ML14184A475
| ML14184A475 | |
| Person / Time | |
|---|---|
| Site: | Robinson  |
| Issue date: | 08/16/1983 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML14184A474 | List: |
| References | |
| NUDOCS 8308260323 | |
| Download: ML14184A475 (7) | |
Text
SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO CAROLINA POWER AND LIGHT COMPANY H. B. ROBINSON STEAM ELECTRIC PLANT, UNIT NO. 2 DOCKET NO. 50-261 INTRODUCTION By letter dated July 8, 1983, Carolina Power and Light Company submitted the results of the May 1983 interim cycle steam generator (S/G) inspec tion program which the staff required as a result of the evaluation of the 1982,refueling outage steam generator inspections.
The 1982 refueling outage S/G inspection was started in March 1982 and the plant went back into operation in July 1982. The inspection covered 100% of the tubes in all three S/G's. A revidw of the eddy current data revealed that defective tubes in "B" and "C" S/G's were primarily the result of phosphate thinning above the tubesheet on the cold and hot legs with some intergranular attack in the tubesheet crevice region on the hot legs of both S/G's.
All tubes with eddy current indications greater than 47% (defective tubes) were taken out of service by mechanical plugging. This is consistent with the requirements of Technical Specification 4.2.5.2.
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-2 The staff required that the licensee operate for only 6 EFPM after restart from the refueling outage at which time an inspection of the S/G's should be performed to ensure that further progression of phosphate thinning does not become excessive. The May 1983 steam generator inspection was dpnducted to meet the staff imposed con dition of operation.
DISCUSSION Plant performance at H. B. Robinson Unit 2 has been closely monitored by NRC'for the past four years. The facility has had defects at U-bends, at or above support plates, inside tubesheet, at top of tubesheet, and above the top of the tubesheet. The degradation above the tubesheet and at or slightly above the support plates is believed to be phosphate induced thinning. Robinson Unit 2 is one of two PWR units which continues to operate with phosphate secondary water chemistry control.
The degradation within the tubesheet is believed to be intergranular corrosion phenomenon which has been observed at other units.
The May 1983 inspections were performed using multi-frequency eddy current equipment. The inspection frequencies utilized were 400 KHz differential, 200 KHz differential, 10 KHz differential and 100 KHz absolute. Various signal frequency mixes were utilized to aid in the detection of tube wall degradation.
-3 The results of the eddy current inspections indicated the presence of degraded and/or defective tubing in four distinct regions of the.S/G tube bundles, namely:
- 1. Top of the Tubesheet/Above the Tubesheet (TTS/ATS) Region
- 2. Crevice Region
- 3. Tube Support Region
- 4. U-bend Region Review'of the inspection results and calculated corrosion rates revealed a continuation of tube wall corrosion in some of the regions where corrosion and tube degradation have occurred in the past. With the exception of the tubesheet crevice region, the ongoing corrosion appears to be a thinning process which was now observed on tubes which had no previous indications of. attack. The cracking phenomenon previously experienced above the tubesheet appears to have been completely arrested by the current low temperature opera tion program.
The December 1982 inspection results indicated that the most limiting corrosion rate, calculated from the eddy current data of the 1982.
inspection versus the previous inspections in 1981, was 1.14% per EFPM for the.C S/G cold leg TTS/ATS region.
4 Comparing the May 1983 eddy current data with the December 1982 eddy current data gave the following calculated corrosion rates:
Number of S/G Indications Corrosion Rate (% Degradation/EFPM)
A 194 HL 1.34 182 CL 4.07 B
232 HL 1.59 472 CL 2.82 C
499 HL 2.81 975 CL 4.61 These data show that the highest corrosion occurred in the cold leg of steam'generator C.
All tubes with eddy current indications in excess of the 47% plugging limit were plugged. A total of 16 tubes were plugged in A S/G, 139 in B S/G and 208 in C S/G. The plugged tubes included all tubes with detectable indications in the tubesheet crevice.
In addition to the eddy current inspections, an inspection of the secondary side of A and C S/G's was performed with fiberoptics to identify any foreign objects capable of causing damage to the tube bundle. No s.uch foreign objects were detected, nor was there any evidence of mechanical damage on the exterior of the tubes inspected.
The B S/G had been inspected during the December 1982 inspection outage.
-5 In order to justify a 4.5 EFPM operating period, the licensee contends that the Regulatory Guide 1.121 criterion that requires a factor of safety against tube burst failure of not less than 3 under normal conditions is excessively conservative and that a factor of safety of 2.3 is reasonable and adequate. A factor of safety of 3 for normal operation allows for 58% maximum tube wall degradation while a factor of safety of 2.3 supports a maximum tube wall degradation of 70%. With a 47% plugging limit, there is an operational allowance of 11% for the factor of safety of 3 case and a 23% operational allowance for the factor of safety 2.3 case. Using a 68% maximum wall penetration and a 21% operational allowance and the most restrictive corrosion rate of 4.61% EFPM (in the cold leg of C S/G),.the licensee contends that 4.5 EFPM would be a conservative operational interval till the next.steam generator inspection.
EVALUATION The H. B. Robinson Unit No. 2 steam generators were subjected to a state-of-the-art eddy current inspection to detect and quantify tube wall corrosion on essentially 100% of the unplugged tubing.
This inspection revealed tube degradation in several distinct regions of each steam generator where degradation has been observed in the past. While no new corrosion phenomenon was observed, there was an increase in phosphate wastage attack rate in some regions.
-6 The licensee's evaluation of eddy current inspection results to determine corrosion rates and to establish a safe operating period to maintain tube integrity under normal and accident conditions was done in a manner such that an upper bound corrosion rate was established for the most restrictive case. However in order to justify a 4.5 EFPM operating period using the upper bound corrosion rate of 4.61% per EFPM, the licensee has taken exception to the required safety factor of 3 against burst for normal operation which is used to calculate maximum allowable tube wall degradation according to R. G. 1.121. The licensee uses a safety factor of 2.3 against burst for normal operation to project a maximum allowable tube wall degradation of 70%; hence, a 23% corrosion allowance is available for further operation based on a 47% plugging limit.
Using a conservative 21% operational allowance with a 4.61% EFPM corrosion rate, the licensee would be permitted to operate for 4.5 EFPM.
We have also noted that the licensee has taken steps to reduce the corrosion rate by performing sludge lancing and crevice flushing during the May 1983 inspection outage.
-7 CONCLUSIONS We have concluded, based on a one time acceptance of the 2.3 safety factor, that H. B. Robinson Unit No. 2 may operate up to 4.5 EFPM from start-up after the May 1983 interim cycle inspection without undue risk to the public health and safety. This conclusion is based on an upper bound corrosion rate of 4.61% per EFPM and a conservative operational allowance of 21% when using a maximum degradation allowance against burst of 68% and a plugging limit of 47%.
The reduced limits on primary to secondary leakage rate limits imposed since August 1981 shall be continued and an eddy current inspection shall be conducted after the 4.5 EFPM of operation.