ML20062H713

Discusses Weld Drawbead Realignment of 6-inch Pressurizer Relief Pipe.Points Raised in J Halapatz 800228 Memo Are Tenuous & Not Applicable

ML20062H713
Person / Time
Site:	Sequoyah
Issue date:	07/24/1980
From:	Danni Smith Office of Nuclear Reactor Regulation
To:	Noonan V Office of Nuclear Reactor Regulation
Shared Package
ML20062H675	List: IR 05000327/1980012 ML20062H670 ML20062H676 ML20062H682 ML20062H690 ML20062H693 ML20062H695 ML20062H700 ML20062H704 ML20062H707 ML20062H710 ML20062H713 ML20062H719
References
NUDOCS 8008210281
Download: ML20062H713 (4)

Text

'

, f nua je UNITED STATES

,,,_ g y e g NUCLEAR REGULATORY COMMISSION

, t WASHINGTON D. C. 20566 o I

\.,,,./ JUL 2 41c80 MEMORANDUM FOR: V. S. Noonan, Assistant Director for Materials and Qualifications Engineering Division of Engineering THRU: S. S. Pawlicki. Chief Materials Engineering Branch A

Division of Engineering FROM: D. E. Smith Materials Application Section Materials Engineering Branch

SUBJECT:

SEQUOYAH UNIT ONE WELD DRAWBEAD REALIGNMENT OF 6" PRESSURIZER RELIEF PIPE

REFERENCES:

a. Memorandum J. Halapat: to S. S. Pawlicki of February 28, 1980, same subject.

b. TVA letter with enclosures of January 24, 1980, L. M. Mills to L. S. Rubenstein, "In the Matter of the Application of Tennessee Valley Authority Docket Nos. 50-327 and 50-328."

The important and relevant reports on the Secuoyah Unit One weld drawbead realignment of a 6" pressure vessel relief pipe have been reviewed. The conclusion of this review is that the technical coments of Reference (a) are of no significance.

The points raised in Reference (a) may be summed up as follows:

1. Inert gas shielding (backing) of the root of these welds was not provided.

The thicker oxide film of such a weld would be more susceptible to corrosion.

2. This thicker oxide film would explain the differences in perfornance of the two test methods to detect integranular corrosion. The tests performed on the welding procedure qualification did indicate material susceptible to .

sensitization, and more metallography is in order.

3. The oxygen content of the steam may cause problems when the interior pipe

"....sur# ace corrosion product sloughed off as crud, the material disclosed as sensiti:ed by Method (a) would be exposed to'a steam environment contain-ing as much as .2 ppm oxygen....".

4 The ASME Code semits base metal repairs limited to 1/3 of thickness. The question is raised if the ASME would pernit this field fix as the weld grooves used were more than 1/3 of the thickness of tne base metal.

That inert gas backing was not used on the pipe interior is of no significance.

Full penetration butt welds without inert gas backing is the standard shop and 80(C210281

V. S. Noonan JUL 21 meu field weld assembly method for nuclear piping. The procedure used was qualified without inert gas backing. The American Society for Mechanical Engineering's Boiler and Pressure Vessel Code,1977,Section IX for gas tungsten are welding allows the d61etion or addition of gas backing without -equalification of the procedure. The acd Mion or deletion of gas backing is a non-essential variable of a weld procedure qualification and has been in the ASME Boiler and Pressure '

Vessel Code as such since 1974. To put this in perspective, under the ASME Code, the fabricator with a weld procedure which had been qualified with gas backing may make the weld in production without gas backing. However, the fabricator would be required to use welders who had performance qualifications for welding without gas backing. The mock up welding perfomed exceeded what was required by the code or any other requirements of the NRC. The field welding was performed with very restrictive controls on welding variables within the limits of the qualified procedure, particularly heat input, which also was more restrictive than required by the code or the NRC.

The question of whether the pipe wall was penetrated, i.e., whether the heat of welding melted the metal on the inside surface of the pipe, is also of no significance. This occurs everytime a full penetration butt weld is welded without inert gas backing. If such penetration occurred with the production job as it did in the mockup, there is no violation of code requirements as outlined above. In addition, subsecuent radiography of the production weld cemonstrates that full penetration had not occurred.

The next concern was that the absence of gas backing caused the interior surface of the pipe to,be oxidi:ed. Reference (a), paragraph 2 states: "If oxidized beyond a temper film, such surfaces are not necessarily corrosion resistant and may actually rust and slough off in service." The corrosion resistance of stainless steels is due to their feming an adherent oxide film. The good performance of existing welds which have a much more severely oxidized root than these non-penetration groove welds proves this issue of slight oxidation of the inside pipe surface area under these welds is unimportant.

The next issue raised was the response of the samples tested to ASTM A-262 and linking their response to alterations in base metal chemistry due to oxidation.

Oxidation of stainless steel alloys is a surface pnenomena. The oxidation film at the root of a weld developed during welding is limited in thickness to i

no more than .002 inches. The bulk material's response to intergranular cor-rosion would not be affected by such a film. ASTM A-262 specifies Method (a) as a screening test, and in the event of an unacceptable microstructure by Method (a) (not a cause for rejection of the material) then Method E is to be perfomed which detamines its acceptability or not in tems of susceptibility to intergranular cracking. Method A can accept material but not reject it.

In Metnoo E, edge cracks are specifically coverec in that they should be excluded from consideration per Footnote 22. In the event of questionable aopearance, metallograpny is required. The bend specimens had an acceptable appearance. However, over and above A-262 recuirements, metallography was perfomed and no fissuring was present. The soecification (A-262) requirements were met and the weld procedure was demonstrated not to make the material

V. S. Noonan -3 JUL 2 i 1::cu susceptible to intergranular cracking in accordance with Regulatory Guide 1.44,

" Control of the Use of Sensitized Stainless Steel." The field metallographic examination of the heat affected zones of the drawbead welds was over and above the code and NRC requirements. The results of these examinations showed that the heat affected zones of these welds had not been sensitized.

Mention is made of the mat"tal being exposed "to a steam environment contain-ing as .2 ppm oxygen" and coupled with the sensitization detected by Method A, it is intimated that a problem could occur.

The Technical Specifications for Pressurized Water Reactors has a maximum oxygen content limit for nomal running operations in the primary loop of .1 ppm.

For transient, the oxygen content is allowed to increase to 1.0 ppm and may remain above the 0.1 ppm limit for a maximum of 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />. When the oxygen content exceeds the transient limit of 1.0 ppm, the reactor must be shut down.

The recort submitted by the Tennessee Valley Authority (Reference b) states:

"The highest calculated oxygen concentration will not exceed .2 ppm." This condition is a transient response during complete loss of water chemistry control. The engineering of this particular reactor's water chemistry control will have a normal operating oxygen content in the steam above the pressurizer of approximately .005 pom oxygen. The transient response oxygen content of

.2 ppm is not expected to cause intergranular stress corrosion cracking of stainless steel. On this issue of oxygen content being of concern because of intergranular stress corrosion cracking of stainless steel and its welcments, the present Technical Specifications and the oxygen content control of this installation show conditions to be acceptable with a good margin of conservatism.

In regards to the code limiting base metal repairs to 1/3 thickness and intimat-ing tnis limit should apply to these drawbead welds, the code clearly applies this limit to material suppliers and is not relevant to this situation. This was not a base metal repair of a surface defect which is the item addressed in NB-2539. If the incorrect logic of Reference (a) is followed to its conclusion, you would not allow weld repairs of weldments over 1/3 their thickness. You would then not be able to build any complex welded project as invariably, you will have weld repairs of weldments which are over 1/3 their thickness.

A basic objection to the weld drawbead technique is the avoidance of a hydro-static pressure tast because with this technique, the pressure boundary is not disturbed. This cojection does not appear in any of the correspondence but has been expressed verbally more than once to the author. It is believed that this tecnnique is viewed by a few members of the NRC staff as a " cheap trick" to avoid an excensive hydrostest procedure. It is the author's ocinion that this

• eld drawbead technique should be positively viewed as a practical, sound engineering approach to avoid tne unnecessary expense of hydrotesting.

Standard non-destructive testing techniques are more than adecuate to establish tne soundness and adecuacy of these welds, and demonstrate tnis to a higher oegree of confidence than a hydrotest which coes not even exceed yield strength sf the base metals.

V. S. Noonan JUL 2 4 b50 Regarding the actual production work, these welds need not be made special in any sense, and consideration should be given to not making them subject to augmented inservice inspection. The logic is that the full penetration circumferential butt joints on this pipe are of a much more critical service and would serve as an adequate monitor for these welds of less severe service and of less criticality. These welds are of less severe service because they are only partial circumference and are not full penetration welds. For the same reasons, they have less severe service than full circumference, full penetration butt welds in the same pipe. If inservice inspection is done on a sampling basis, it is reconnended that these welds be counted for the initial determination of population, but as they do not see the most severe service, be excluded from the sample to be inspected in the interest of conservation.

It must be emphasized that these welds meet all applicable code and regulatory guide requirements.

In conclusion, the points raised in Reference f =) were tenuous or not acplicable.

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Y l' D. E. Smi th Materials Application Section Materials Engineering Branch Division of Engineering Office of Nuclear Reactor Regulation cc: S. S. Pawlickt R. M. Gamble W. S. Hazelton F. M. Almeter D. E. Smith l

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