ML20003G079

Updates Response to IE Bulletin 81-01, Surveillance of Mechanical Snubbers. Util Will Visually Examine & Stroke Test All Accessible Snubbers by 810320

ML20003G079
Person / Time
Site:	Millstone
Issue date:	03/11/1981
From:	Counsil W NORTHEAST NUCLEAR ENERGY CO.
To:	Grier B NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION I)
References
A01531, A1531, IEB-81-01, IEB-81-1, TAC-08952, TAC-8952, NUDOCS 8104280202
Download: ML20003G079 (5)

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g N March 11, 1981 -

Docket No. 50-336 8 f A01531 A 6, f "s4p " Usa =

Mr. Boyce H. Grier , Director Region I 4hr '

Office of Inspection and Enforcement U. S. Nuclear Regulatory Commission b

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631 Park Avenue King of Prussia, PA 19406

References:

(1) B. H. Grier letter to W. G. Counsil, dated January 27, 1981, transmitting I&E Bulletin No. 81-01.

(2) W. G. Counsil letter to B. H. Grier , dated February 19, 1981.

(3) D. G. Eisenhut letter to All Power Reactor Licensees (except SEP licensees), dated November 20, 1980.

Gentlemen:

Millstone Nuclear Power Station, Unit No. 2 I&E Bulletin No. 81-01, Mechanical Snubbers In Reference (1), the NRC Staff provided Northeast Nuclear Energy Company (NNECO) with a description of failures of mechanical snubbers supplied by International Nuclear Safeguards Corporation (INC) snd Pacific Scientific Company. The Staff requested that NNECO take certain actions to determine the condition of the mechanical snubbers, specifically those manufactured by INC, at Millstone Unit No. 2. ,

In Reference (2), NUECO provided the Staff with an outline of the approach by which the Reference (1) concerns would be addressed. Subsequent to the docketing of Reference (2), verbal communications with the Region I Staff were held on February 26 and February 27, 1981. During these conversations, our respectite Staffs agreed to a program by which IDECO vould respond to the Reference (1) concerns for Millstone Unit No. 2. This included an extensien until March 20, 1981, to perform the stroke testing of all accessible INC mechanical snubbers.

The mutually acceptable approach which NNECO is cur'--tly i=plementing at Millstone Unit No. 2 to respond to the Reference (1, uancerns is sunmarized below:

1. By March 20, 1981, UNECO vill visually examine and stroke test all accessible INC mechanical snubbers. The results of these examinations will be verbally conmunicated to the Staff,by March 20, 1981 with written followup correspondence.

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2. Operability testing, as described in Reference (1), of a representative sample of accessible INC mechanical snubbers will be performed during the next outage of greater than thirty (30) days duration. .

3 All inaccessible INC mechanical snubbers will be visually examined and manually stroke tested during the first outage of greater than five (5) days duration.

4. Operability testing, as described in Reference (1), of a representative sample of inaccessible INC mechanical snubbers will be performed prior to startup frcm the next scheduled refueling outage.

In addition, NNECO will be proposing Technical Specifications addressing inservice inspection for mechanical snubbers as requested by Reference (3). NNECO has determined that the response to Reference (3) will fulfill the requirements delineated in Item 3 of Reference (1) for Millstone Unit No. 2.

In fulfillment of the commitments made to the Staff on February 27, 1981, NNECO proceeded with a program to stroke test the accessible INC mechanical snubbers. .

To date, thirty nine (39) snubbers have been inspected. NNECO has identified fourteen (14) mechanical snubbers which could not be stroke tested satisfactorily.

The systems'on which the inoperable snubbers are located are described below:

SYSTDi/ DESCRIPTION INOPERAELE SNUEEERS Main Steam Supply 6 to Terry Turbine Service Water System 5 Safety Injection 3

, Pumps Suction All inoperable snubbers were replaced within the time constraints of the appropriate Technical Specification action statements and the affected systems were returned to service.

Of the systems on which inoperable mechanical snubbers have been identified, NNECO has determined that the =ain steam supply to the terry turbine would be the most limiting with respect to stresses generated as a result of thermal

growth restricted by locked snubbers. The basis for this determination is that this piping experiences large ther=al growth as well as higher than average vibration during operation. In addition, this system has experienced the highest failure rate , to date, of mechanical snubiers. Tne high failure rate of techanical snubbers on the main steam supply to the terry turbine can be attributed to the snubber location and the environment to which they have been exposed. The six (6) failed snubbers are located in the penetration rooms and have pr;viously been exposed to adverse environments consisting of elevated temperatures and humidity. These conditions resulted from stea

leaks in the penetration rooms.

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Snubber lock-ups restricting thermal growth will result in high stresses within the piping system.

As such, a stress analysis was performed for that portion of the main steam supply to the terry turbine which included the six (6) inoperable mechanical snubbers. The results of this analysis demonstrate that the stresses generated by ther=al growth of the system, compounded by the locked snubbers, remain below the yield strength of the piping material. NNECO concludes that the integrity of this system has not been compromised as a result of the failure of the mechanical snubbers and that this system vill continue to perfom its intended design function.

It is NNECO's engineering judgment that the operating conditions which the main steam supply to the terry turbine experiences as well as the high failure rate of the mechanical snubbers on this system represent the vorst case which is expected to be identified at Millstone Unit No. 2.

In addition to the information obtained to date on the accessible INC

, mechanical snubbers, the following data on snubbers located in inaccessible areas of Millstone Unit No. 2 is provided.

The systems on which the inaccessible mechanical snubbers are located are listed below:

SYSTEM PIPE SIZE (in) NUMBER OF SNUBBERS Safety Injection 6 h Pressurizer Relief 2h lh h 6 Pressurizer Spray 3 7 Shutdown Cooling 12 2 Containment Spray 8 6 Safety Injection 12 2 Tank Outlet Walkdowns cf safety related piping systems conducted in response to I&E Bulletin No. 79-14 did not identify any visual ane" lies with the pipe supports, hangers, or associated snubbers located in inaccessible areas of the plant. These walkdowns were completed during the 1980 refueling outage.

Recent maintenance performed on a pressurizer safety valve and the associated discharge piping during January,1981, included the stroking of four =echanical snubbers in the pressurizer blockhouse. All four snubbers were detemined to be operable.

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-k-Inservice inapections of velds in the pressurizer relief valve discharge piping conducted during the 1980 refueling outage revealed no indications.

This supports the assertion that this system has not been degraded by high stresses due to snubber lock-up.

It has been determined that the environment to which the inaccessible snubbers are exposed is less severe than that seen by those snubbers located in the penetration rocms. Generally, one of the most adverse environments within containment would exist within the pressurizer blockhouse. The temperatures in this area are high (150 - 170 F),

however, the relative humidity vithin the containment is much lower than that evidenced in the penetration room during steam leaks.

Eased on the results of the analysis perfomed on the main steam system and the available information on the inaccessible snubbers, their en-vironment, and the systems on which they are located, NNECO has determined that continued operation of Millstone Unit No. 2 is justified and can be conducted safely.

According to the provisions of Item le of Reference (1), NNECO is required to commence with a plant shutdown within thirty (30) days after discovery of the first inoperable mechanical snubber at Millstone Unit No. 2, unless justification for continued operation is provided to the NRC. Currently, NNECO vould be required to shut down Millstone Unit No. 2 on or about April 1,1981, to conduct examinations of the inaccessible INC mechanical snubbers. As was discussed above, NNECO has evaluated the failure of mechanical snubbers identified to date as well as the available information on the inaccessible mechanical snubbers and determined that continued operation of Millstone Unit No. 2 is both safe and justified. As such, NNECO has responded to Items ld and e of Reference (1).

NNECO does, however, acknowledge the fact that there exist concerns related to the ability of INC mechanical snubbers to perform their design functions. Although NNECO has determined that continued safe operation-can be assured, it is our intention to commence with a shutdown of Millstone Unit No. 2 to perform a complete replacement of all inaccessible INC mechanical snubbers with units of a superior design. Due to the extremely strained financial condition of Northeast Utilities, it is highly desirable to defer this program until the restart of Millstone Unit No. 1, currently scheduled for the first week in April,1981. In any event, the shutdown for snubber replacement at Millstone. Unit No. 2 vill commence no later than .

May 1,1981, a maximum of one month delay in the schedule proposed in Reference (1).

The good faith effort by NUECO to comply with the Reference (1) requirements demonstrated to date as well as our commitment to replace the inaccessible INC =echanical snubbers in spite of serious financial burdens, represents the optimum response to the issues identified above.

To further substantiate the acceptability of continued operation, it is noted that snubber lock-up resulting in a calculated over-stress condition does not necessarily render the system inoperable. Such a system would remain functional

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provided the operational = ode of the plant and system is unchanged. . Deferring -

this mode change from April 1,1981 to May 1,1981 poses no additional threat to system integrity.

The operability testing requirenents for any INC mechanical snubbers remaining at Millstone Unit No. 2 following the changeout program vill be conducted in accordance with the program described herein. The surveillance program for mechanical snubbers produced by other manufacturers required by Item 3 of Reference (1) vill be provided in response to Reference .(3).

We trust you find this information satisfactory and conclude that the intent of the Reference (1) requirements are fulfilled by the above-described program.

Specifically, it is our position that the extension to a date not later than May 1, 1981 has been , justified, and the option of Item le of Reference (1) has been exercised. The absence of Staff action on this letter will result in implementation of the above program. In the event that the Staff identifies a concern, it is NUECO's intention to pursue this matter to the extent required to obtain concurrence in this approach.

Very truly yours, NORTHEAST INCLEAR ENERGY COMPANY

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W. G. Counsil Senior Vice President b

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