ML20118D256
| ML20118D256 | |
| Person / Time | |
|---|---|
| Site: | Dresden  |
| Issue date: | 11/17/1965 |
| From: | Fraley R Advisory Committee on Reactor Safeguards |
| To: | Advisory Committee on Reactor Safeguards |
| Shared Package | |
| ML093631134 | List:
|
| References | |
| ACRS-GENERAL, NUDOCS 9210120042 | |
| Download: ML20118D256 (12) | |
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SUBJECT:
STATEMENT BY T. J. THOMPSON RECARDING REACTOR PRESSURE VESSEL FAILURES Attached for use at the ACRS meeting on November 22, 1965 is a statement by Dr. Thompson regarding reactor pressure vessel failures.
Attachment:
A Review of Pressure Vessel Failures as it May Affect Future Reactor Designs and Particularly Dresden II, by T. J. Thompson, dated 11/16/65 s
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A REVIEW OF PRESSURE VESSEL FAILURES AS IT MAY AFFECT FtriURE REACTOR DESIGNS AND pARTICULARLY DRESDEN_I_I__
The discussions which have developed on tha Dresden II Reactor are basically the c:ilmination of a growing concern on the part of a number, if not all, of the Ccmsnittee Members as to whether or not a pressure vessel of a water-cooled power reactor-41ther pressurized water or boiling water--
can fail in such a way as to breach the containment during an accident.
I know of no. way of stating conclusively that this problem is more serious i
with one type of water-cooled power reactor than the other. There may be, l
in specific designs, some difference in the probabilities of such an occur-rence or difference in the -hitity of the system, as designed and built, to withstand at least the more minor incidents of this type.
However, I do not believe that these differences are m re ths.n a fine structure to the overall problem. It may be that there will be differences in the cost to remedy the situation v.ith the different kinds.of reactors, but I have not looked into this and, again, I believe that it is a function of the specific design of the system. Therefore, in the discussion below I will assume that we are dealing here either with a pressufized or a boiling water reactor.
It is clear, and I believe it has been from the beginning, that the likelihood of such an accident is extremely small.
I personally became aware of this problem when I was first introduced into the field of power reactors in 1955. Since that time the general problem haa been of concern to me and I have continued to question my more knowledgeable peers in metal-lurgy and in the presaure vessel business as to the likelihood of such a failure.
Before coming on the Comittee and since, as a meder of the Com-mittee, I have received in reply to my questions a steady stream of reassur-ances that such an occurrence was incredible.
At this point it is perhaps worthwhile to digress suf ficiently to point out that the word " incredible" differs from the word " impossible".
l The dictionary defines incredibility as the state or quality of being un-believable or hard to believe--an unbelievable thing. In my mind, and I believe in most others in the nuclear industry, the word " incredibility" has taken on a dif ferent connotation than " impossible". Harold Etherington in questioning Mr. Joslin of Commonwealth Edison it, regard to Dresden II defined " incredibility" as being something that would be unbelievable
'to a reasonable man" and differentiated it from something that was physi-cally impossible under the laws of nature. Obviously, a high pressure cannot exist within any confinement in such a way that it will be physi-cally impossible for this pressure to relieve itself into a surrounding low pressure.
Potential energy will always tend towards a minimum and, therefore, the fracturing of a pressure vessel is certkinly not impossible--
even ductily.
It is therefore certainly not impossible that such a rup-ture could occur in such a manner as to breek the containment existing around the reactor.
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!E At various periods during the past ten years I have spent considerable time questioning various reactor groups concerning their methods of selecting bolts, their methods of tigl.taning bolts includ-ing use of bolt beaters and torsion wrenches and impact wrenches of various types, the care with which belt tightening was carried out, and the methods of selecting and checking the performance of bolts.
It has always seemed to me that this is the most vulnerable area and I continue to believe that thir, is probably true. More knowledgeable people than I in this field have continued to assure me that bolt fail-ures in such systems will normally first be observed by a leaking in
<.a ragion and that this leaking is of such a nature as to give a fore-warning of an impending difficulty. It has beco.ne com:non practice in i.he reactor industry to provide open microphones in isolated areas to insure that audibis means exist for hearing the initiation o f such leaks.
In at least one reactor, the VBWR, such a microphone was instrumental in detecting a cracked major primsry pipo.
Prior to the DresJen II case there had been at least one yesr during which no major reactors had been considered for construction per-mits. Ecs.ever, during this same period several evsnts have occurred c
wi.ich, mhi,'y or not, have led to a growing concern on the part cf the Membership the Conunittee.
First, there was the failure at a tempera-ture near NW of a very large heat exchanger under test by the Foster Wheeler Corporation.
(On the plus side, it is clear that failure started at a highly stressed weld point and further, that the failure did occur at or near the appropriate N M temperature. Further, since the N M tem-perature in general is in the region from 0 to 1000 F the stored energy in the system at that time in the reactor is relatively small and, hence, the brittle fracture of a reactor vessel at temperatures below the boiling point of water at atmospheric pressure would normally not be expected to have sufficient energy to rupture the containment structures of any exist-ing water-cooled reactors to date. However, when the NM temperatures with appropriate safety margins began to get in the region above 2000 F, it is absolutely essential to insure that the vessel is well above unsafe tem-peratures before high pressures are applied if brittle fracture is to be factored out.) The review which the Comittee held in March of 1965 in regard to pressura vessel and piping failures and to methods of measuring the nil ductility temperature was, I believe, disconcerting to all members who attended. For my own part, my most serious worries arising fro:n that meeting concerned the apparent inability to determine with any kind of accuracy the existing HM in thick slabs of steel.
Further, I personally am now having some difficulty in knowing what N M is the limiting N M in slab in which the NDT my vary through the thickness of the slab.
Is it the surface or quarter thickness NM that is the correct one in trying to judge the possibility of brittle fracture or is it some other value? These con-cerns, brought about by the Foster Wheeler failure, by the meeting of the Consnittee in the spring, and by recent experience with pressure vessels and
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piping in other reactors, the increasing radiation exposure on those reactors
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now in pertormance have att teacoa to point to cna nesa for a generai ruvio 0FTI(Tp ${g_ggag _
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g Initially, the reactor sites chosen for power reactors were relatively remote and it could be argued that the likelihood of en-dangering any human lives, or at most very few, would be an acceptable risk in the event that the incredibile happened and that somehow the containment vessel vere ruptured. Since the initini reactors have been built, however, power levels and inventories have been going up as much as a factor of ten or more, the sites chosen have been in more populous districts, and even near old sites already considered populatio-s have been growing rapidly. All of the factorn cited above point to the fact that this is a good time fo a review of the general situation. It is unfortunate 'that Dresden II happened to be the first new major power reactor to be considered for a construction permit since this concern has been reopened. It is particularly unfortunate that the final re-view for this reactor should occur at the same aession when another pressure vessel problem had to be considered. I am confident that the existence of one such problem in an operating reactor will not away the judgments of the Cemittee a regard to the case of Dresden II.
It seems to me that there are a number of fundamental questions which should be 6sked before we reach a solution to the problem now f acing us.
These include:
1.
Has the evidence developed in the last few months or year
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shown that we have underestimated the likalthood that a failure of a pressure vessel may occur in a catastrophic way such es to rupture the containment?
2.
Are the present or future site locations, the increasing fission produer inventortes, and other f actors sufficiently different from tne situations maidered in the earlier reactors that we should require e:nplete protection from all missiles, including the pressus, vessel itacif, of the containment?
3.
Should this containment be required of Dresden II?
Concerning question No.1, I do not have a definite positior. There are two facets to this problem, it seems to me.
First of all, we may have been underestimating the likelihood of this type of accident from the very beginning. If the under-estimate is serious, perhaps all power reactors should be reviewed to see wha. can be done. Since there appear to be no reported accidents of this type from i.ajor pressure vessels in the industry, the likelihood must be small, although clearly not zero. Therefore, we probhbly do not have any basi.a for a complete review. The x
second facet of the problem concerns the changes in technology which have occurred since the first reactors were reported on.
Clearly, the understanding available in the field of metallurgy has increased greatly during the past few years. It is also clear that the techniques and the facilities avsilable for I.
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New codes have been prepared and these codes provide better definitions and require more careful analysis than the older codes. At the same timo, the reactor vessels have become larger and the required thickness has become greater. %e discussions we heard last spring would indicate that the NDT temperature is a function of the depth at which the sample was taken through the thickness of the plate. While Section III of the code is indeed;soreostringent in its requirements on stress analysis, 3
at the same time it has reduced the factor of safety from fcur to three. Improvements in the understanding of metals and in the methods of fabrication and in specifying and inspecting such vessels have clearly been nade. he problem that I have is in dstermining whet fraction of these improvemente have gone into increased safety and what fraction hava gone into helping the economics, as for instance by reducing the required thickness of 5
pressure vessel walls and hence reducing costs. I have no way of knowing what this split may be, but I am under the general impression that the pressures during the past few years have been primarily those of economics since there have been no serious failures within the pressure vessel industry or reactors to indicate that the codes then in existence or now to existence are not sufficiently ccuservative. It is for this reeson that I made the statement at the last meeting that I would have to judge on the basis of my limited knowledge that a pressure vessel or a reactor to be fabricated in the near futtr e might be considered to be in general as safe as some of those fabricated for earlier power reactors. I was discussing this matter on Wednesday, November 10, with one knowledgeable member of the Committee who stated that he was not sure what a factor of three or four in safety really ecant in regard to the safety of the vess21. For instance, as I understand it, the factor of safety in design on many key components in aircraft is two.
A factor of safei.y of three or four might very well mean that no airplane could fly.
On the other hand, it might be that a system with a factor of safety of say 1.5 or even less might be operated forever with cceplete safety. Much boends upon the methr,d of use of the system, the likelihood c over-stressing and u:any other f acters.
Thus, when I said that it seemed to me that it was lilely that the old vessels might be as safe as the new ones so long as both
~ were operated properly within their known bounds of capability and performance, I was saying that the tsw vessel designed under Section III of the code with its reduced factor of safety might indeed be either more or less safe than an old vessel designed under a less stringent code but with a factor of four for safety and that the likelihood of one being a f actor of two safer than the other is probably quite small.
I have stated these things in order to delineate my position as an admitted non-expert in the field. While I am eeither a metallurgist nor a mechanical design expert, I have been forced lthrough the yeqrs by pressu3e of circumstances as a reactor i
" *designeurarve11 mrthmber-of-this-Comnittev-to -lam-sWWg about these fiailda.- I would be particuintly interestsid to have
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a the followin; points. All of these get.ilemen are much trors ccanpetent than I in those areas and could give definitive comments. Specifically, 1 vould ask, "Do you feel that we can safely take credit for increased safety of the new vossein which vill be fabricated in the near future over the earlier fabricated vessels, assuming that both the nav and t.he old vessels are operated appropriately at the proper temperatures and prescures?" If such a factor cf increased safety e;;iste, it should certainly ba taken into account in considering Dresden II and I would be most happy if ry more knowledgeable colleagues can assure m that indeed it does.
It may also be that the testing which we vill attend on November 23td and 24th vill throw further light on this topic.
If the decision of the Comittee is that we have indeed under-estimated the likelihood of the accident being considered, it vould seem prudent to reviev all of the power reactors presently I
existing with the folleving rem objectives in mind:
a.
Are th(.re feasihte ways in which the possibility of a contattrent rupture could be made physically impoedMe?
y b.
Are the consequences of a rupture of the cont.wmens during an accident such that, coupled uith the incredibility of such an accident, they do not " constitute an undue hazard to the health and safety of the general public"?
Is it possible to ascure adequate core cooling in event of c.
cuch an accident by the existing means or should other additional means be introduced?
d.
Is it possible that certain reaccore should be chut down?
If it is indeed conclud2d by the Comittee that we have undepestdosted the likelihood of 'this accident, then it seems to c:e that the Cecnittee muut at the same time take the position that Dresden II should have protection against this accident.
In considering principal question No. 2, if we conclude l
that there is no reason for changing our past vievo on the eubject, we must still deride that point at which the likelihood of this accident, even tht...gh it be deemed incredible, combiNd with the population density existing; or predicted fission product inventories, and the continued growing ppopulation lead us to believe that the situation iu intolerable. It nay be that on a careful review of thir part of the subject we may indeed find that we have alrevy passed the point where we are now comfortable and may find it nccenary to go back and place additional requirements on aircady approved cases.
It may also be that we can fcresse ahead a developing intolerable situation and that a warning letter of the type written by Dave Okrent vill be most appropriate. I believe all of us
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h I did not-believe--that there were sufficient f acts.before us conce{cing I
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II site was a "re:no% site".the arguments advanec4 by Dave Okrent in his letter that I am hopeful that we will find an important difference between the sites of the general type occupied by Dresden II and
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those being considered for the future, so that we can provide by a Ictter such as that written by Dave Okrent an adequate and sufficient verning to all reactor fabricators and owners that such provisions vill be necessary at scene point.
letter we can even provide s logical basis for the line ofPerhaps, in that demarcatics.
Otrviously, this will be extrewly difficult since we have drifi.ed frcma rather good Sites to rather bad sites by easy stages.
It is my own tarsmal belief that unless the tons of the letter is made very strong, potential applicants will continue to let contracts and make agreements with fabricators and designs will continue to be started on the basis very nach resembling the present basis before they are brought before iche AEC.
i have not had a chance to discuss this within the Coussittee, While we Dr. Etheris gton and I attendod an Indian point meeting at Bs: hue,da on November 2.
Thero, in a rdther poor way I tried to indicate that the Westinghouse Corporation, tM company involved with Indian Point, should try to use neore imagination in their use of containment in order to provide more surety against very serious accidents and that this could hopefully ba done without greatly 4
increased costs and perhaps at a saving mr present designs With these thoughts in mind I believe that we should:
(a) be certain that we do have a demonstrabic basis for drawing a line at some definite point.
Dresden I recournend that a letter even stronger than Dave Okrent'sIf that poi be written to insure that there can be no question about our views concerning the subject.
I would lika to suggest that Frank Gifford and population distributions and fission peoduct inventories fora those reactors we hwe reviewed in the recent past or will review in the near future so that they asy give us the benefit of their special knowledge in these areas at the next meeting.
In regard to question No. 3, I do not have my mind made e
up in ei*.her direction.
I do not know whether Dreseen II should be required to provida prote: tion for this incredible at.;ident or not at the present time.
The information which I lack is discussed above and I believe that that coupled with the pressuro vessel meeting will provide sufficient input so that we can all arrive at a well-considered and judicious opinion.
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-m Additional Note Ten years ago at the time the first uster-cooled coranercial power plants were beginning to be designed and constructed, the question was very much, "Could any power plant be designed nd butitt" There was a great deal of enthusinem after the declassification of existing reactor information and because of the thrust of the gisenhower Atoms for Peace progra:n.
We emphasis on safety was certainly me strong then as it is now and people wantsd first to insure that these early plants would be safe.
There were worries about the stability which might exist in boiling and the response of various types of plants to Iw ' changes; and many other probleme which we now consider solved and noL too important.
People were trying to design *.he perfect control rod mechanism and they are still trying to design the perfect control rod== % ism.
%e nureber of unknowns and the lack of knowledge caused everyone to be very conservative in establishing their perfortsence limits and in carrying out their initial operations.
It was recognized that planto were probably overconservative and as years Wve gone by and as knowledge has improved some of this overconservatism has relaxed.
We power distribution within the core has been flattened, multi-region cores j
are being used, fuel elements Love snach longer lives than were predicted in those days, and burnout safety factora have been continually lowered.
All of these factors tend to reduce the margin of conservatism.
It is our duty to insure that this arrgin of conservatism is related appropri-stely as new knowledge comes in that provides sufficient assurance that there continues to exist an adequate margin of safety. h is continual review and updating is a dynamic process and the reactor designer is responsive to the pressures which are applied. During the past few years the economic factors have received the most emphasis.
Even five ynars ago it was not clear whether coal er r>il or nuclear power would in the long run be more economic. Now, I believe, it is quite clear that nucicar power can hold its own end ct the economic race with more con-ventional fuels.
We economic pressures now are those of an extremely ccupetitive market both between reactor fabricators and types and between public and private power.
gin of safety conservatism.These pressures tend always to reduce the mar-No reactor fabricator and no reactor owner will purposely design, construct, or operate a reactor in an unsafe manner if they, in their own conscience, believe this to be the fact.
On the other hand, all of these organizations receive considerable assurance from the AEC--
o both the staff and the emed ttee -- in the fact that these reactors are continuing to be approved and that, therefore, the covernecnt shares the safety responsibility.
In fact, as the reguistory process has become more detailad and its methods more refined, the utility users, especially the inexperienced ones, tend to believe that the problems of reactors are routine and so long as the reactor is approved by the Atomic Energy Com-
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h is view ta either consciously or unconscicosly aided end abetted by fatricators anxious to sell resctors.
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eg:-:e ;p; I!N rH ' 9 g ity,g y01.,U M.I At the same time it should again be indicated that the fabricators of reactors in a conscientious way are doing their best, as they see it, to insure that reactors a;a safe. f t u-ust be recognized, hovaver, that they are urwier strong. Occutuic 4:eseures to respond to a keenly coepetitive situation. Theafor% Nrsonally, I would have great difficulty in arriving at the ccnclusion that rs. actors today are per se safer than they were say five years ago.
It is obvious that we beve caught a number of weak peints in our earlier reaccor designet perhaps a good example is the 17-4 pH pt oble:n. On the other hand, as we reduce the mergins of conservatism othe. probleens will arise and ecos of these have already been predicted. Sorne of the new problaea may indeed be much more serious than those which we have faced to date. As yet, it has not been necessary to shut down any major power reactors for reasons of safety.
It is still conceivable that this situation may arise, even with a coupletely new plant.
If this situation should ari.ae, it in cicar that the embarrassment tc the industry and the enguished nashing of teeth from the econornic view-oint will be awescane to behold.
Additional Hote Another point that should be considered is the effect on the general public and on the industry of the public introduction of additional concern regarding this incredible, but still possible, accident.
If the final Cocraittee decision is that existing reactors or future reactora at some definite Icvel of population or other critation must have such provisions, care must be given to the phrasing with which such a letter is written.
It is necessary that l
the htter be firm in its intent to the industry and yet not unduly l
alarming to the general public.
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ADVISORY Co?G1ITTEE ON REACTOR SAFEGUARDS REES j
RECOMMEND 1TIONS ON N1] CLEAR POWER PLAliT DESIG1; 1
The Atomic Energy Commission has received-a report from its Advisory Committee on Reacter Safeguards outlining the Committee's recommendations concerning pressure vessels and engineered safeguards for pressurized water and boiling water nuclear power plants.
A copy of the Committee's letter together wich a state-ment by the Commission on th' safety recearch program is attached.
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STATEMENT BY AEC ON SAFETY RESEARCH PROGRAM The Atomic Energy Ccmmission has received a report from 4
its Advisory Committee on Reactor Safeguards which comments i
on many improvements made in pressurized and boiling water
- i reactors with consequent marked reduction in the risk of significant radic. tion exposure to the public.
The report L
then outlines certain technical areas relating to design, construction and operating surveillance of pressure vessels for water reactors on which the committee recommends that additional work be done.
The ACES reconfirms its belief that no undue hazard to p
the health and safety of the public exists in water-type i
reactors.
However,- t he - committee notes that _ "or ;erly: growth of the industry" is occurring, with increase-in the number, 4
y size, power level and proximity of reactors to large-popula-F tion centers, which will in the future make it desirable and k
prudent to incorporate in many reactors the design approaches I
whiah it recommends be developed.
Inspection techniques, f
struss analyses, flaw propagation during vessel use, protec-j'.
tion of the containment against missiles (equipment parts or fragments) and pressure in the unlikely case of vessel fail-j 4
ure, and other related matters are recommended by the ACRS S
for increased attention.
I The Commission wulcomes these specific recommendations I*k 4
of the ACRS as being 1-n keeping witn and supplementar r to statements nade by the Commission to the Congressiona Joint fi Ccmmittee on Atomic Energy in June of this year.
At that time the AEC noted that the rapid expansion and development
-f of the nuclear powar industry, including increased' size of proposed plants and the incentives to locate in closer prox--
e imity to metropolitan load centers, have re-emphasized the continuing need for careful attention to all matters which i
potentially could affect the-health and safety of_the public.
.The Commission also stated in its testimony that further important advances in reactor plant design, in theferpability of safety systems and engineered safeguards, in adap:ing
,s critical-components and systems to Lecommodate their inspec-y.
tion and testability, and in practical demonstration of e
dependability of performance of such critical systems, must evolve to keep pace with the development of the nuclear (more) l e
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It also was stated that an augmented and l
reoriented safety research and development program would be undertaken by the Commission, in collaboration with industry, l
to accomplish the improvements which would be required to i
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keep pace with the developments in reactor techralogy, i
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The recommendations of the ACRS on pressure vessels will i
be giver, prompt attention by the Safety Research Steering a
6 Committee in its review and development of the augmented safety research and' development programs.
The Commission
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envisages that steady improvements 1n safety technclogy will
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evolve from these augmented and accelerated programs and 3
1 that the safety of reactors will continue to advance.
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1 adequacy of safety provisions in each reactor will continue to be establiched by thorough and detailed analysis and
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Fl evaluation on a case-by-case basis.
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