ML19308A379
| ML19308A379 | |
| Person / Time | |
|---|---|
| Site: | Green County, New Haven  |
| Issue date: | 07/24/1979 |
| From: | Harold Denton Office of Nuclear Reactor Regulation |
| To: | Brown P MID-HUDSON NUCLEAR OPPONENTS, INC. |
| References | |
| NUDOCS 7908170636 | |
| Download: ML19308A379 (12) | |
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jut 2 41979 Docket ilos. 50-549 50-596 and 50-597 Dr. Peter D. G. Brown Chairman of the Board Nuclear Opponents, Inc.
P. O. Box 666 T!cw paltz, NY 12561
Dear Dr. Brown:
In accordance with your request, we have reviewed the paper by Dr. David M. Pisello, "The Zirconium Connection," transmitted by your letter dated May 1, 19/9.
Dr. Pisello's paper contains a great deal of incorrect information, of which the following are examples:
1.
The charge that NRC concealed the nature of the Zircaloy-water reaction is false. This reaction was discussed extensively in the public hearing on Emergency Co e Cooling Systems, and is so routinely analyzed that it is covered explicitly by a regulation (10 CFR 50, Appendix K, paragraph 1.A.5).
2.
The charge that NRC concealed the source of hydrogen in TMI-2 and only admitted it privately is false. The Zircaloy cladding reaction with water was described as the majt r source of hydrogen in NUREG-0557, " Evaluation of Long-Term Pest-naident Core Cooling of Three Mile Island, linit 2," May 1979 and in other public references.
3.
The decay 1.e -
as not "only about one fifth of what is would be in a n ature cor.
Because the decay heat of concern (i.e., within a few hours afte: shutdown) depends primarily on short-lived fission products, the decay heat was about 95% that of a mature core at the time of major core damage.
4.
The oxide film on zirconium is not transparent to hydrogen, but is nearly impermeable to hydrogen.
5.
Plutonium dioxide formed in nuclear fuel is a ceramic; it is not volatile.
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i Dr. Peter D. G. Brown Dr. E. A. Culbransen, whose work is referenced by Dr. Pisello, has expressed his concerns about the use of zirconium alloys as reactor fuel cladding. De have recently prepared a response to those concerns and are enclosing a copy of that response for your information.
In summary, our evaluation of Dr. Pisello's paper indicates that it is without substantial technical merit, and that his allegation of a major design flew due to the use of zirconium alloys as cladding material is unsupported and unwarranted.
Sincerely Origba' :iped by E.G. Case Harold R. Denton, Director
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Office of t'uclear Reactor Regulation
Enclosure:
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Gulbransen Response s
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ENCLOSURE Evaluation of Prof. Gulbransen's Letter on Zirconium Hydride in TMI-2 Senator Heinz transmitted a letter from Prof. E. A. Gulbransen (U. of Pittsburgh) expressing concern about the formation of zirconium hydride in the TMI-2 core. A similar letter (Enclosure 1) from Prof. W. E. Wallace (a colleague of Gulbransen at U. of Pittsburgh) also reached the NRC, and a response to that letter was prepared earlier (Enclosure 2). Both Gulbransen and Wallace believe that the hydrogen in the THI-2 " bubble" will react (or i
has reacted) with the remaining unoxidized Zircaloy to form large quantities of zirconium hydride. Wallace believes that the hydride will be finely divided and lying at the bottom of the reactor, and that this hydride pre-sents a grave exolosion hazard if it is exposed to dry air (powdered zir-conium hydride is pyrophoric). Gulbransen, on the other hand, believes that the zirconium hydride will react with water to produce more free hydrogen, and that this cycle will repeat until all of J._ ?# caloy cladding i
is oxidized and the crimary system contains large quantities of free hydrogen.
Both of these concerns are based on Gulbransen's early work on hydrogen reaction with preoxidized zirconium. Gulbransen's experiments were con-ducted in a vacuum furnace into which hydrogen was admitted.
The oxide film was found to be nearly impermeable to hydrogen except at specimen edges where localized attack occurred on chance patches of fresh zirconium.
The hydride would then spall exposing more fresh zirconium such that the reaction continued.
Zircaloy cladding in a reactor, hcwever, is in an oxidizing environment j
(water and steam) such that the fresh edges in Gulbransen's experiments would not be present.
If by chance any fresh metal were exposed in the preoxidized cladding, those places would quickly exidize and seal off the transport of hydrogen.
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. It is clear that hydriding is not a oroblem during normal reactor operation i
even though PWRs are operated with a hydrogen overpressure and cladding temperatures are amply high (about 6007) for hydriding. Some fuel rods 4
i have been kept in reactors for test purposes as long as 15 years without showing evidence of excessive hydriding.
4 Fuel rods that have experienced abnormal hydriding also demonstrate the effectiveness of an oxidizing atmosphere in shutting off the hydriding 4
orocess. Earlier industry problems with internal hydriding occurred only after the oxidizing environment had been removed, i.e., the enclosed mois-ture had been removed by oxidation with zirconium freeing hydrogen. Although i
hydriding in those cases origina ed on the cladding inside surface, the hydride chase extended through the wall thickness and was visable on the outside surface as a blister or " sunburst." Those blisters do not spall or continue to hydride significantly on the outside surface even at operating temocratures.
j It remains, therefore, to examine the conditions at TMI-2 that were signifi-j cantly different than our common experience with Zircaloy.
These conditions were present during (a) the period of the high-temperature excursion, and l
(b) the later low-temperature period during which the bubble containing hydrocen was present. The questions to address are (a) whether large amounts of hydrogen are absorbed during a LOCA-like temperature and oxidation transients, and (b) whether hydrogen permeation through the oxide film is significant at relatively low temperatures but high hydrogen pressures.
Itydrogen absorption during temoerature transients in the presence of steam has been studied by Kawasaki et al.* They shcwed that significant hydrogen absorption occurs only when the hydrogen-to-steam ratio is greater than about 0.2.
During a LOCA-like temperature transient, this condition is satisfied lo. ally (_on the inside of a freshly burst tiube near the burst location] resulting in local concentrations of hydrogen only as high as about 3,000 parts per million (i.e., 0.3%)..This concentration does not
- Recent results from U.S.-Japanese information exchange. See memoranduni dated December 20, 1978 from S. Levine to H. R. Denton (Public Document Rcom accession number. 7902270150).
. constitute the severe hydriding problem of concern to Gulbransen, and it is so low that hydrogen pickup was overlooked in many early LOCA-simulation tests.
The rate of hydriding in oxidized zirconium is controlled by the rate at which hydrogen can permeate the oxide film and reach the metal. The effect of hydrogen pressure on the hydrogen permeability of oxide films on zir-conium at relatively low temperatures has been studied by Smith (Journal of fluclear Materials, Vol.18, p. 323,1966).
In principle, the rate of permeation (and hence the rate of hydriding) is a function of the fraction of available surface (oxide) sites that contain hydrogen, and this in turn depends on the hydrogen overpressure. When the oxide surface is saturated with hydrogen, further increases in the availability of hydrogen, i.e., the hydrogen overpressure, should have no effect. Smith confirmed this effect experimentally c ser the pressure range of 1 to 860 mm Hg of hydrogen and expressed the piessure effect by the function bP/(1+bP), where P is the pressure and b is a constant. Unfortunately, near the high pressure end of the pressure range in Smith's experiment, results were erratic and the permeation rates were all abnormally high. Therefore, we cannot say with absolute confidence that there is no high pressure effect on oermeation, but we believe that to be the case.
The exact time, temperature, and hydrogen pressure conditions are unknown for the TMI-2 accident so we cannot be sure that the TMI-2 conditions were present in the above references. ?!evertheless, it seems most likely that the oxide layer that is formed on all Zircaloy cladding during fabrication (by autoclaving) and the additional oxide formed on the damaged TMI-2 cladding will prevent large scale hydrogen absorption and hydriding in the TMI-2 core. We have seen no evidence to suggest the contrary.
l Professor Gulbransen also stated that circulating cooling water would react with zirconium h'ydride forming more zirconium dioxide. This reaction would continue (if zirconium hydride were present). only to the extent of forming -
an oxide layer on the zirconium hydride. This oxide coating would then inhibit further oxide formation just as it does on the metal.
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_4 In his letter, Professor Gulbransen expressed an additional concern about the'possible effect of hydrogen on the reactor vessel and piping at TMI-2.
At high temperature, hydrogen has the potential for combining with carbon in steels to form methane, which results in internal stresses that can cause cracking. This is sometimes referred to as " hydrogen embrittlement,"
although a more appropriate term might be hydrogen-induced decarburization.
Othc r types of hydrogen embrittlement are found in some high-strength steels, but it is not encountered significantly in reactor pressure-vessel s teels.
Two circumstances should prevent hydrogen-induced decarburization from being significant in the pressure vessel and piping steels at TMI-2.
(1)
Tne pressure vessel and large pipes in the primary system are lined with stainless steel, which acts as an effective barrier to hydrogen and will prevent its contact with the higher-strength alloys.
(2) The effect of hydrogen on steels of the type used in pressure vessels has been found to be unimportant for the combination of pressure and temperature experienced in TMI-2 (even if stainless steel lining were not present).
discusses this topic and shows that these steels can be used indefinitely at temperatures up to 700 F at a pressure of 1000 psi of hydrogen.
Except for short periods during the first day of the accident--periods of time that would not meet incubation requirements for this phenomenon-, total system pressure was 1000 psi or less (hydrogen partial pressure would, therefore, be smaller) and system temperatures were generally below 600 F.
It therefore seems unlikely that hydrogen embrittlement would have occurred in the primary system.
In any event, TMI-2 inspections for damage would be made prior to operating that reactor again.
In conclusion, there is strong evidence that the type of cladding hydriding feared by Gulbransen and Wallace will not occur because of the continued formation of a protective oxide layer. Even if zirconium hydride were formed, there is little danger that it could ignite because the damaged TMI-2 core will not come into contact with dry air until after underwater examinations could determine the presence of significant zirconium hydride.
It will be an. easy matter at the time of examination to see if Gulbransen's prediction of a fully reacted core is valid; we think it is not. The explosion risk of free hydrogen in the primary system is also small.
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. itydrogen in the primary system is being regularly nonitored and found to be very 1cw in concentration--so low, in fact, that hydrogen is being added to adjust the pH. The effect of hydrogen on piping and vessel materials during the period of the TMI-2 bubble's existence shculd also be insignifi-cant since that exposure to hydrogen was within temperature and pressure limits for which no damage is expected.
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Zirccnium Hydride 5=cn
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Richard Vollmer, Asst. Director, Systems and Projects, NRC Robart Arnold, Metropolitan Edison Company, Reading, Pa.
'o' Thecas Gerusky, Director, Sureau of Radiation Protection, DER
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i e ncui Clif ford L. Jones Secretary L
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Department of Environmental Rqso,urc s J
I am enclosing a letter from Dr. W. E. Wallace who is a Distinguished Service Professor at the University of Pittsburgh with a speciality in chemistry.
Dr. Wallace and one of his associates, Dr. Earl Gulbransen, feel that there may be problems with zirconium hydride at the reactor as it is cooled.
Dr. Gulbransen has offered his time without cost as a consultant on this problem.
i These people are reliable individuals who are concerned and have knowledge and experience to offer.
I would appreciate it if you 4
would be in touch with Dr. Earl Gulbransen, his telephone numbers are on the attached letter and see if he can be of help. Thank you.
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Att:
cc: Governor Dick Thornburgh Lt. Governor William Scranton, III William Middendorf t
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Mr. Clif ford Jon s, Secretary repartmcat of Environ.: ental Fesources Corc anvcalth of Pennsylvania brrisburg, PA 17120 RE:
Chemical Explosive Hazard at Three ' tile Island De ir C!iff:
This is to put in writing the substance of cy corrents to you via telephone a few mor.ents ago.
I am reascnably certain that the hydrogen produi.ed at the Three Mile Island accident is still there and represents a gra,ve explosion hazard if inproperly handled.
My authority for this is Dr. Ea71..hUfGansen, Resaarch Professor of Metallurgy and Materials Engineering and of Chemistry, in this 1*n ive r s.i t y.
Clearly the hydror,en was formed by reaction of the zircalloy clodding and teat er in the overheated reactor.
To the hest of my knowledge its dis-appearance, i.e. of the hydrogen bubble, is generally regarded as a m.ystery.
Dr. Culbransen asserts, and I agree with him, that it disappeared as a gas by reaction with zircalloy to form zirconium hydride as the reactor cooled (thermodynamic measurenents of Dr. Gulbransen fully support this idea).
He furthermore holds the opinion that some tons of this exceedingly hazardous hydride are lying at the bottom ot' the reactor where it Js at present covered over with water. As long as this hydride is covered. it nreso_n_t L 1p, b ga.rJ;,
but there is a distinct pessibility of an explosic.n when this finely divided pyrophoric m::ss cor.es in contact with oxygen ia the air.
This pr.'sents a special hazard at the time of the drainage of the reactor.
P strong recommendation is that Dr. Gulbransen be used as a consultant in regard to the c]can-up cperation at this reactor site.
He:had anticipated this " hydrogen problem" some years ago.
My credentials in the hydrogen area are strong also, and I would be willing to give what help I can.
However, I will be in Japan from May 19-June 3, inclusive.
Dr. Culbrancen's address.
and phone numbers are as follows:
N3R ORIGINAL CbE.EIRY Ot"L O.NG_ P*l T SFlenCH P A 15. r0, t t? S 4 9:0 i
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Dr. Ca rl Gul br,a r.sen 63 Hat:nway Ct.
Pittsburgh, PA 15235 Phone:
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Dept. of I:et111urgy and "aterials T.ngineering tJniversity of Pittsburgh Pittsburgh, PA 15260 Phone:
412-624-5312 If you need me, I can be reached in Japan as fo11ews:
-:ay 21-24, Hakone Prince Hotel, Hakone; "ny 25-27, Holiday Inn, 1:y o to, Japan; M.ty 23, 29 in Tokyo with Jim Cont 6 -- Telephone:
03-357-4 758; ::ay 30, 31, June 1, ra ua l Su r f lio t el, Ka na f ; J un e 2 w i th my so n, Wallace in Sta t rie -- Tele;, hone ccatact:
206-631-5336 I will be back at my office (412-62?-5004) on June 4.
Ee cure that someone alive to the chemistry of the probig is f r.volved in the cican-up offort.
I know you will take steps which are appropriate in following up on this letter.
Sincerely yours, I
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W. E. k'allace Distinguished Service Professor W::li/ a =s PODRORGINA!.
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Paul S. Check, Chief, Reactor Safety Eranch, D]R
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Kris I. Parc:cwski, Reactor Safety Branch, 00P,
SUBJECT:
FOR!% TION OF ZIRCONIUM HYDRIDES IN THE THREE ?,ILE ISLAND-2 INCIDENT Introductio_n The Secretary of the Department of Environmental Rar urces, Comonwealth cf Pennsylvania transmitted to us a letter Trom Professor W. E. Wallace of the University of Pittsburgh in which he draws attention to the fact that during the TMI-2 accident large amount of generated hydrogen may have caused for-nation of zirconium hydrides which, if not handled properly, can under certain circumstances cause a violent reaction. Prof. Wallace quoted the work of Professor E. Culbransen, also from the University of Pittsburgh, who for the last 25 years was studying the kinetics of fomation and decomposition of zirconium hydrides.
The purpose of this meno is to evalua e, in light of the presently a'ailable infornation, the concerns brought by Prof. Wallace.
Available Infomation The information used in evaluating the problem of zirconium hydrides came from the following sources:
(1) Telephone conversation with Prof. Gulbransen (06/04/79). -
(2)
Conversatiors with several members of the NRC Staff (F. D. Coffman, M. L. Picklesimer, D. A. Powers).
(3)
"The Matallurgy of Zirconium," by B. Lustman and F. Kerze, Jr., Mc Graw-Hill Book Company, Inc.,1955.
(4) "The Metallurgy of Zirconium," by D. L. Douglass, IAEA, Vienna,1971.
(5) "The Encyclopedia of the Chemical Elements," by C. A. Hampel, Reinhold Book Corporation, 1968.
(6) " Dangerous Properties of Industrial Matert'als," by N. I. Sax, Van Nostrand Reinhold Company,1975.
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KN 0 6"b73 Evaluation of the Problem Prof. Gulbransen (Source 1) had indicated that uhan Zr comes in contact with hydrogen at certain pressures two types of zirconf un hydride are formsd:
7.r Hl.4 and Zr Hl.g.
At about 500*C the equilibrita hydrogen pr rrsures for these compounds are few hundreth of tra Hg cnd few mm Hg, respectively.
Thise hydrides are formed despite the existence of protective Zr02 because, eccced-ing to prof. Gulbransen, Zr02 cannot stop completely per.atration of h,'d: ogen into metallic Zr.
This is a controversial point since in tha,pir. fen of other people (Source 2) Zr02 could completely prevent hydrogan from coming in c:ntact with metallic Zr.
The information from the literature (Sources 2 and 3) also confirmed the view that Ir02 would very significantly limit hydrogan penetration.
Prof. Gulbransen pointed out that Zirconium hydride fon?.ed on Ze surfaces may spall off forming a highly divided mass at the bottom of the reactor vessel.
This point was also challenged by other people (Source 2) who did not believe that Zr hydride could ever assume a highly divided fonn.
According to Prof. Galbransen the presence cf zirconium hydride in the reactor vessel in TMI-2 could cause two problems:
(1)
In centact with water at Icuer pressures hydrogen gas can be released.
Although the rate of release woul.d be slow the existence of this source of hydrogen should be taken into consideration.
(2)
Zirconium hydride in powdery form is pyrophoric and when exposed to air may ignite and The information obtained from other sources (produce violent reaction. Source 6) shows that the auto-ignition temp Zirconium hydride is 270'C in air.
It is, however, very much dependent
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on the physical form of the hydride.
As a ramedy Prof. Gulbransen has suggested a method for decomposing zirconium hydrides by circulating hydrogen free water at low pressure and preferably containing some oxidizing agent (e.g. dissolved air). The rate of deccmposition wil) be slow because of a slow rate of reaction and it would take a long time to deccapose all hydrides.
In order to determine the maximum amount 'of zirconium hydride which could theoretically be formed during the accident it was assumed that 30% of. Zr in the core reacted with steam or water and that 30% of the hydrogen generated in this reaction formed hydrogen hydride. With these assumptions about 2500 lb of zirconium hydride would be formed in the reactor vessel during the accident.
It should be realized however, that this.is an upper theoretical limit and it is most unlikely that such large amount of zirconium hydride would ever be pro-duced.
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scme.. hat controversial, however, because of the possibility of existence of this hazardous caterial in the reactor vessel the following precautions are reco..r. ended:
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To monitor the presence of hydrogen in the prim 3ry coolant in order to (1) establish if the decomposition of zirconium hydride takes place.
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tihen opening the reactor vessel for cleaning assure that the debris at (2) the bottom of the vessel are not exposed to the oxidizing enviccre.ent (e.g. dry air).
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John R. Weeks
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SUBJECT:
Discussions Rel,tive to the Three Mile Island Incident 1.
Hydrogen in Containment Walter Sutler of NRC asked me to estimate being possible the build-up of hydrogen in the containment by radiolysis of water in a high y field.
I in turn discussed it with Dr. Harold Schwar: of the BNL Chaaistry Department.
His r ough guess was that the hydrogen may build-up to several percent which should be approaching the ignition point.
The higher the temperature (above C
100 C), however, the greater would be the recombination rate and the less the build-up of hydrogen.
2.
Discussions Concerning the Hydrogen Subble in the Reactor Vessel Wa rren RazcIton asked me s.ha t 1ifor=atien I had on the theraodynmics and kinetics of the reaction of hydrogen at a high temperature and pressure inside the reactor vessel on the possible decarburization of and methane formation in the vessel material.
I discussed this subject with David Curinsky and J. Chow of ENL, M. Censamer, Professor Emeritus at Columbia and A. Ciuff reda of Laon Rcsearch.
"Ihe stainless steel cladding on the inner surf ace of the vessel would be a partial barrier to hydrogen provided it were intact. There is enough of a chance of a flaw in this cladding, however, that no credit should be taken for it in estimating the performance of the reactor vessel material.
The reactor vessel is :rade of a pressure vesse.1 steel (AS'IM A-533-B) which centains 1% Mn, 0.5% Ni and approximately 0.5% Mo.
The oil industry is continuously concerned about hydrogen induced decarburization of steels in their refinery equipment.
They have prepared a graph stating the safe te perature and pressure for steels (Nelson Diagrm) in the kserican Petroleum Institute report API-941, which was most recently modified in 1977.
A steel of the ccmposition used in the Three Mile Island vessel should be safe from decarburization by,1000 psi of hydrogen at t emp era tures up to 700 F for indefinite use.
Exceeding this temperature or pressure for short periods vould not cause serious damage as there is a definite incubation time, of a matter of several days, before problems begin to develop. Mo appears to be even core effective than Cr in retarding th.is decarburization although the reasons are not clear. The same steel without the Mo would only be safe up to 500 F at 1000 psi of hydrogen. I think the upper part of the reactor vessel should be carefully checked for any possible dannge from decarburization prior to its return to service.
A copy of the curve showing this relationship as revised in 1977-4s this me-orandum.
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W.Y. Kato April 4, 1979 Hazelton also asked whether r.ndiolysis of the water within the vessel could add oxygen to the hydrogen gas bubble.
In =y opinion, it should not.
Radiolysis of water proceeds by a cocplex chrin reaction and can be prevented even by e overpressure of hydrogen in an operating PWR.
small The high hydrogen pressures over the coolant at Three Mile Island should totally prevent oxygen formation.
In fact, Harold Schwarz stated it cxygen slowly to the coolant;may be feasible to r emove the hydrogen by simply adding be very careful not this could, admitcedly, be riaky.
I thiak we should to use chemicals such as sulfate or sulfur bearing compounds to react with the hydrogen since these can be reduced by the excess hydrogen t sulfides wh!ch are very harmful to a number of the ca terials in the system, o
especially the Inconel steam generator tubes.
It cight cc=plicate the return of the unit to service.
I recenmended that a nitrate be used if one wishes to go by this route.
(such as potassium nitrate)
Mcwever, I think the best means of hydrogen re=cval would be through venting it from the primary coolant into the c on ta inacn t where it can be recombined with oxygen.
3.
Some Crude Calculaticas of the Amount of Zircaloy that Participa ted in a Zr-R O Reaction During the Incident 2
I estimate that as =u-S as 3200 lbs. of Zr may have reacted with water to produce the hydrogen bubble, assuming it occuppied 750 cu. ft. at 500 F and 3 000 psi, as stated by Hazelton.
This suggests that cladding in the core was converted to oxide by reaction with the water.over 10% of the Zircaloy hydride formation is notthe recaining Zircaloy could act to remove hydrogen from the water by Whether or not clear.. However, the hydrogen overpressure during normal TWR operation does not cause significant hydriding of the fuel cladding so that hydrogen removal from the bubble by this mechanism see=s unlikely.
hydrogen (10-50cc STP/kg H O) amounts to a maximum of 3.24 lb. in the pri=ary This 2
coolant (329,200 kg) so clearly, the majority of the hydrogen bubble came f rom seme other source such as Zr-H O reactions, if the bubble was as large as 2
described by Hazelton on 3/31/79.
JRW:ob Distribution BNL J. Chow D. Curinsky H. Kouts NRC V. Noonan W. Hazelton F. Almeter H. C,
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