ML19347C550
| ML19347C550 | |
| Person / Time | |
|---|---|
| Site: | North Anna  |
| Issue date: | 12/08/1980 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML19347C549 | List: |
| References | |
| NUDOCS 8012310019 | |
| Download: ML19347C550 (3) | |
Text
t SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION VIRGINIA ELECTRIC AND POWER COMPANY NORTH ANNA POWER STATION, UNIT NO. 1 DOCKET NO. 50-338 4
INTRODUCTION In a letter dated December 10, 1979 Virginia Electric and Power Company (VEPCO) comitted to installing inspection ports just above the upper support plate in all three steam generators at North Anna Unit 1.
These inspection ports were to be installed during the second refueling outage in late 1980. VEPC0 now believes that installation of the inspection ports is unnecessary at this time and provided its technical justification by letters dated September 26, 1980 and October 20, 1980.
BACKGROUND Steam generator inspections perfomed during the first refueling outage at North Anna Unit 1 during September 1979 indicated that approximately 34% of the steam generator tubes had experienced early stages of denting.
In addition, the inspec-tion reveiled throughwall defects at the U-bend tangent points of two Row 1 These defects had caused minor leakage prior to the shutdown. Because tubes.
the inspections showed no evidence of support plate "hourglassing" (deformation) of the flow slots, the U-bend defects'were not believed to be denting related.
As a precautionary measure, all Row 1 tubes in each of the steam generators were plugged. Among other corrective actions taken by VEPCO were the institution of a boric acid treatment program to prevent further corrosion of the support plates (denting), and implementation of a hydrogen monitoring program to judge the effectiveness of the ' boric" ' acid t'reat'ments.
DISCUSSION VEPCO's justification for not installing the inspection ports at this time is as follows:
1.
The plugging of all Row 1 tubes hn reduced the imediate need for inspection ports.
Preliminary indications as to the effectiveness of the boric acid treatment 2.
program will not be available until the upcoming steam generator inspections The results of this are perfomed during the second refueling outage.
inspection would prove valuable in assessing the future advisability of installing inspection ports.
3.
Installation of inspection ports would not provide significant additional benefit from an operational or safety standpoint.
Eddy current inspections The effectively reveal the degree of denting at every support plate level.
eddy current inspections are supplemented by visual inspection of the lower support plates through the lower steam generator hand hole penetrations.
Installation of upper inspection ports during the second refuelin t
would result in additional personnel exposure (about 7.5 man-rem 4.
FU123100l(
-2 EVALUATION Inspection ports located above the upper support plate provide access for visual inspection of the upper support plate and tube U-bends. The motivation to install these ports developed primarily out of concerns for stress' corrosion cracking of the small radius U-bends in Row 1 tubes.
Denting induced " hour-glassing" (deformation) of the flow slots in the upper support plate can lead directly to stress corrosion cracking in the small radius U-bends. This phen-omenon was responsible for a major tube rupture occurrence in 1976 (80 gpm primary to secondary leak) at Surry Unit 2 which affected a Row 1 tube at the apex of the U-bend.
Inspection ports provide the means for early detection of flow slot hourglassing in the upper support plate, permitting the timely plugging of all Row 1 tubes which could potentially develop U-bend cracks due to this phenomenon.
The fact that all Row 1 tubes have already been plugged at North Anna Unit i removes the imediate concern regarding the need for visual inspection of the upper support plate and U-bend region.
Laboratory examinations and analysis of tubes that have been removed from units that have experienced flow slots hourglassing and operating experience indicate Row 1 tubes to be the most susceptible to this U-bend cracking phenomenon. U-bend cracks or leaks due to this hourglassing phenomenon have not been observed at other units beyond Row 1 tubes. Although unrelated to flow slot hourglassing (or denting in general),
it should be noted that stress corrosion cracks at the tube U-bend tangent points which have been observed recently at Trojan Unit 1, Farley Unit 1, and North Anna Unit 1 (and which have resulted in small leaks) have also been limited to Row 1 tubes. The cause of these non-denting related cracks has not yet been established; however, the Row 1 tubes experienced more cold working during fabri-cation (and thus higher residual stresses) than the larger radius tubes from other rows.
Although a large number of tubes at this unit are known to contain dents, the degree of denting is minor at this time.
Plugging of the Row 1 tubes has elimi-nated the immediate concern for denting or non-denting related stress corrosion cracking in the U-bends.
Eddy current inspections of the upper as well as the lower support plate intersections, and visual inspections of the lower support plate through existing hand holes will provide adequate indication of the onset Therefore, we of significant denting and extensive support pla' e cracking.
find that there is adequate basis for not installing the inspection ports during i
the upcoming refueling outage.
1 l
For plants with severe denting, upper inspection ports may prove useful for monitoring the condition of the upper support plate (for example, the extent of support plate cracking).
However, pending the completion of our evaluation l
regarding the future need for inspection ports at North Anna Unit 1, we believe that North Anna Unit 1 should comit to' installing these ports during the third refueling outage.
CONCLUSION Imediate concerns regarding the potential for denting or non-denting related stress corrosion cracking in the tube U-bends at North Anna Unit 1 have been eliminated by the plugging of all Row 1 Mbes. Consequently, we find there
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- g is adequate basis for not installing upper inspection ports at this time.
However, we are continuing to review the long term need for inspection ports at this unit. We believe that pending the outcome of our evalu tion the licensee should in the meantime corrit to installing these inspection ports during the third refueling outage.
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