ML13333A337
| ML13333A337 | |
| Person / Time | |
|---|---|
| Site: | San Onofre  |
| Issue date: | 01/09/1979 |
| From: | Baskin K Southern California Edison Co |
| To: | Ziemann D Office of Nuclear Reactor Regulation |
| References | |
| TASK-06-04, TASK-6-4, TASK-RR NUDOCS 7901170221 | |
| Download: ML13333A337 (4) | |
Text
Southern California Edison Company P. 0. BOX 800 2244 WALNUT GROVE AVENUE ROSEMEAD, CALIFORNIA 91770 K. P. BASKIN January 9, 1979 TELEPHONE MANAGER, GENERATION ENGINEERING 213-572-1401 Director of Nuclear Reactor Regulation Attention:
Mr. D. L. Ziemann, Chief Operating Reactors Branch #2 U.S. Nuclear Regulatory Commission Washington, D.C.
20555 Gentlemen:
Subject:
Docket No. 50-206 Containment Purging San Onofre Nuclear Generating Station, Unit 1 Your letter of November 29, 1978 requested that we provide either a commitment to cease all containment purging during operation or a justification for continuing purging at San Onofre Unit 1.
In addition, your letter requested that we review the design of all safety actuation signal circuits which incorporate a manual override feature.
This letter is submitted in response to your requests.
Containment Purge System The containment purge system at San Onofre Unit 1 consists of two 24" diameter lines.
Both of these lines have an automatic air operated valve (POV 9 or POV 10) and a manual backup valve (9A or 10A), all of which are located outside containment.
During power operation (i.e., when the reactor is critical) the containment purge system is not required since containment pressure is maintained within the Technical Specification limits by use of a six inch equalizing line.
Therefore, all four of the purge line valves are maintained in the closed position. However, during plant cooldown when the reactor is in a shutdown condition, these valves are opened and following sampling in accordance with station procedures, the purge system is used to reduce the airborne radiation level inside containment such that personnel access can be accomplished in a timely manner with doses as low as possible.
REGULAICVf?7T7 7901 1702~a
-2 In the event of an accident during a plant cooldown while the containment purge system is operating, the ability to restore containment integrity is ensured by the fact that the purge and exhaust line isolation valves (POV 9 and POV
- 10) will automatically close upon receipt of an isolation signal from any one of the following:
(1) high containment pressure (2 psig) from either of two pressure switches (PS 23 or PS 117), (2) high radiation from the containment sphere and plant stack gas monitor (R1212), or (3) a safety injection actuation signal from the sequencer. None of these automatic isolation signals to the purge line valves can be manually overridden, except for the sequencer signal as discussed below.
In addition, the valves can be closed remote manually.
As stated in Section 4.3.6.3 of the Final Safety Analysis these valves were designed to close against a differential pressure of 50 psi.
However, since the valves receive an automatic isolation signal when containment pressure reaches 2 psig, the differential pressure across the valve when it closes will be significantly less than the design value of 50 psi. The ability of these valves to close in an accident environment was further discussed in our Amendment 30 to the Docket, submitted July 1, 1974.
The analysis included in that submittal was found to be acceptable by the NRC Staff in their Safety Evaluation Report dated July 18, 1975.
Since the containment purge system is only required during plant cooldown at San Onofre Unit 1, use of this system during operation (hot shutdown, hot standby, startup and power operation) is only infrequently necessary. Over the past 11 years of operation, San Onofre Unit 1 has averaged a little less than 3 plant cooldowns per year with a maximum of 5 during one year.
Since each plant cooldown requires operation of the purge system for approximately 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> before cold shutdown is reached, it is reasonably certain that the purge system has never been used during operation for more than 90 hours0.00104 days <br />0.025 hours <br />1.488095e-4 weeks <br />3.4245e-5 months <br /> in a year. However, at your request, a proposed Technical Specification change which limits operation of the containment purge system to less than 90 hours0.00104 days <br />0.025 hours <br />1.488095e-4 weeks <br />3.4245e-5 months <br /> per year will be submitted through our review committees following review of this matter by the Regulatory Staff.
Pending completion of the staff review, SCE will institute written procedural controls to limit use of the containment purge system during operation to less than 90 hours0.00104 days <br />0.025 hours <br />1.488095e-4 weeks <br />3.4245e-5 months <br /> per year.
Safety Actuation Signal Circuits At the system level, the only manual override feature at San Onofre Unit 1 is the safety injection system block circuit.
-3 A safety injection actuation signal (SIAS) is automatically initiated on receipt of 2 out of 3 low pressurizer pressure signals coincident with. 2 out of 3 low pressurizer level signals.
In addition, safety injection can be actuated manually from the control room.
During plant cooldown, when reactor coolant system pressure and temperature are reduced, the automatic SIAS is blocked to prevent inadvertent automatic actuation of the safety injection system. The block is accomplished in the control room by manually operating a double action block switch for each safety injection train.
Control room annunciation is provided which indicates that the SIAS is blocked.
The SIAS block signal is automatically reset during heatup at a pressurizer pressure of 1900 psig.
A containment spray actuation signal (CSAS) is initiated by receipt of 2 out 3 high containment pressure signals coincident with an SIAS.
Therefore, manually blocking the SIAS will also prevent initiation of a CSAS.
Containment spray can also be actuated manually from the control room. In addition, individual components in the containment spray system can be bypassed to permit the operator to take the components offline.
Specifically, the refueling water pumps, hydrazine additive pumps, spray bypass valves and hydrazine isolation valves can be prevented from actuating on receipt of a CSAS by placing a push button in the control room in the bypass position. An indicating light on the containment spray system panel in the control room identifies the override status of individual components.
Removal of these components from service during operation is limited to only one train for a duration of less than 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> in accordance with the Technical Specification Limiting Conditions for Operation.
The sensor signals (low pressurizer level, etc.) cannot be overridden.
If one channel is not functional it fails in a fail-safe or tripped condition.
In addition, as described above, the containment purge system receives automatic isolation signals on high containment pressure, high contain ment radiation or SIAS.
Therefore, even if the SIAS is blocked, the containment purge valves will still close on receipt of either a high containment pressure or high containment radiation signal.
As a result of our review of these systems, it is concluded that San Onofre Unit 1 has the protection required during postulated accident conditions.
No design changes are required.
However, as described above, procedural controls on operation of the containment purge system will be implemented.
-4 If you have any additional questions on this matter, please let us know.
Very truly yours, Manager, Generation Engineering