ML20235E066

Safety Evaluation Re Corrosion of Drywell Shell.Licensee Meets NRC Requirements as Specified in 861224 Safety Evaluation

ML20235E066
Person / Time
Site:	Oyster Creek
Issue date:	09/22/1987
From:	Office of Nuclear Reactor Regulation
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ML20235E022	List: ML20235E019 ML20235E066
References
NUDOCS 8709250470
Download: ML20235E066 (3)

Text

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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR. REGULATION RELATED TO CORROSION OF THE DRYWELL SHELL RPU NUCLEAR CORPORATION OYSTER CREEK NUCLEAR GENERATING STATION DOCKET NO.50-219 1.0 -INTRODUCTION-In late November 1986, GPU notified the NRC of' a condition involving corrosion of the outir surface.of the drywell shell. Three meetings were held between the staff and GPU on December 1 10, and 19,1986 to discuss this subject.

By letter dated December 19, 1986,.GPU provided, for staff review, justi-fication for interim operation for Cycle 12R. Based on our review of this information, the staff concluded in our letter and Safety Evaluation dated December 29, 1986 that the original code allowables for the drywell are met should the drywell plates be locally reduced in'the sand cushion area to-0.700 inch. Therefore, with a conservatively assumed corrosion. rate, the drywell has adequate design margins for operation during Cycle 12.

We, however, required GPU to take the following actions:

1. GPU has comitted "to continue the UT shell thickness test programs at future outages of opportunity including forced outages otherwise requiring drywell entry during the next cycle." The staff requests that GPU provide the NRC with the plans for the intended inspection and, should there be no ,

forced' outages of sufficient duration prior to.the mid-point of the

-0 operating cycle, perfonn the inspection no later than September 30,1987.

This activity is necessary to assure the corrosion rate assumed by the licensee is bounding.

2. GPU shall, by June 30, 1987, provide comprehensive plans for mitigating the corrosive attack on the drywell shell and any other intended long term corrective actions it may deem necessary for staff review and approval.

As a fulfillment of the NRC requirement and of its comitment, GPU made a presentation on June 11,.1987 to the NRC staff of the actions taken and plans under study to: resolve the staff's concerns.

2.0 EVALUATION Two additional UT measurements of the wall thickness were made by GPU, one in February, another in May 1987. On the basis of the data so far obtained, GPU concluded that the previous safety evaluation is not invalidated, and planned outage in September 1987.for UT as specified by NRC is not required, especially with the possibility of more opportunities to obtain data during the current operating cycle. The staff has reviewed the presentation--(Ref. 2) and the information (Ref.1) on UT measurements and concurred with GPU's conclusion.

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GPU together with its consultants made a presentation on' corrosion mechanism and corrosion arrest. From the results of tests of' samples taken, GPU -

concluded that the wastage of the drywell shell is an aqueous general.

corrosion process and there is no evidence of the. involvement of bacteria even though viable bacteria have been identified.' On the basis' of this-finding, GPU has studied various alternatives.for corrosion arrest, ranging from cathodic protection., chemical treatment, drying the sand cushion, and removal of sand and coatings. ' Among the alternatives, GPU has selected cathodic protection as the means of corrosion' arrest. GPU has not provided to the staff the definite date of implementing the cathodic protection. GPU should advise the staff when the system is. installed and operational. The staff has reviewed.

.the information given at the presentation .(Ref.2) and in' reports'(Ref.1)-

submitted to the staff which are related to corrosion, and found GPU's findings and the planned action for corrosion arrest are acceptable'.

GPU's consultant, the Chicago Bridge and Iron made'a more detailed stress analysis of the drywell shell. Seven different shell and sand configurations and twelve load cases were considered in the analysis. On the basis of the results of this more detailed analysis, GPU concluded that the analysis continues to demonstrate code allowable stresses are met and original design stress margins are maintained. On the basis of the review of the infomation presented, the staff concurs with GPU's conclusion.

Since it has been determined that the moisture in the sand patch is the major cause to corrosion of the drywell shell, GPU continues their efforts to identify and minimize future water incursion into the sand bed.

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3.0 CONCLUSION

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-0 From the various actions taken by GPU as described above, the staff-concl6 des that GPU has met staff's requirements as specified in staff's December 24,.

1986 Safety' Evaluation. The staff approved the actions taken and those planned including the commitments, (1) to implement cathodic protection as a means of corrosion arrest, (2) to monitor wall thickness by UT at outages of opportunities for several years, and (3) to identify'and minimize future water incursions into the sand bed.

4.0 REFERENCES

1. Letter to NRC from J.R. Thorpe of GPU dated May 29, 1987 forwarding two reports on drywell corrosion mechanism.

2. GPU presentation to NRC on subject issue on June 11, 1987.~ Details of the presentation were documented in the meeting summary dated June 18, 1987.

Principal Contributors:

C. P. Tan F. B. Litton F. Y. Witt Dated:

The staff has completed its review of the information presented. Our Safety '

Evaluation is enclosed. Based on our. review, the staff concludes that the licensee has met the staff's requirements as specified in the staff's December 29, 1986' Safety Evaluation. The staff also a those planned including the commitments, (1)topproved thecathodic actions protection taken and i

implement as a means of corrosion arrest, (2) to monitor wall thickness by UT at outages  ;

of opportunities for several years, and (3) to identify and minimize future -;

water incursions into the sand bed.

You are requested to advise the staff when the cathodic protection system is )

installed and operational. '

Sincerely, .

/s'/

John F. Stolz, Director Project Directorate I-4 t Division of Reactor Projects I/II  ;

Enclosure:

As stated cc w/ enclosure: l See next page J l

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Distribution:  !

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