ML20148C490

Safety Evaluation Supporting Postponement of Permanent ECCS Single Failure Fixes at Facility.Postponement of Installation of Redundant Undervoltage Relays Unacceptable

ML20148C490
Person / Time
Site:	San Onofre
Issue date:	10/16/1978
From:	Office of Nuclear Reactor Regulation
To:
Shared Package
ML20148C471	List: ML20148C466 ML20148C490
References
TASK-06-07.C2, TASK-6-7.C2, TASK-RR NUDOCS 7811020059
Download: ML20148C490 (10)

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. UNITED STATES p- g

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,j NUCLEAR REGULATORY COMMisslON

, WASHINGTON, D. C. 20066

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9 SAFETY EVALUATION BY THE OFFICE OF NUCLEAR ' REACTOR SUPPORTING ACCEPTANCE OF POSTPONEMENT OF'CERTAIN PERMANENT ECCS SINGLE FAILURE FIXES

_ SAN ONOFRE NUCLEAR GENERATING STATION, UNIT 1

_ DOCKET NO. 50-206 Introduction

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By letter dated August 10, 1978, Southern California Edison Company (the licensee) proposed to defer four pending modifications to the ~

Emergency. Core Cooling Evaluation Program, SEP. System for incorporation into the Systematic The purpose of the proposed postponement was to permit evaluation of the potential impact on other plant modifications which may be considered as part of the integrated assessment of the facility in the SEP.

The four modifications in question resulted from the licensee's ECCS '

single failure analysis. Our evaluation of.the modifications is described 25 on in our 1,1977.

Safety Evaluation supporting the issuance of Amendment No. April inttiationofSEP. TheseThis evaluation was performed prior to l j

modifications involve the following:

L Nistallation of redundant undervoltage relays on each of the t 4160V operation.busses 1C and 2C during the refueling outage for Cycle 7 2.

Installation 1100C during of thearefueling redundant outage valve in series for Cycle with valve 7 operation. MOV/LCV 3.

Modifications to provide independent and redundant hot leg recirculation outage for Cycleflow7 control operation. capability during the refueling 4.

Installation of redundant control power and instrument air for flow control valves FCV-11150 E and F during the outage scheduled for Winter 1979-1980 for refueling for Cycle 8 operation.

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Evaluation Item 1 The licensee identified a situation in which a failure in the loss of power (LDP) logic could preclude automatic start of the dies'el generators required to power safety related equipment following a loss-of-Coolant Accident (LOCA). To provide independent and redundant LOP logic on an interim basis. .the undervoltage relays on the two redundant 4160V busses were modified to deenergize to activate before startup for Cycle 6 operation. This assures that an interrupted circuit' on the contact side of the relay causes the relay to perform its intended function. , However, this modification did not preclude failure of diesel generators to automatically start if mechanical failure of

• a relay occurred. Because of the low probability of such a failure during the limited time interval, we accepted the interim modification until the end of Cycle 6 operation. Installation of a second under-voltage relay on each of the two 4160V busses 1C and 2C was deemed necessary to assure automatic start of the diesel generators and the sequencers when required to perform a safety function. "

In our evaluation of the licensee's proposal to postpone the installation of a second undervoltage relay on each diesel bus, we considered the ability to manually start the diesel generators and sequencers. However, credit for manual operation is not consistent with the LOCA analysis

.which assumes a diesel starting time of 10 seconds. Because the likelihood of a mechanical failure of an undervoltage relay must be considered during a longer time interval, we cannot justify post-ponement of this modification beyond the end of Cycle 6. We have, therefore, concluded that the licensee's proposal to postpone the above described modification is not acceptable. The licensee has agreed to implement this modification during the. refueling outage for Cycle 7 operation.

Item 2 -

The licensee identified a situation in which the failure of one valve, MOV/LCV 1100C could result in the loss of both charging pumps. The position of this valve in the system is shown in Figure 1. MOV/LCV 1100C is normally open. Upon ECCS initiation MOV/LCV 1100C is - -

required to close. Failure of MOV/LCV 1100C to close may cause ~

cavitation of the two charging pumps downstream of the valve because hydrogen from the Volume Control Tank could enter the pump suction. -

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l To eliminate this problem the licensee disabled the automatic feature from the transfer switch which connected the valve to one of two

power supplies (Motor Control Center 1 or Motor Control Center 2).

A manual-transfer capability, requiring two separate operator actions l (activating a control switch and closing a normally racked out breaker j to the alternate power source) was retained. Elimination of the  !

j' automatic transfer switch means that loss of the switch would not  !

result in loss of both power sources to valve MOV/LCV 1100C. g ,

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In-addition, as an alternate method, ane of the two Charging Pumps was removed from equipment automaticaQ started by the sequencers l following postulated accidents. This interim measure ensures that l with the postulated loss of power to MOV/LCV 1100C, one Charging .

Pump remains available for service during recirculation; Charging Pump service is not required during safety injection and.only one Charging Purn is required during recirculation. During the recirculation  ;

phase, the Charging Pumps are provided recirculation flow from the Recirculation Pumps whose discharge pressure closes a check valve  !

between MOV/LCV.1100C and the suction of the Charging Pumps, preventing -

' flow from the Volume Control Tank to the Charging Pumps regardless of the position of MOV/LCV 1100C.

Changes in station operating procedures were also m6de which require that an operator be dispatched to manually close MOV/LCV 1100C if it fails to close following signals to do so from the control room.

Our. review of these interim procedures identified;no problem which sho'uld limit the duration of the interim fix to less than the time required to complete the SEP. Therefore, the alternate procedure descr.ibed above may be used until the SEP review is completed. ,

Item 3 h The licensee's submittal dated March 25, 1977, described an alt ernate

' flow path for hot leg ' recirculation following a LOCA which' could eliminate loss of the normal hot leg recirculation flow path due to failure of any of the five valves in the system (FCV 1112, CV 304, CV 305, PCV 430H and PCV 430C). The prime flow path, containing these valves, takes water from the Sphere Sump and discharges it into the pressurizer spray to hot leg B. This is shown in Figure.2. 1 The alternate system transports water from the Sphere Sump through '

the path shown in Figure 3 to hot leg C.

The permanent fix proposed in the March 25, 1977 submittal and accepted by us is the addition of redundant valves for the five valves subject to single failure as shown in Figure 4 (compare with Figure 2).

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• 1 We requested the licensee to demonstrate that loss of one diesel train (also called a Division) would not result in failure of both the_ primary and alternate flow paths. The licensee's response to the request is shown in Table 1 which gives the valve in the primary hot leg recirculation flow path and the alternate hot leg recirculation flow path. Figure 2 shows the position of the valves of.the primary hot leg recirculation system which are listed in Table 1. Failure of one of these valves to open er close could prevent this system from

! perfoming its function. If Division 2 were lost, the primary system

' could still perfom its function since valve CV 305 and CV 304 can be macually transferred to Division 1 and all the other valves in '

the primary flow path use Division 1 power.

If Division 1 were to fail, the primary flow path would still be able.to deliver flow sincs the only valves connected to Division 2 are not required to change position for hot leg recirculation use.

Therefore, failure of a diesel train wauld not eliminate both hot leg recirculation flow paths.

The alternate flow path injects 'into hot leg C rather than through

' the pressurizer spray to hot leg B. It therefore provides not only a redundant flow path but a redundant injection point. In addition,.

construction was done with materials and metheds which meet the same Codes and Standards.as the equipment to which the new components were added. .

We have reviewed the alternate flow system and have concluded that it woutd.be available for performing the intended function under all likely conditions during the interim period until the SEP review is completed.

Therefore, deferral of the pemanent modification for this period of time is acceptable.

By letter dated March 25, 1977, the licensee recomende 'lat redundant control power and instrument air be provided for valves e J-lll5D, E and F or that their operators be modified from open to close on loss of power or instrument air. These valves are normally closed. When opened, they provide a flow path from the charging pumps to the three cold legs for long term cooling. These valves presently fail closed upon loss of air. The licensee does not this, thus, another supply of pressure is necessary. plan to change The pemanent fix described in our Safety Evaluation issued April 1, 1977 with Amendment No. 25, is to supply a redundant air supply and control system. As an interim system,. the licensee installed a nitrogen supply system which can be used to supply press.ure to open

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i the three valves. The nitrogen systs consists of a separate nitrogen header for each of the three valves and a separate power source. The nitrogen system is of all metal construction.

A three way solenoid allows the operator to choose either the air supply or the nitrogen supply. ,

' The power supply for the air system is the 115 volt vital Bus 4 and the power supply for the nitrogen system is the 125 volt DC Bus

4. Both systens are completely separate. .

We have concluded that the licensee's request to use the nitrogen system as a backup, if required, until the completion of the SEP review is acceptable. Consequently, the addition of the backup air system for Cycle 7 operation as described in our April 1,1977 Safety Evaluation may be deferred to the SEP review.

Conclusions -

In summary we have concluded:

1.

Postponement of. installation of redundant undervoltage relay is not acceptable as discussed in Item 1 above. The licensee has agreed to complete the installation during the refueling outage for Cycle 7 operation.

2. The installed alternate systems described in Items 2,3 and 4

' provide adequate protection should this be required by a LOCA, tfntil the SEP review is completed. Therefore, implementation of these pennanent modifications may be deferred until theytave -

been considered as part of the integrated assessment of the facility in the SEP review.

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1. Figures 1 through 4

2. Table 1 Date: October 16, 1978 '

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