ML20045E060
| ML20045E060 | |
| Person / Time | |
|---|---|
| Site: | Cooper  |
| Issue date: | 06/23/1993 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20045E058 | List: |
| References | |
| NUDOCS 9307010051 | |
| Download: ML20045E060 (3) | |
Text
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UNITED STATES j
NUCLEAR REGULATORY COMMISSION f
WASHINGTON, D.C. 20555-0001 Q
gw.....f SAFETY EVALUATION BY THE OFFICE OF h0 CLEAR REsCTOR REGULATION RELATED TO SCHEDULAR EXEMPTION FROM APPENDIX J TO ALLOW REVERSE-DIRECTION LOCAL LEAK RATE TESTING 0F 10 CONTAINMENT ISOLATION VALVES NEBRASKA PUBLIC POWER DISTRICT LOOPER NUCLEAR STATION DOCKET NO. 50-298 l.0 INTRODUCTION 1
By letter dated June 7, 1993, the Nebraska Public Power District (the licensee) requested a schedular exemption from Appendix J to 10 CFR Part 50 to allow Type C (local leak-rate) testing of 10 containment isolation valves in the reverse direction until the next refueling outage, currently scheduled to j
begin in the fall of 1994.
Section III.C.1 of Appendix J requires that for Type C testing, the test pressure must generally be applied to the valve from the same side or direction as pressure would be applied from during an accident.
That is, the pressure is usually applied from the inside-l containment side.
This is called the accident direction, or the forward direction.
However, the regulation allows testing in a direction other than the forward direction if it can be shown that such testing is equivalent to, l
or more conservative than, testing in the forward direction.
As a result of testing conducted during the 1993 refueling outage, the licensee has determined that its previous determination that reverse direction i
testing is acceptable may not be correct for certain valves at the Cooper Nuclear Station (CNS). The licensee has also determined that 10 of the CNS containment isolation valves cannot be shown at this time to be acceptable -
when tested in the reverse direction and cannot be tested in the forward direction without plant modifications. Therefore, the licensee has requested that a schedular exemption be granted for these 10 valves, exempting them from the forward-testing requirement of Appendix J until the next refueling outage.
At that time, the licensee will make the plant modifications necessary to perform the forward-direction testing if it has not been able to establish that reverse-direction testing is acceptable.
2.0 EVALUATION The licensee is applying for schedular exemptions for each of the following valves at CNS:
PC-MOV-305MV, PC-H0V-1304MV, PC-H0V-1306MV, RCIC-M0V-M015, 9307010051 930623 PDR ADOCK 05000298 P
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l RHR-MOV-M016A, RHR-H0V-M016B, RHR-MOV-M021A, RHR-M0V-M021B, RHR-MOV-M031A, and RHR-MOV-M0318.
PC-MOV-305MV, PC-M0V-1304MV, and PC-MOV-1306MV are solid wedge gate valves; the others are flexible wedge gate valves.
For their containment isolation function, these valves are required to isolate flow away from the primary containment utilizing the disc seating surface on the opposite side i
from which the primary containment pressure would be applied (outboard seating surface). To adequately test this outboard seating surface in the reverse direction, the valve stem thrust must be sufficient to seat this side of the disc against the test pressure. Unless the seating force provided by the stem thrust is significantly greater than the force from the test pressure applied i
in the reverse direction, the measured leakage may be through the seating surface on the opposite (inboard) side of the disc. Contrary to the i
requirements of Appendix J, the test results from the reverse direction testing of these valves may not provide equivalent or more conservative results.
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In order to justify exemption for the individual valves, the licensee has identified factors based on calculations of seating force ratios to accident pressure and on evaluation of past Type A (integrated leak-rate) and Type C test results.
For valves that do not have thrust test data available, seating-force-to-test-pressure-force ratios are based on comparisons to similar valves for which such thrust data is available.
Thus, the 10 valves may be divided into two categories:
those for which valve-specific data is available to determine the ratio'of seating force to test pressure force, and those for which the ratio is estimated using data for similar valves.
The table below lists the valves in these two categories and also provides the ratio for each valve:
Valves with Ratio. seatino force specific data to test pressure force RCIC-MOV-M015 5.9 to 1 RHR-MOV-M031A 1.28 to 1 RHR-MOV-M031B 2.0 to 1 RHR-MOV-M016A 3.27 to 1 RHR-MOV-M0168 3.86 to 1 PC-M0V-305MV 8.75 to 1 Valves with estimated data t
RHR-M0V-M021A 3.56 to 1 l
RHR-MOV-M021B 3.56 to 1 PC-MOV-1304HV 24 to 1 PC-MOV-1306MV 24 to 1 Although the ratio for the last two valves in the table is high enough to assure equivalent seat leakage in either direction, the information is only an estimate, so these valves are included in the schedular exemption request.
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As a general guideline, the staff considers reverse-direction testing conservative for gate valves if the seating force is 10 times the calculated test pressure force.
While this ensures that the leak geometry is dominated by the seating force instead of the test direction, there is no rigorous calculation for determining what other seating force may be acceptable.
For the interim of the schedular exemption, the staff believes that with the above seating forces these tests will provide reasonable leak-rate data. Although the ratios for valves RHR-MOV-M031A and B are quite low, an additional factor supports the granting of the schedular exemption for these two valves. They are part of the residual heat removal system, which can be considered to be a closed system outside containment since leakage through these valves during an accident would enter the closed system and likely be contained there rather than entering the environment.
Further, all 10 valves were tested in the accident direction during the 1991 Type A (integrated leak-rate) test.
Although the Type A test is not performed as frequently as Type C tests, the Type A test does provide an indication of overall containment leak tightness, including penetrations.
Even if the forward-direction leak rates of these 10 valves were greater than the measured reverse-direction leak rates, there is a considerable margin for error.
The sum of the leak rates for the 10 valves, from 1993 Type C testing, is 15.21 standard cubic feet per hour (scfh), and some fraction of this may actually be going through 12 other valves that arc tested simultaneously with the 10 valves.
The 10 CFR Part 50, Appendix J limit for the sum of all local leak rates (Types B and C) is 189 scfh (0.6 La).
The 1991 Type A leak rate was 102.5 scfh, and the most recently available sum of local leak rates (Types B and C) was 99.11 scfh.
It is apparent that there is considerable margin for potential error in the measurement of the leak rates of the 10 valves in question.
Therefore, the staff finds that there is reasonable assurance that, in the interim until the next refueling outage, the 10 valves in question will be sufficiently leak-tight that the. leak rate objective of Appendix J will be satisfied, even with the 10 valves being tested in the reverse direction.
3.0 CONCLUSION
Based on the above evaluation, the NRC staff finds that, for good cause shown, CNS containment isolation valves PC-M0V-305MV, PC-MOV-1304MV, PC-MOV-1306MV, RCIC-MOV-MOIS, RHR-MOV-M016A, RHR-MOV-M016B, RHR-MOV-M021 A, RHR-MOV-M021B, RHR-MOV-M031A, and RHR-MOV-M031B may be exempted from the forward-direction testing requirement of Appendix J for Type C tests until restart following the next refueling outage. At that time the valves will be tested in the forward direction if the licensee cannot establish that reverse-direction testing is equivalent to or more conservative than forward-direction testing and that the valves therefore meet Appendix J requirements.
Principal Contributor:
J. Pulsipher Date: June 23, 1993
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