ML20003H649
| ML20003H649 | |
| Person / Time | |
|---|---|
| Site: | Millstone  |
| Issue date: | 04/16/1981 |
| 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 8105060569 | |
| Download: ML20003H649 (6) | |
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//6 NORTHEAST IFI1EJTIES 1
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April 16, 1981 g
g Docket No. 50-336
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Mr. Boyce H. Grier, Director Region I Ng -
Office of Inspection and Enforcement U. S. Nuclear Regulatory Commission
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631 Park Avenue King of Prussia, PA 19406 Referances:
(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 March 11, 1981.
(3)
W. G. Counsil letter to B. H. Grier, dated March 27, 1981.
Gentlemen:
Millstone Nuclear Power Station, Unit No. 2 I&E Bulletin No. 81-01, Mechanical Snubbers In fulfillment of the requiremeMs of I&E Bulletin No. 81-01, Northeast Nuclear Energy Ccmpany (NNECO) performed visual exa-inations and manual stroke tests of accessible mechanical snubbers manufactured by International Nuclear Safeguards Corporation.
During these inspections, NNECO identified several saubleu vLich could not be stroked through their full range in both compression and t ension. These snubbers were declared inoperable, and in accordance with Item le of Reference (1), NUECO provided the required justification for continued operation of Millstone Unit No. 2 in References (2) and (3). At that time, it was NUECO's intention, as concurred in by your Staff, to commence a plant shutdown by May 1, 1981, to replace all inaccessible INC mechanical snubbers with snubbers of a different design. However, as reported in Reference (3), evaluations were in progress to support operation of Millstone Unit No. 2 beyond May 1,1981. These evaluations have been completed and the results, presented herein, support NUECO's conclusion that Millstone Unit No. 2 can be operated safely until the next cold shutdown at which time the snubber replacement program will co=mence.
Millstone Unit No. 2 utilizes forty-one (kl) INC mechanical snubbers on safety related systems within the containment. The systems and the number of snubbers by model are presented in Table 1.
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2-UNECO has performed stress analyses and fatigue evaluations for all the systems within the containment on which ETC mechanical snubbers are in-stalled. The analyses conservatively assumed that all the HTC mechanical snubbers have been locked in the cold position of the piping systems since the initial operation of the plant. This assumption is extremely conservative compared to the data obtained to date on failures of INC snubbers at Millstone Unit No. 2 as well as other nuclear plants.
This data demonstrates much lower failure rates than that assumed herein. The results of the evaluations for each system are discussed below.
Pressurizer Safety and Relief Systems A fatigue analysis was performed for those systems assuming all twenty (20) INC snubbers were locked in the cold condition from initial plant operation. The analysis incorporated all plant thermal cycles experienced to date and in addition, postulated both a safety and relief valve operation concurrent with a Safe Shutdown Earthquake (SSE).
The results of this analysis demonstrate a maximum usage factor of 0 7 This value re=ains below the code allowables for these systems. These systems experience the most severe thermal transients of all the containment systems analyzed. The results support the contention that Millstone Unit No. 2 can be safely operated at least until the next cold shutdown.
Pressurizer Spray System The pressuriz6r spray system was analyzed assuming all the INC snubbers have been inoperable since the initial plant operation.
In addition, all postulated operating cycles, including an SSE, were assumed.
The results of this analysis demonstrate a minimal increase in the stress level for this system. The original fatigue analysis was scaled to illustrate a conservative usage factor significantly below code allowables, j
Safety Injection System The safety injection system was analyzed assuming all INC mechanical snubbers i
l have been frozen since initial plant operation. The fatigue evaluation in-cluded all the original postulated mechanical loading and actual thermal cycles experienced to date.
The maximum usage factor calculated utilizing these conservative assu=ptions was 0.26 which remains below code allowables.
Containment Spray System The containment spray system was analyzed assuming all INC snubbers were inoperable and included either a post accident operation of the system or an SSE. The thermal stresses calculated were below the fatigue endurance limit and the l
. stresses resulting from the seismic event were within code allowables.
The results of these analyses demonstrate that the integrity of this system would not be compromised in the event of a postulated accident requiring this system to be functional or following an SSE.
Shutdown Cooling System i
The shutdown cooling system was analyzed assuming all the INC snubbers were inoperable and frozen in the cold position since initial p1 ant operation. In addition, all thermal and mechanical cycles assumed in the original design were incorporsted into the analysis. The increase in thermal stresses due to the inoperable snubbers is minimal and the fatigue analysis results demonstrate a conservative usage factor of 0.25.
NNI:C0 has reviewed the results of inservice inspections performed at Millstone Unit No. 2.
Inspections conducted on the pressurizer safety and relief, pres-surizer spray, and shutdown cooling systems during outages in 1980 and 1977 4
confirm the absence of structural anomalies.
Six (6) welds have been inspected in both the pressurizer safety / relief system and the pressurizer spray system and three (3) welds were inspected in the shutdown cooling system. The ultra-sonic examinations revealed no recordable indications in any of the velds inspected.
Based on the results of the analyses and evaluations performed to date, NNECO has determined that all piping systems located within the containment at Millston,e Unit No. 2 are acceptable with inoberable INC mechanical snubbers.
The results of these analyses are not time dependent.
The thermal state and, therefore, the stress levels of the systems involved will remain constant until the plant is brou6 t to a cold shutdown condition.
h The conservatisms assumed in the analyses presented herein include a 100%
snubber failure rate, SSE seismic loading, and actual plant thermal cycles i
experienced to date. NNECO concludes that the integrity and operability of l
the systems on which INC mechanical snubbers are installed will not be com-promised by continued operatica of Millstone Unit No. 2.
A cold shutdown l
for the purposes of examining the INC mechanical snubbers within containment would unnecessarily add to the fatigue usage of the effected plant components.
Therefore, it is NNECO's intention to operate the Unit until the next cold shutdown, during which the INC mechanical snubbers located in the containment l
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4 Will be replaced with snubbers of e. different design. All INC mechanical snubbers will be replaced prior to startup from the next refueling outage.
We trust you find this information satisfactory and concur in this approach.
Very truly yours, NORTHEAST NUCLEAR ENERGY COMPANT i
W. G. Counsil Senior Vice President Director of Nuclear Reactor Regulation Attn: Mr. Robert A. Clark, Chief Operating Reactors Branch #3 U. S. Nuclear Regulatory Ccxamission Washington, D.C.
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STATE OF CONNECTICUT )
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COUNTY OF HARTFORD
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Then personally appeared before me W. G. Counsil, who being duly sworn, did state that he is Senior Vice President of Northeast Nuclear Energy Company, a Licensee herein, that he is authorized to execute and file the foregoing information in the name and on behalf of the Licensees herein and that the statements contained in said information are true and correct to the best of his knowledge and belief.
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Table 1 INC Mechanical Snubbers in Containment Millstone Nuclear Power Station, Unit No. 2 Number Of Snubber System Snubbers Model Pressurizer Relief 1
MSVA-1 19
- MSVA-2 i
Pressurizer Spray 7
MSVA-2 Containment Spray 4
MSVA-1 2
MSVA-2 Shutdown Cooling 2
MS?A-3 Safety Injection 3
MSVA-2 j
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