ML19261E773
| ML19261E773 | |
| Person / Time | |
|---|---|
| Site: | Crane  |
| Issue date: | 07/23/1979 |
| From: | Gossick L NRC OFFICE OF THE EXECUTIVE DIRECTOR FOR OPERATIONS (EDO) |
| To: | Heinz J SENATE |
| Shared Package | |
| ML19261E774 | List: |
| References | |
| NUDOCS 7909110554 | |
| Download: ML19261E773 (6) | |
Text
Distribution Central files Coinell SECY (79-1792)
(3 copies)
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ENCLOSURE Evaluation of Piof. Gulbransen's Letter on Zirconium Hydride in TMI-2 Se,ator 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 colleagte of Gulbransen at U. of Pittsburgh) also reached the NRC, and a response to that letter was prepared earlier (Enclosure 2). Both Gulbransen and Hallace believe that the hydrogen in the TMI-2 " bubble" will react (or 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 ore-sents a grave exolosion hazard if it is exposed to dry air (oowdered zir-conium hydride is oyrophoric). Gulbransen, on the other hand, believes that the zirconium hydride will react with water to produce more free hydrogen, and that this cycle will receat until all of the Zircaloy claddino is oxidized and the crimary system contains large cuantities of free hydrogen.
Both of these concerns are based on Gulbransen's early work a 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 imoermeable to hydrogen except at specimen edges where localized attack occurred on chance patches of fresh zirconium.
The hydride would then spall exoosing more fresh zirconium such that the reaction continued.
Zircaloy cladding in a reactor, however, is in an oxidizing environment (water and steam) such that the fresh edges in Gulbransen's experiments would not be cresent.
If by chance any fresh metal were exposed in the preoxidized cladding, those places would quickly oxidize and seal off the transport of hydrogen.
2158, 267 e
It is clear that hydridino is not a orablem during normal reactor coeration even though PWRs are operated with a hydrogen overpressure and cladding temperatures are amply high (about 600 F) for hydriding. Some fuel rods have been kept in reactors for test purposes as long as 15 years without showing evidence of excessive hydriding.
Fuel rods that have exoerienced abnormal hydriding also demonstrate the effectiveness of an oxidizing atmosphere in shutting off the hydriding 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 hydrocen. Although hydriding in those cases originated 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 temoeratures.
It remains, therefore, to examine the conditions at TMI-2 that were signifi-cantly different than our common experience with Zircaloy.
These conditions were present during (a) the period of the high-temoerature excursion, and (b) the later low-temperature oeriod during which the bubble containino hydrogen was present. The questions to address are (a) whether larce amounts of hydrogen are absorbed during a LOCA-like temperature and oxidation transients, and (b) wnether hydrooen Dermeation through the oxide film is significant at relatively low temoeratures but high hydrogen cressures.
Hydrogen absorption during temoerature transients in the cresence of steam has been studied by Kawasaki et al.* They showed 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 locally (on the inside of a freshly burst tube near the burst location) resulting in local concentrations of hydrogen only as high as about 3,000 carts oer million (i.e., 0.3%). This concentration does not
- Recent results from U.S.-Japanese information exchange. See memorandum dated December 20, 1978 from S. Levine to H. R. Denton (Public Document Room accession number 7902270150).
2158 268 O
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 hydrocen permeability of oxide fi'ms on zir-conium at relatively low ten.peratures has been studied by Smith (Journal of Nuclear Materials, Vol. 18, p. 323, 1966).
In principle, the rate of cermeation (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 over the pressure range of 1 to 860 mm Hg of hydrogen and expressed the pressure effect by the function bP/(1+bP), where P is the cressure and b is a constant. Unfortunately, near the high pressure and of the pressure range in Smith's experiment, results were erratic and tne 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 hydrocen 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. Nevertheless, it seems mo;+ likely that the oxide layer that is formed on all Zircaloy cladding during ubrication (by autoclaving) and the additional oxide formed on the damaged TMI-2 cladding will prevent large scale hydrogen absorotion and hydriding in the TMI-2 core. We have seen no evidence to suggest the contrary.
Professor Gulbransea also stated that circulating cooling water would react with zirconium hydride 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 f6rmation just as it does on the metal.
2158 209 O
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 corbining 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 acoropriate term might be hydrogen-induced decarburization.
Other tyoes of hydrogen embrittlement are found in some high-strength steels, but it is not encountered significantly in reactor pressure-vessel steels.
Two circumstances should prevent hydrogen-induced decarburization from being significant in the pressure vessel and piping steels at TMI-2.
(1)
The 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 pressurL and temperature exoerienced in TMI-2 (even if stainless steel lining were not present). Enclosure 3 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 chenomenon-, 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 fomed, 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 ri,k of free hydrogen in the primary system is also small.
2158 270 Hydrogen in the primary system is being regularly monitored and found to be very low in concentration--so low, in fact, that hydrogen is being added to adjust the pH. The effect of hydrogen on pioing and vessel materials during the period of the TMI-2 bubble's existence should 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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" Richard Vollmer, Asst. Director, Systems and Projects, NRC Robert Arcold, Metropolitan Edison Company, Reading, Pa.
Thomas Gerusky, Director, Bureau of Radiation Protection. DER S
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Clifford L. Jones, Secretary
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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.
These people are reliable individuals who are concerned and have knowledge and experience to offer.
I would appreciate it if you 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.
Att:
cc: Governor Dick Thornburgh Lt. Governor William Scranton, III William Middendorf 2
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Fay 18, 1979
- 0 D '9' Y f tir. Clifferd Jones, Secretary j
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,j, repartnant of Environ: ental % sources Cr.monwealth of Pennsylvaria arrisburg, PA 17'20 RE:
Cher-ical Explosive Hazard at Three Mile Island
Dear Cliff:
This is to put in writing the substance of my comments to you via telephone a few menents ago.
I am reasonably certain that the hydrogen produced at the Three Mile Island accident is still there and represents a crave exolosion hazard if ieproperly handled.
'!y autherity for this is Dr. Earl Galoransen, Research Professor of '!etallure and Materials Engineering and of Chemistry, in this University.
Clearly the hydrogen was formed by reactien of the zircal:oy cladding ind ' cater in the averhe.- ed reactor.
To the best of my knowledge its dis-appearance, i.e. of the hydro;;en bubble, is generally regarded as a mystery.
Dr. Culbransen asserts, and I agree with him, that it disappeared as a gas bv rcaction with.:irealley to form zirconium hydride as the reactor cooled (thermodynamic measurements of Dr. Gulbransen fully support this idea).
He furthermore holds the opinien that some tons of this exceedingly hazardous hy' ride are lying at the bottom of the reactor where it.is at present covered over with water. As long as this hydride is covered, it niasants nr S - rd-but there is a distinct possibility of an explosion wnen this finely divided pyrophoric mass cones in contact with oxygen in the air.
This presents a special hacard at the time of the drainage of the reactor.
My strong reconnendation is that Dr. Culbransen be used as a consultant in regard to the cican-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 he willing to p,1ve what help I can.
However, I will be in Japan froe May 19-June 3, inclusive.
Dr. Cu]bransen's address.
and phone nu:-bers are as follows:
2158 273
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412-624-5312 If you need me, I can be reacht ? in Japar. as follows- ::a:.- 21-24,
- 4ukone Prir.cc H>tel, Hakone; ' day 25-27, Holiday Inn, }:y o t o, Japan; May 23, 29 in Tokyo with Jin Cont 6 -- Telephone:
03-357-4758; iday 30, 31, June 1, Fauai Surf Hotel, Kaual: June 2 with y son, Wallace in Scsttle -- Telephone contact:
206-631-5336.
I will be back at ny office (412-624-50C4) on June 4.
Ee sure that screone alivo to the chenistry of the prnblen is involved in the clean-up effort.
I know you will take steps which are appropriate in following up on this letter.
Sincerely yours,
.(
b'. E. Wallace Distinguished Service Professor
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PEM0FANDUM FOR:
Paul S. Ch :'r., Chief, Reactor Safety Er nch, COR g
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Carl H. Berlinger, Section Leader, F.Eactor Safety Branch, D0R g,
FROM:
Kris I. hr::cwski, Reactor Safety tranch, DOR
SUBJECT:
FOR"ATIC', GF ZIRCONIUM HYDRIDES IN THE THREE MILE ISLAND-2 INCIDE'iT Introduction The Secretary of the Department of Environmental Resources, Cor.monwealth of Pennsylvania transmitted to us a letter from 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-mation of zirconium hydrides which, if not handled properly, can under certain circumstances cause a violent reaction. Prof. Wallace quoted the work of Professor E. Gulbransen, also fmm the University of Pittsburgh, who for the last 25 years was studying the kinetics of fomation and deconposition' of zirconium hydrides.
The purpose of this memo is to evaluate, in light of the presently a'ailable information, the concerns brought by Prof. Wallace.
Available Infomation The infor.ation used in evaluating the problem of zirconium hydrides came from the following sources:
(1) Telephone conversation with Prof. Gulbransen (06/04/79)..
(2)
Conversations with several members of the NRC Staf f (F. D. Cof fman, M. L. Picklesimer, D. A. Powers).
(3)' "The Metallurgy 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.
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(6) " Dangerous Properties of Industrial Materials," by N. I. Sax, Van Nostrand Reinhold Company,1975.
2158 275
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Evaluation of the Proble-Prof. Gulbransen (Source 1) had indicated that when Zr comes in contsc:,
.h hydmgen at certain pressur2s two types of zirconium hyd. ide are formid:
Zr Hl.4 and Zr Hl.g.
At about 500*C the equilibrium hydrogen pr2ssures.~ce these compounds are few hundreth of n. Hg cnd few mm Hg, r2spectively.
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Ise hydrides are formed despite the existance of protective Zr02 because c:.ed-ing to Prof. Gulbransen, Zr02 cannot stop completely per.etrGn of hyd.cgen into metallic Zr. This is a controversial point since in :.n3 opinion of :ther people (Source 2) Zr02 could completely pr2 vent hydrogen from coming in coatact with metallic Zr. The infomation from the literature (Sources 2 and 3) also confimed the view that Zr02 would very significantly limit hydrogen penetration.
Prof. Gulbransen pointed out that Zirconium hydride fomed on Zr surfaces may spall off feming 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 form.
According to Prof. Gulbransen the presence of zirconium hydride in the reactor vessel in TMI-2 could cause two problems:
(1)
In contact with water at lower 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 produce violent reaction. The infomation obtained from other sources (Source 6) shows that the auto-ignition temperature of Zirconium hydride is 270*C in air.
It is, however, very much dependent f
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on the physical form cf the hydride.
As a remedy Prof. Gulbransen has suggested a method for decomposing zirconium hydrides by circulating hydrogen free water at low pressure and preferably The rate of decomposition containing some oxidizing agent (e.g. dissolved air).
will be slow because of a slow rate of reaction and it would take a long time to decompose all hydrides.
In order to detemine 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 fomed hydrogen hydride. With these assumptions about 2500 lb cf zirconium hydride would be fomed in the reactor vessel during the accident.
It should be realized however, that this is an upper theoretical limit and it is nost unlikely that such large amount of zirconium hydride would ever be pro-duced.
2158 276 6
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3 tion and behavior of zirconium hydrid: :ca The existing informition on for:
sc.ewhat controversial, however, because of the possibility of exister e of this hazardous material in the reactor vessel the following precautions tre recor.: ended:
To monitor the presence of hydrogen in the primary coolant in orhr to (1) establish if the decomposition of zirconium hydride takes place.
Uhen opening the reactor vessel for cleaning assure that the dabris at (2) the bottom of the vessel are not exposed to the oxidizing environment
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(.1 hA Kris I. Parczewski Reactor Safety Branch Division of Operating Reactors cc:
C. Berlinger F. Coffman S. Weiss R. Yollmer
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BROOKHAVEN NATIONAL LABORATORY MEMORANDUM DATE: April 4, 1979 To:
W.Y. Ka to
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FROM:
John R. Weeks sueJECT: Discussions Relative to the Three Mile Island Incident 1.
Hydrogen in Containment Walter Butler 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 Schwarz of the BNL Chemistry Department.
His rough guess was that the hydrogen may build-up to several percent which should be approaching the ignition point.
The higher the temperature (above 100 C), however, the greater would be the recombination rate and the less the build-up of hydrogen.
2.
Discussions Concerning the Hydrogen Bubble in the Reactor Vessel Warren Hazelton asked me what information I had on the thermodynamics and kinetics of the reaction of ~ hydrogen at a high temperattre and pressure inside the reactor vessel on the possible (acarburization of and methane formation in the vessel material.
I discussed this subject with David Gurinsky and J. Chow of BNL, M. Gensamer, Professor Emeritus at Columbia and A. Ciuff reda of Exxon Research. The stainless steel cladding on the inner surface 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 made of a pressure vessel steel (ASIM A-533-B) which contains 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 tecperature and pressure for steels (Nelson Diagram) in the American Petroleum Institute report API-941, which was most recently modified in 1977.
A steel of the composition used in the Three Mile Island vessel should be safe from decarburization by 1000 psi of hydrogen at temp eratur es up to 700 F for indefinite use.
Exceeding this te=perature or pressure f or short periods would not cause serious damage as there is a definite incubation time, of a matter of several days, bef ore problems begin to develop. Mo appears to be even more effective than Cr in retarding this decarburization although the reasons are not clear.
The same steel without the Mo would only be saf e 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 damage from decarburization prior to its return to service.
A copy of the curve showing this relationship as revised in 1977 is appended to this memorandum.
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"D W.Y. Kato ad es April 4, 1979 Hazelton also asked whether radiolysis of the water within the vessel could add oxygen to the hydrogen gas bubble.
In my opinion, it should not.
Radiolyr is af water proceeds by a complex chain reaction and can be prevented even by a small overpressure of hydrogen in an operating PWR.
The high hydrogen pressures over the coolant at Three Mile Island should totally nrevent oxygen formation.
In fact, Harold Schwarz stated it may be feasible to rer.sve the hydrogen by simply adding oxygen slowly to the coolant; this could, admitcedly, be risky.
I think we should be very careful not to use chemicals such as sulfate or rulfur bearing compounds to react with the hydrogen since t hese can be red uced by the excess hydrogen to sulfides which are very harmful to a number of the materials in the system, especially the Inconel steam generator tubes.
It might complicate the return of the unit to service.
I recommended that a nitrate (such as potassiu= nitrate)
F e used if one wishes to go by this route.
However, I think the best means of hydrogen re= oval would be through venting it from the primary coolant into the c on ta inment where it can be recombined with oxygen.
3.
Some Crude Calculations of the Amount of Zircaloy tha t Participa ted in a Zr-H O Reaction During the Incident 2
I estL= ate that as much as 3200 lbs. of Zr may have reacted with water tw produce the hydrogen bubble, assuming it occuppied 750 cu. ft. at 500 F and 1000 psi, as stated by Razelton.
This suggests that over 10% of the Zircaloy cladding in the core was converted to oxide by reaction with the water. Whether or not the remaining Zircaloy could act to recove hydrogen f rom the water by hydride formation is not cl ea r.
However, the hydrogen overpressure during nor=al PWR operation does not cause significant hydriding of the fuel cladding so that hydrogen removal from the bubble by this mechanism see=s unlikely. This (10-50cc STP/kg H O) amounts to a maximum of 3.24 lb. in the primary hydrogen 2
coolant (329,200 kg) so clearly, the majority of the hydrogen bubble ca=e f rom some 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 BKL J. Chow D. Gurinsky H. Kouts 2158 280 NRC V. Noonan W. Hazelton F. Almeter N. 0,. A nd 4