

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DEC-17-1996 10217 FRO 1 UCSB EN3R. RESEFRCH CTRS. TO 672091912005266249 P.05  ;

l;', l Ju, F. D. and T. A. Butler, " Review of Proposed Ductile Failure Criteria for Ductile Materials," Los Alamos National Laboratory report LA-10007-MS (NUREO/CR 3544),

April 1984.

Mirza, M. S. and D. C. Bartoa, "The Effecc of S:ress Triaxiality and Strain-Rate on the ,

4 Fracture Characteristics of Duetile Metals," Jottnal of Materials Science, Vol. 31 (1996) 453-461.

Server, W. L. and W. Oldfield, "Nuc! car Pressure Vessel Stect Data Base " Electric Power Resean:h Institute report EPRI-NP-933, December 1978.

4 TOTAL P.05

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. THE UNIVERSITY OF SYDNEY l

Y [Cl yemm DEPARTMENT OF CHEMICAL ENGINEERING l

NSW 2006, AUSTRALIA g

4.> 3,yf - l TELEPHONY.: 61 2 9351 4147 FAX: 61 2 935l 2854 j DX: 1154SYmTY 1 EMAR.:

davidf@cheuteng.usyd.edu.au j Dr. David F. Fletcher

. Samar Resterch fedo'r Dr. L.W Deitrich, Argonne National Laboratory, 9700 South Cass Avenue, a

Argonne, IL 60639.

USA.

Dear Walter,

Please find enclosed my review of the DOE project on " Lower Head Integrity Unde In-vessel Steam Explosion Loads" by Theofanous and co-workers. As you will see from the review I judge it to be an excellent study in tenns of its depth, scope, technical quality and shear volume of work. I fully agree with the conclusions dra the authors.

Yours sincerely,

,pd F. N~

David F. Fletcher (Dr.)

cc. Prof. T.O. Theofanous RECEIVED

, REACTOR ENGINEERING D!VIC ~ ;l

! -DIRECTOR'S OFFICE- .

1 NOV 20 996

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