ML18051B441
| ML18051B441 | |
| Person / Time | |
|---|---|
| Site: | Palisades  |
| Issue date: | 06/21/1985 |
| From: | Bordine T CONSUMERS ENERGY CO. (FORMERLY CONSUMERS POWER CO.) |
| To: | Office of Nuclear Reactor Regulation |
| References | |
| NUDOCS 8506250167 | |
| Download: ML18051B441 (5) | |
Text
i' consumers Power company General Offices: 1945 West Parnell Road, Jackson, Ml 49201 *.{517). 786-0550 June 21, 1985
- Director, Nuclear Reactor Regulation US Nuclear Regulatory Commission Washington, DC 20555 DOCKET 50-255 - LICENSE DPR PALISADES PLANT -
ADDITIONAL INFORMATION -
AFW SYSTEM TECHNICAL SPECIFICATION CHANGE REQUEST Consumers Power Company submitted on September 17, 1984 a proposed Technical Specifications Change Request to reflect modification to our Auxiliary Feedwater System.
The request included a provision to extend the maximum period of inoperability of an auxiliary feedwater (AFW) pump from 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> to 7 days.
In response to NRC staff concerns regarding the Station Blackout Event, the following additional information is provided to support the 7 day limiting condition for operation (LCO) for the steam driven auxiliary feedwater pump (P-8B).
The auxiliary feedwater pumps are important for a wide spectrum of accidents and transients among which is loss of offsite power (LOSP).
The importance of the steam driven auxiliary feedwater pump has been substantially reduced for these events (including the LOSP) by the installation of the second motor driven auxili_ary feedwater pump.
Prior to this modification, removal of the steam driven auxiliary feedwater pump from service left the motor driven pump (and its diesel generator for LOSP events) as a single train of equipment on which automatic makeup to the steam generators relied.
The existence of the additional motor driven auxiliary feedwater pump (powered by the second diesel generator during LOSP) now requires multiple equipment failures to occur in.
order to fail the auxiliary feedwater system during periods when the steam driven pump is removed from service.
This improvement in reliability of the system is greater than the approximate factor of two increase in permissible outage time being requested for the auxiliary feedwater pumps.
To demonstrate the approximate importance of the steam driven pump with respect to other equipment associated with the operations of the auxiliary feedwater system, reliability analyses of the system as it now exists have been performed. uses the Risk Ratio method specified in NUREG/CR-3082 "Probabilistic Approaches to LCOs and Surveillance Requirements for Standby Safety Systems." A risk ratio of 1 is considered to be acceptable for
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. Director, Nuclear Reactor Regulation Palisades Plant Add'l Info -
AFW TSCR June 21, 1985 2
choosing an LCO period (reference pg 32 of the NUREG).
A risk ratio of 1 means that the risk to the public during the period of time that the component in question (here the steam driven AFW pump) is out of service is no more than the risk to the public over the remainder of the base time period (taken here to be 1 year).
The calculations presented in Attachment 1 show that the risk ratio is less than 1 and therefore having the steam driven AFW pump out for 7 days (as described in the propose9 Technical Specification Change Request) is acceptable.
The attachment further indicates that the probability of the loss of the auxiliary feedwater system during.a loss of offsite ac power with the steam driven AFW pump out of service is approximately equal to the syst'em failure probability given a diesel generator out of service.
This is because each diesel generator is a power supply for a motor driven AFW pump.
In other words, removing the steam driven AFW pump from service for a length of time equal to that allowed for the diesel generator introduces no more risk than does removing the diesel generator from service.
Therefore, since it is acceptable to have a diesel generator out for 7 days, it should be acceptable to have the steam driven AFW pump out for 7 days (as long as it is not the same 7 days).
Based on the attached results, Consumers Power Company will commit to revise plant operating procedures pending receipt of the NRC Amendment to our Technical Specification Change Request.
The revised procedures will prohibit intentionally taking the Steam Driven AFW Pump out of service if an emergency diesel generator (EDG) is also out of service.
We conclude that thi~
commitment will satisfactorily address the issue of reliability of our AFW system against the station blackout event.
Thomas C Bordine Sta.ff Licensing EnKineer CC Administrator, Region III, USNRC NRC Resident Inspector - Palisades Attachment OC0685-0224-NL04
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IC0685-0224-NL04 ATTACHMENT Consumers Power Company Palisades Plant Docket 50-255 STEAM DRIVEN AUXILIARY FEEDWATER PUMP LCO June 21, 1985 2 Pages
- ATTACHMENT 1 STEAM DRIVEN AUXILIARY FEEDWATER PUMP LCO NiJREG/CR-3082 "Probabilistic Approaches to LCOs and Surveillance Requirements for Standby Safety Systems" provides a method on page 39 called the Risk Ratio Method.
The method is as follows:
The risk during the outage of, say, the kth component is the probability of the initiating event occurring during the o¥fiage, times the unavailability of the system with the k component out of service, times the unavailability of other systems that contribute to mitigation of the accident sequence:
Rt =
where:
Rt =
'-1 t
""I (t)(Qk) Q risk during outage th of k component of duration t
= initiating event frequency
= outage duration
= system average unavailability with the th k
component removed from service Q = composite average unavailability of other systems on the accident sequence.
The risk evaluated over a baseline period T *assuming that the outage does not occur is:
= '-1 (T)(Q) Q where RT
= risk during baseline period T, outage Q
= system average unavailability The ratio of risks is:
R = Rt/RT
= t (QK)
T (Q) and solving for t results in:
t =
R (T) (Q)
QK assuming no T is chosen to be one year.
R is chosen to be equal to 1 (the risk should equal RT).
Therefore t = Q/Qk NU0685-0224A-NL04
2 In the case here, QK is the system unavailability with the steam-driven auxiliary feedwater out. of servic_e.
The value of QK changes for different initiating events depending on if AC power is available to power either or both of the motor-driven pumps, if the valving or piping from the motor-driven pump fails, etc.
Therefore, the equation should be written:
I R =
~AI(t)(Qk) Q
~I(T)(Q) Q Where the summation is over all initiating events and Al' Q,Q,QK are functions of the initiating event.
For conservatism on y the worst case will be considered, the loss of offsite AC power coupled with the failure of both diesel generators to start (i.e., station blackout).
Here QK = 1.0 since without any AC power the auxiliary feedwater system fails with a probability of 1.0 when the steam-driven pump is out of service.
The system unavailability with the steam-driven auxiliary feedwater P!!1pP in service is 0.03 probability of pump failure to start) + (3.8E-4 h
) (24 hr)
(probability of pump failure running times the required run time) which equals 0.04.
Therefore R = 7 days (1.0)
= 0.5 365 days (0.04)
That is, the risk associated with the steam driven pump out of service for 7 days does not exceed 1/2 of the annual system failure probability.
Note that existing Technical Specifications permit one diesel generator to be out of service for seven days.
From the PRA on the cost-benefit of a second Main Steam Isolation Valve (reference our submittal dated May 23, 1985, MSLB Single Failure Backfits) the probability of a Loss of Offsite AC Power is 0.6 per year, the probability of the diesel generator failing to start is 0.02 and the probability of the steam-driven auxiliary feedwater pump to run is 0.04.
Therefore, the probability of a core melt due to a station blackout with the steam-driven auxiliary feedwater pump_~ut of service for seven days is (0.6)(0.02)(0.02)(1.0)(7/365) = 4.6E-6 yr.
The probability of a core melt due to a station blackout with one dies~t generator out for seven days is (0.6)(0.02)(1.0)(7/365)(0.04) = 9.2E-6 yr The above numbers suggest that the steam driven auxiliary feedpump is no more important during a loss of offsite power than a diesel generator.
This makes sense in that the diesel generator is the power supply for running an AFW pump.
Removing a diesel generator from service is equivalent to removing an AFW pump should a loss of station power event occur during maintenance.
Two AFW pumps remain for makeup of inventory to the steam generators whether the steam driven AFW water pump or one of the diesel generators is removed from service.
The risks associated with removing the steam driven AFW pump from service are therefore no greater than removing a diesel generator from service.
NU0685-0224A-NL04