ML16342C717
| ML16342C717 | |
| Person / Time | |
|---|---|
| Site: | Diablo Canyon  |
| Issue date: | 10/12/1994 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML16342C716 | List: |
| References | |
| NUDOCS 9411030244 | |
| Download: ML16342C717 (10) | |
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UNITED STATES NUCLEAR REGULATORY COMMISSION WASHINGTON, D.C. 20555-0001 ENCLOSURE SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATING TO RE UEST FOR RELIEF FROM STOP VALVE CONTROL AND INTERLOCK RE UIREMENTS OF SECTION III SUBSECTION NC-7142 OF THE ASME BOILER AND PRESSURE VESSEL CODE PACIFIC GAS AND ELECTRIC COMPANY DIABLO CANYON POWER PLANT UNITS 1
AND 2 DOCKET NOS.
50-275 AND 50-323
- 1. 0 INTRODUCTION In an April 13, 1993, letter, Pacific Gas and Electric Company (the licensee) requested permanent relief from Section III, Subsection NC-7142 of the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code.
The relief would allow the licensee to use specific administrative controls on block valves CVCS-1-831 and CVCS-2-831.
These valves are installed in the downstream piping of the positive displacement (charging) pump (PDP) relief valves at Diablo Canyon Units 1
and 2.
The licensee requested expedited NRC review and approval of this relief request by April 18,
- 1993, so Unit 2 could transition to Mode 4 during the Unit 2 fifth refueling outage.
In our April 15, 1993, letter, we granted interim relief from Subsection NC-7142 requirements until we could complete this in-depth evaluation.
The licensee's February 11, 1994, letter responded to our December 6,
- 1993, request for additional information.
The attached figure shows a schematic of the PDP block valve/relief valve configuration, which is typical for both units.
The block valves were installed locked open in the discharge path of relief valves 1-RV-8116 and 2-RV-8116.
The relief valve protects the positive displacement pump (PDP 1-3 for Unit 1 and PDP 2-3 for Unit 2) and the safety-related discharge piping.
The relief valve discharges to the volume control tank (VCT).
All components are part of the chemical and volume control system (CVCS).
ASME Section III, Subsection NC-7142, "Provisions When Stop Valves Are Used,"
indicates the following:
No stop valve or other device shall be placed relative to a pressure relief device so that it could reduce the overpressure protection below that required by the rules of this Article, unless such stop valves are constructed and installed with controls and interlocks so that the relieving capacity requirements of NC-7300 are met under all conditions of operation of both the system and the stop valves.
Means shall be provided such that the operation of controls and interlocks can be verified.
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The ASHE further issued Code Interpretation No. III-1-80-67R to clarify what is meant by controls and interlocks.
The Interpretation specified that controls and interlocks were pressure sensing devices that would activate the stop valve to provide fluid access to the relief valve.
The Interpretation also made it clear that administrative control of block valve positions was not acceptable.
The ASHE incorporated these provisions into the 1989 Code Edition.
Therefore, the CVCS-1-831 and CVCS-2-831 block valve installations do not meet ASHE Section III requirements.
- 2. 0 EVALUATION Paragraph 10 CFR 50.55a gives the NRC two methods for granting relief from ASHE Code requirements.
The NRC may grant relief pursuant to 10 CFR 50.55a(a)(3)(i) or 10 CFR 50.55a(a)(3)(ii).
Paragraph 10 CFR 50.55a(a)(3)(i) requires the licensee to demonstrate that their proposed Code alternative would provide an acceptable level of quality and safety.
Paragraph 10 CFR 50.55a(a)(3)(ii) requires the licensee to demonstrate that complying with Code requirements would result in hardship or unusual difficulty without a compensating increase in the level of quality and safety.
We evaluated the licensee's relief request based on the licensee demonstrating that complying with Code requirements would result in hardship or unusual difficulty without a compensating increase in the level of quality and safety.
Demonstratin Hardshi or Unusual Difficult The licensee would have to make costly backfit design modifications to install controls and interlocks for the subject block valves to comply with Code requirements.
The block valves also enhance maintenance flexibility. Relief valves in high pressure service applications are subject to a number of potential problems such as seat
- leakage, setpoint drift, gasket
- leakage, and failure to reseat.
Without block valves, relief valve RV-8116 maintenance requires one of the following:
(1) Freeze sealing the valve discharge line to the VCT to allow relief valve removal, inspection, repair, testing, and replacement while the plant is at power.
This option creates an unnecessary risk of a VCT spill or airborne release.
I (2) Declare the PDP inoperable, remove this portion of the system from
- service, and take appropriate compensatory measures.
Repairs would then be delayed until the plant was in a mode allowing the VCT to be depressurized and drained.
The licensee considers both of these options to be undesirable.
The licensee considers the block valve to be beneficial because it allows for an on-line relief valve repair.
No Com ensatin Increase in the Level of ualit and Safet There are no adverse safety consequences for accident scenarios associated with the installed block valve configuration.
The PDP is used for normal charging and has no ECCS function for accident mitigation.
However, its pressure boundary integrity must be maintained to assure centrifugal charging pump (CCP) safety injection capability.
The CCPs automatically start on a
safety injection signal to fulfillthe high head safety injection ECCS function.
However, operators can use the PDP for boration flow.
The PDP is required to be operable only when credit is taken for its use as a boration flow path.
The FSAR update takes credit for the PDP for post-fire safe shutdown to meet 10 CFR Part 50, Appendix R requirements.
A fire in the CCP room could render both CCPs inoperable.
In plant Nodes 1 through 4, the utility takes credit for the PDP providing both pressurizer level control and boration to achieve cold shutdown.
A single failure that would render the PDP inoperable is not considered as part of this Appendix R scenario.
Therefore, there is no need to have backup equipment to the PDP for Appendix R fire protection.
The licensee evaluated potential means of overpressurizing the PDP discharge line to determine a postulated failure mechanism.
They determined the postulated failure mechanism would be having the block valve inadvertently closed and RV-8116 lifting as a result of a high pressure condition.
The licensee calculated that either the bonnet of drain valve 54 or the piping flange on the RV-8116 discharge would deform once the discharge pressure reaches between 430 to 500 psig.
The licensee further determined that the 500 psig maximum back pressure would not prevent RV-8116 from opening to adequately relieve the PDP discharge pressure.
This results in the loss of diaphragm or gasket sealing which relieves system pressure and prevents failure of the piping.
RV-8116 will reseat once pressure falls below the relief valve setpoint, thereby maintaining PDP discharge piping system pressure boundary integrity.
Inadvertently closing a downstream remotely operated charging line isolation valve and overpressurizing the PDP discharge piping is unlikely because PDP discharge flow to the reactor coolant pump seals continues.
Inadvertently closing valves 8388A or 830 and overpressurizing the PDP discharge piping is also unlikely.
The licensee has administrative controls to make sure these block valves are locked open at all times unless the PDP is out of service.
Operating Procedure K-IOA9, "Sealed Valve Checklist for Charging Pump 1-3 (2-3 for Unit 2)" contains this requirement.
The Operations Department controls the single key to the padlock on the valve to minimize the potential for inadvertent operation.
If the pump is removed from service, Operating Procedure 8-IA:IX, "CVCS Alignment Verification for Plant Startup," gives instructions to ensure that the block valve is correctly positioned to the locked open position prior to pump operation.
Additionally, operators must complete the Sealed Valve Checklist for all sealed
- valves, including the block valve, prior to declaring the PDP operable.
This includes independently verifying the valves are in their required position.
Surveillance Test Procedure (STP) I-lD, "Routine Honthly Checks Required by Licenses,"
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requires verifying that the block valves are in their required open position on a monthly basis.
However, if the PDP discharge piping should fail as a
result of closing valves 8388A or 830, the CCPs would be isolated from the failed portion of piping and would remain operable.
Therefore, based on this evaluation, we conclude that overpressurizing the PDP discharge line is highly unlikely.
Even if overpressurization
- occurred, the resulting postulated failure mechanism would not result in the loss of the PDP discharge piping integrity or the simultaneous loss of the CCPs.
As a result, there are no adverse safety consequences for accident scenarios associated with the current block valve configuration.
Thus, requiring the licensee to remove the installed block valves would result in hardship or unusual difficulty without a compensating increase in the level of quality and safety.
3.0 CONCLUSION
We have determined that the CVCS-1-831 and CVCS-2-831 block valve installations do not meet ASME Section III requirements.
- However, we agree with the licensee that complying with all of the ASME Code requirements as specified in 10 CFR 50.55a(a)(2) in this instance would result in hardship or unusual difficulty without a compensating increase in the level of quality and safety of the plant.
We also find that mispositioning the manual block
- valves, and subsequent overpressurizing of the PDP discharge line does not result in the licensee's inability to safely shut down the plant.
Therefore, pursuant with 10 CFR 50.55a(a)(3)(ii),
we find that it is acceptable to grant the licensee's request for relief from the ASME requirements regarding control and interlock requirements for relief valve discharge line stop valves.
Principal Contributor:
L. Padovan G.
Hammer Date:
October 12, 1994
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