ML19308B814
| ML19308B814 | |
| Person / Time | |
|---|---|
| Site: | Maine Yankee, Crane |
| Issue date: | 03/14/1975 |
| From: | Anders W NRC COMMISSION (OCM) |
| To: | Muskie E SENATE, GOVERNMENTAL AFFAIRS |
| References | |
| TASK-TF, TASK-TMR NUDOCS 8001170347 | |
| Download: ML19308B814 (6) | |
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M UNITED STATES
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>E NUCLEAR REGULATORY COMMisslOs.
OCTG[g Record gopy 8
wasmucTon. o. c. 20sss MAR 1 4 1975 Senator Edmund S. fluskie Chairman, Subcommittee on Intergovernmental Operations Conmittee on Government Operations United States Senate
Dear Senator !!uskie:
In your letter of February 24, 1975, you expressed concern regarding the relationship between the fluclear Regulatory Commission's recent action directing special inspections of 23 boiling water reactors and the con-tinued operation of the liaine Yankee pressurized water reactor plart in Wiscasset, liaine.
You posed three specific questions and asked that our reply be included in the record of the February 5 hearings of the Senate Committee on Government Operations and the Joint Committee on Atomic Energy.
- I would like to summarize the detailed answers to your questions provided in the enclosure to this letter. Although the exact cause of the small hairline cracks in certain stainless steel piping of boiling water reactors has not yet been determined, the evidence at hand indicates that the basic mechanism involved is a phenomenon called "intergranular stress corrosion."
The susceptibility of stainless steel piping material to crackina by this mechanism is known to be 00nendent on a number of factors. The specific conbinations of factors contributing to stress corrosion are not present to the same degree in the twc types of light water reactors.
In particular, the chemical content of the coelant is different, and the level of dissolved oxygen in the water is higher in boiling water reactors.
In addition, no similar cracking phenomenon has been observed in the reactor coolant piping of pressurized water reactors.
Because of these identified differences, we concluded, on the basis of current information, that there was no need to require special inspections of pressurized water reactors.
Should the results of our ongoing investi-gations of this natter suggest the existence of a problem in the piping of pressurized water reactors, the Commission will, of course, consider
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the need for special, or more frequent, inspections of these reactors.
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MAR *4 53 Senator Edmund S. 'tuskie 2
I would like to point out that all operating reactors are required by flRC regulations to perform periodic inspections of their ofping systems.
These periodic inspections provide for early detection of potential crack-ing problems.
If through-wall leaking cracks should occur in a pipe prior to their detection by the periodic inspection progran, leakage detection systems would provide timely indication.
In accordance with the limits specified in the plant's operating license issued by flRC, shutdown would be required to determine the source of the leakage, and appropriate correc-tive action must be taken. The properties of the stainless steel piping materials involved are such that small cracks would grow slowly, and detectable leakage would occur well before complete pipe severance.
In addition, the redundant emergency core cooling systems provided on both boiling water reactors and pressurized water reactors protect against the consequences of complete pipe severance ranginq from the smallest to the largest pipes in the primary systen.
In each of the cases where cracks have been observed, the leakage from reported through-wall cracks in the stainless steel piping was insufficient to cause any significant drop in coolant level in the reactors or to involve actuation of an emergency core cooling system.
. I hope that the material provided in the enclosure answers your cuestions regarding the recent snecial inspections required for boiling water reactor plants. Should you have further questions, we would be pi'.:M to provide additional information.
Sincerely, lliMiam A. Anders Chairman
Enclosure:
As stated
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I NRC RESPONSE TO SENATOR hDSKIE'S QUESTIONS In a letter to Chainnan Anders, dated Febntary 24, 1975, Senator Edmund S.
hhskie inquired as to what specific differences exist between the boiling water reactors (BhR's) that were requirec to conduct special piping inspec-tions and the pressurized water reactors (PhR's), such as Maine Yankee, that would preclude the development of similar crack problems in the PhR's.
Before addressing the three specific questions posed by Senator Muskie, it is useful to describe some of the differences between BhR's and PhR's that are relevant to the recent crack problems, and to summarize the factors considered by the Commission in deciding to require special inspections of BhR piping.
The cooling water in boiling water reactors operates at a pressuge of about 1050 pounds per square inch and a temperature of about 550 F.
This water is heated as it passes through the reactor and is converted into steam within the reactor vessel The steam goes directly to the turbine-generator to produce electrici+.y.
1 Inthepressurizedwaterreactor,thecoolingwateroperatesatapgessure of about 2250 pounds per square inch and a temperature of about 600 F.
The
-water is heated as it passes through the reactor core, but is not converted into steam. The heated water is circulated from the reactor vessel to steam generators and returned to the reactor in a closed system. Steam is gen-ersted in a separate secondary system as the reactor coolant flows through the steam generators.
Because the prim 9ry coolant water in the pressurized water reactor is not boiled to produce steam, but recirculates in a closed system, it is possible to control the chemical content of the water more precisely than is the case for boiling water reactors.
In particular, the amount of dissolved oxygen in the water in pressurized water reactors can be maintained at a very low level by the use of additives.
In contrast, the oxygen content of the water in the boiling water reactors is not as readily controlled and attains a higher level during operation.
In consequence, the piping materials in these two types of plants are subjected to different water environments.
The portion of the core spray system in which cracks were found in Dresden-2 is connected to the reactor coolant system and is subjected to the same pressure, temperature, and other conditions as is the reactor coolant system. The discovery of these cracks in Januar/1975 followed;the as yet unexplained occurrences of small cracks.in four-inch diameter stainless steel bypass lines in the reactor coolant system in six BhR's, beginning in September 1974.
It was this series of occurrences and the lack of definitive explanation of the specific causative factors, k
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i that led the NRC staff and the Commission to the conclusion that, although the circumstances indicated no immediate safety hazard, prudence dictated that prompt inspections of piping in operating BWR's should be performed to assure that the cracking problem was not nore widespread than had been reported. The directive issued on January 30, 1975, required the performance of the special inspections within a period of 20 days.
It was necessary tha: the plants be shut down only for the period of time required to perform the inspections.
Although metallurgical examinations of all the pipes where cracks have occurred have not been completed, the evidence at hand on January 30, when the NRC directive was issued, indicates that the cracks found in the stainless steel pipes of several boiling water reactors were caused by a phenomenon called intergranular stress corrosion.
The stainless steel used in these pipes is subject, in varying degree, to a condition called " sensitization" as a consequence of fabrication and welding processes.
This condition renders the material more susceptible to intergranular stress corrosion cracking when the piping is stressed in service, and is exposed to certaia levels of oxygenated water. As already noted,
- the coolant in boiling water reactors contains higher levels of oxygen than does that of pressurized water reactors.
No similar cracking phenomenon has been observed in the reactor coolant piping of pressurized water reactors. The specific combinations of f M urs contributing to stress corrosion are not present to the same degree in this type of reactor.
In particular, the chemistry of the coolant is different and the level of dissolved oxygen is lower.
Because of these identified differences and the disparate service experience, the NRC Regulatory staff has not identified a need to require the special inspection and tests of the piping systems of pressurized water reactors.
Question (1)
"What are the differences between the emergency core cooling systems used in the reactors closed by NRC and the reactor in Wiscasset, Maine? Do these differences insure that cracks similar to those found in the Dresden-2 emergency core cooling system uill not occur in the ECCS's of high pressure reactor?"
Although there are differences in the designs of the emergency core cooling systens between the two types of plants', it is not these difference: in design that led to the distinction the staff made between boiling and pressurized water reactors with regard to the need for special piping inspections. The different water environments noted above and the absence of incidences of intergranular stress corrosion cracking in pressurized water reactors, are the reasons for that distinction.
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Question (2)
"Has NRC considered that similar problems may arise with the high pressure reactors such as the one in flaine? lihat action is NRC taking to insure that these dangerous conditions will not arise?"
In recommending that the Comission direct a prompt inspection of selected piping in boiling water reactors, the staff did consider whether such action was also needed for pressurized water reactors, such as the Maine Yankee plant. For the reasons previously discussed, the staff concluded that the situation did not require such special inspections of pressu'rized water re.1ctors. Should the results of the ongoing investigations of this matter susgest the er.istence of a problem in the piping of pressurized water reictors, the need for special, or more frequent, inspections of these reactors will be considered.
All operating reactors are required, in compliance with NRC regulations, to perform periodic inspections of their piping systems, as prescribed in Section XI of the Boiler and Pressure Vessel Code of the American Society of Mechanical Engineers. Such periodic inspections provide for
~ early detection of potential cracking problems.
If through-wall leaking cracks should occur in a pipe prior to their detection by the periodic inspection program, leakage detection systems installed in nuclear power plants would provide timely indication.
In accordance with the limits specified in the plant's operating license, shutdown would be required to determine the nature and location of the leakage, and appropriate corrective action must be taken. Tha stainless steel piping materials involved exhibit ductile properties so that growth of such cracks would proceed slowly, and detectable leakage would occur well before complete pipe severance.
In addition, the redundant emergency core cooling systems nrovided on both boiling water reactors and pressurized water reactnrs protect against the consequences of complete pipe severance ranging from the smallest to the largest pipes in the primary system.
It should l
be realized that, in each case where cracks have been observed, the leakage from reported through-wall cracks in the stainless steel piping was insufficient to cause any significant drop in coolant level in the reactors or to involve actuation of an emergency core cooling system.
Question (3)
"Has NRC discovered tne exact cause of the cracked pipes? Has it ruled out cause; such as the method in which pipes are walded together, and may not the problen, if it is the welding for instance, extend to other reactors of other types?"
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l The exact cause of the crack occurrences has not yet been determined, l
although, as noted previously, the mechanism of stress corrosion crack-i ing appears to be involved. A special study group within NRC is looking into the method by which pipes are welded together, as well as other factors that may be causative of cracking.
Several factors including reactor water chemistry, the imposed service stresses, and sensitization of austenitic stainless steels associated with fabrication and welding, acting in concert, could contribute to the initiation of cracking in stainless steel pipes.
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