ML19309C233
| ML19309C233 | |
| Person / Time | |
|---|---|
| Site: | Surry  |
| Issue date: | 02/05/1980 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML19309C232 | List: |
| References | |
| NUDOCS 8004080327 | |
| Download: ML19309C233 (5) | |
Text
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UNITED STATES
[ $ ey( j NUCLEAR REGULATORY. COMMISSION
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C WASHINGTON. D. C. 20555
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w SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO AMENDMENT NO. 55 TO LICENSE NO. OPR-32 VIRGINIA ELECTRIC _AND POWER COMPANY SURRY POWER STATION UNIT NO. 1 DOCKET NO._50-280 Introduction By letters dated April 6,1979 and May 31, 1979, Virginia Electric and Power Company (licensee) submitted results of a steam generator inspection performed in April 1979 (during the March 15, 1979 outage) and requested a license amendment to allow seven months of equivalent operation from the date of this inspection after which the unit would be shut down for steam generator replacement.
Following telephone discussions with the NRC staff the licensee agreed to reduce this to six months of equivalent operation. Surry Unit i returned to power October 25, 1979, following the March 16, 1979 shutdown.
Amendment No. 45 to Operating License DPR-32 for Surry Unit I authorized six months of equivalent operation (above 350*F) from December 29, 1978 following which the unit was to be brought to a cold shutdown for steam generator inspection. Seventy One (71) days of the authorized operating interval had been completed at the time the unit was shut down on March 15, 1979 as a result of the seismic related Show Cause Order issued by the NRC. Therefore, the April 1979 inspection was based on somewhat less than half of the authorized interval.
Discussion Inspection Program The April 1979 steam generator tube inspection included programs to assess the conditions associated with both the denting and " wastage" phenomena.
For denting, tube gauging was performed using.540",.610" and.650" diameter eddy current probes in all three steam generators to assess the extent and pattern of tube denting. On the hot leg side, all tubes near the tube lane which were predicted to be bounded by the 17.55 hoop strain contour were gauged. Surry Unit I has operated approximately 13 i
l 3004080%
. equivalent months since full closure of the support plate flow slots was first observed during the April 1977 inspection. The tube hoop strains calculated by finite element analysis for 18 equivalent months were used to determine the 17.5% inspection boundary. The 15% strain boundary which was used as the basis for previous inspections at Surry Unit 1, is predicted to cover almost the entire tubesheet at 18 equivalent months beyond full closure of the flow slots. The licensee maintains that extensive plant specific information for Surry Unit 1 indicates the adequacy of the 17.5% boundary and notes that significant tube restriction activity remains confined to areas immediately adjacent to previous activity.
Gauging within the 17.5% strain boundary was supplemented by additional gauging in the wedge and patch plate regions where significant activity could be expected based on plant experience at this and similar units.
Additionally, when a restricted tube was found near the inspection boundary, the inspection was expanded in that area.
Gauging was also performed on cold leg tubes in all three steam generators in conjunction with the U-bend inspection program conducted from the cold leg side.
Eddy current testing (ECT) was performed through the first support plate in the control region of the tube bundle (" kidney" region) in each steam generator where previous inspections have indicated wastage corrosion activity and tube leaks. This inspection was identical to the December 1978 inspection and included all tubes for which the May 1978 Regulatory Guide 1.83 inspection had indicated greater than 20% wastage degradation. In addition, all tubes one and two tubes beyond the May 1978 Regulatory Guide 1.83 inspection boundary were inspected to determine whether the area of significant tube wastage had expanded beyond that boundary.
Using photographs, handhole inspections of visible tube support plates were performed in all three steam generators in order to assess the support plate conditions. Also, wrapper to shell annulus measurements were made in steam generator'C to monitor the support plate growth due to the continuing formation of magnetite in the tube / tube support plate crevices.
Insoection Results Results of the tube gauging inspection did not indicate any increase in rate of tube restriction activity since the previous inspection in December 1978. No tubes in any of the steam generators restricted passage of the.540" eddy current probe. All tubes restricting passage of the
.610" and.650" probes were adjacent to areas where denting activity was observed in previous inspections. Tubes inspected on the cold leg side in all steam generators met.the gauging criteria with a.610" probe and U-bend inspections revealed no defects.
. Two of the tubes with.650" restrictions leaked during hydrotesting.
These tubes, (at Row 20, Column 8 and Row 5, Column 75 in the hot leg of steam generator C), showed slow drips at a 200 psi differential pressure. A very small reactor coolant to secondary side leak, estimated in the range of 0.6 to 0.8 gpd had been identified in steam generator C prior to shutdown on March 15, 1979.
The results of the handhole inspections revealed that the condition of the visible support plates had not changed since previous inspections and no new phenomena were observed.
Plugging Program The plugging criteria for dented tubes implemented by the licensee during the April 1979 inspection were the same as those discussed in the SER attached to the Order of December 3,1977. These include the plugging of leaking tubes.540" and.610" probe-restricted tubes,.650" probe-restricted tubes in the periphery of the hot leg wedge region, and preventa-tive plugging criteria to preclude tube leaks resulting from the progression of denting. Additionally, those tubes inspected for wastage in the kidney regions that exhibited either significant wastage (not necessarily in excess of the 40% plugging limit) or some wastage in proximity to a dent of sufficient si:e to mask the defect were plugged.
The plugging totals for the April 1979 inspection were as follows:
Plugged this Outage Total Plugged to Date Denting Related Wastage Related SG A 3
2 27; SG B 1
0 644 SG C 20 2
986 A total of 2504 or 24.6% of the tubes in the three steam generator tubes have been plugged to date.
Evalua tion Surry Unit No.1 is one of the six lead PWR facilities that have suffered moderate to extensive tube denting and that have been under close monitoring by the NRC staff following the September 15, 1976 tube failure at Surry Unit No. 2.
The April 1979 inspection program is the sixth such program for this unit and follows by only 71 days of equivalent operation the previous inspection performed in December 1978. A discussion on the
. technical background and safety evaluation of the denting related phenomenon was presented in an SER dated February 11, 1977, in support of the NRC Order for Modification of Facility Operating License No. DPR-32 dated February 8,1977, and in an SER attached to a later NRC Order affecting Surry Unit No.1 dated May 6,1977. The background information contained in the February 11, 1977 and May 6,1977 SERs remains valid and is incorporated in this Safety Evaluation by reference. This report is an update on the condition of the steam generators at Surry Unit No.1.
The April 1979 gauging and plugging program performed at Surry Unit I was similar to previously implemented programs at this unit, Surry Unit 2, and Turkey Point Units 3 and 4 These gauging programs have been develop *d over the course of time in consultation with the NRC staff and have been determined adequate to support operation of these facilities for six effective full power months.
Whereas the predicted location of the 15% tube hoop strain. contour provided the basis for the gauging boundary used in the December 1978 inspection, the predicted location of the 15% contour at the time of the April 1979 inspection encompassed almost all of the tube support plate. However, significant tube restriction activity occurred well within the imple--
mented inspection boundary based upon the predicted 17.5% strain contour.
Therefore, we agree that the inspection results confirm that the 17.5%
strain inspection boundary adequately bounded the region of active tube denting.
Based upon our review of gauging results, we find that tube restriction activity has continued to progress in a stable manner at a rate consistent with that observed previously. We also note, however that, in the December 1978 and April 1979 inspections, five tubes identified as leaking tubes were apparently able to pass a.610" eddy current probe, and that two of these were also apparently able to pass a.650" probe.
Past experience (prior to December 1978) had shown that dent related leaks involved only.540" and.610" restricted tubes which are preventively plugged under current criteria. Stress corrosion cracking is a function of time as well as strain level and environment. The recent appearance of leaks in tubes which pass a.610" probe, and which are not otherwise subject to preventative plugging under the current criteria, indicates that the time parameter is becoming significant.
The preventive plugging criteria implemented in April 1979 and in previous inspections has proven successful in removing from service severely restricted tubes which are the most likely candidates to develop inservice l ea ks. Through-wall cracks wnich have occurred at dented locations have been small and stable (no rapid failures). The Technical Specification 0.3 gpm leakage rate limit provides adequate assurance that even if through-wall cracks and leaks occur, they will be detected and appropriate corrective action will be taken before any individual crack becomes sufficiently large as to be unstable under normal operating, transient, or accident conditions.
With regard to the wastage phenomenon in the kidney region, the April 1979 wastage inspection and associated plugging' criteria were identical to those implemented in the December 1978 inspection. The small number of tubes plugged (two tubes in steam generators A and C, respectively) as a result of wastage is consistent with the short period of operation between the December 1978 and April 1979 inspections. We concluded that the wastage inspection in April 1979 was adequate to establish the current extent and magnitude of wastage in the Surry Unit I steam generators and that there is reasonable assurance that unacceptable wastage degradation will not occur during the next operating interval.
Summarv On the basis of the above evaluation, we conclude that Surry Unit 1 may be operated for six (6) equivalent months from the April 1979 inspection without adversely affecting public health or safety and that paragraph 3.E of Facility Operating License No. OPR-32 should be revised accordingly.
Environmental Consideration We have determined that this amendment does not authorize a change in effluent types or total amounts nor an increase in power level and will not result in any significant environmental impact. Having made this determination, we have further concluded that the amendment involves an action which is insignificant from the standpoint of environmental impact, and pursuant to 10 CFR 551.5(d)(4) that an environmental impact statemsnt, or negative declaration and environmental impact appraisal need not be prepared in connection with the issuance of this amendment.
Conclusion We have concluded based on the considerations discussed above, that (1) because the amendment does not involve a significant increase in the probabil*ity or consequences cf accidents previously considered and does not involve a significant decrease in a safety margin, the amendment does not involve a significant hazards consideration, (2) there is reasonable assurance that the health and safety of the public will not be endangered by operation in the proposed manner, and (3) such activities will be conducted in compliance with the Commission's regulations and the issuance of this amendment will not be inimical to the common defense and security or to the health and safety of the public.
i Da te : February 5,1980
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