ML20135D357

Rev 1 IPEEEs for Maine Yankee Atomic Power Station

ML20135D357
Person / Time
Site:	Maine Yankee
Issue date:	11/30/1996
From:	Maine Yankee
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ML20135D343	List: ML20135D341 ML20135D357
References
NUDOCS 9612090350
Download: ML20135D357 (26)

Text

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! ATTACHMENT B i

INDIVIDUAL PLANT EXAMINATION EXTERNAL EVENTS, REVISION 1 i NOVEMBER 1996

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ANALYS$ ANALY3s 2

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. ANALYWB

.As this figure illustrates, core daerJe sequences are developed by j " running" initiating events through the event sequence model. Initiating I event frequencies are developed either directly from historical data l (e.g., general transients), through external events analysis (e.g.. external

! . floods), or from systems / operator analyses (e.g., loss of support systems).

The event sequence model then determines the probability that each' initiating .

event will either be successfully mitigated or will resul't in core damage.

Components of the event sequence model include support and front-line system i availabilities, as well as operator error probabilities, These , components are j developed based on systems analysis, operator analysis, and external events  ;

i analysis. Direct impacts of the initiator on mitigating system availability '

and operator response are handled explicitly in the model. All analysis is

! supported by data snalysis. For the Maine Yankee IPEEE. data includes site-specific initiating event frequencies developed from generic data, i .These blocks were built and then combined to effectively create a I

single, large event tree from initiating event to plant damage state within

!- the computer model. The software allows linking of all scenarios with i significant frequency without the need for support states or impact vectors to

! accomplish the linking.

1 This systematic, structured approach to constructing the risk model

! (which'is further described in Reference 3) allows for a high level of

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completeness and permits unraveling the results to understand the key risk-j controlling factors that drive the results. This, in turn, supports the

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development of engineering insights needed to use the PRA as a risk management l tool.  !

l The exception to the above described methodology is for seismic events.  !

Reference 5 describes in detail, and Section 3 describes in summary, the '

seismic IPEEE effort for Maine Yankee.

1.5 Summary of Maior Findinas The external events related portion of calculated core damage risk is i dominated by fire, internal flood and High Energy Line Break (HELB) events. I Since seismic events were addressed via a margins program, the seismic core damage frequency was not calculated. All other events are screened out. The risk from fires, internal floods and HELBs is due to events which disable multiple trains of equipment or result in spurious actuation of SG isolation signals. For all quantified hazards, the risk is due in large part to uncertainties in the likelihood of the event and the impact on equipment and operator performance. See Table 1.2 for a more detailed breakdown of external event risk.

The examination identified several significant improvement opportunities which have been, or will be. implemented to increase Maine Yankee's resistance to external events. These are as follows:

. Ensure plant procedures provide adequately detailed guidance in l response to fires. l

. Improve flood protection in the Turbine Building.

Remove water line in the Control Room (Complete).  ;

. Leak before break program for MSVH.

. Analyze Turbine Building HELBs - make modifications as necessary.

. Reroute cable for HPSI suction valve HSI-M-51 outside of MSVH.

. Upgrade RWST to a HCLPF of .279

. Upgrade HCLPFs of equipment required to reacn cold shutdown.

. Upgrade mounting of two outside containment isolation valves to HCLPF of > .3g.

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.I TABLE 1.3 PEEE Identified Enhancement Docortunities Issue Enhancement Disposition i IPEEE FIRES 4

1. The guidance Add additional Maine Yankee has

provided in the fire guidance. committed to

procedures for implement

. various actions is procedure changes not as detailed as by the end of the that provided for next refueling 1 I similar actions outage. l i credited and l guantified for the

Internal Events PRA.

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2. Nonseismically- Isolate hydrogen Plant procedures l supported hydrogen supply after filling require hydrogen -I l piping. the generator and to be isolated l volume control tank when not in use. i (VCT).

INTERNAL FLOODS l

i 3. Turbine Building Install trip Maine Yankee has

Floods can circuits, level committed to i potentially affect alarms. SW expansion implement the l

the Cable Vault, joint bands changes by the ,

t Control Room and end of the next 1 other important weatherstrippg, isolation o room refueling outage. ,

, areas. drains, and l t

berms / door

modifications.

, Implement Procedure

! Controls.

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4, Water line in the Remove water line. Water line has Control Room ceiling been removed.

can affect important equipment. Effects i are difficult to j analyze.

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TABLE 1.3 (Continued)

IPFFF Identified Enhancement Gooortunities Issue Enhancement -

Disposition IPEEE HELBs

5. In'itiating Event Perform leak before Maine Yankee has I Frequency for MSVH break analysis. Take committed to l HELBs should be necessary actions to complete by the reduced. A well ensure that piping end of the next designed leak before meets leak before plant refueling break program should break criteria (e.g., outage.

reduce the initiator walkdowns, erosion-frequency by a corrosion factor of 10. examinations etc.)

6. SWGR Room Implement procedural Maine Yankee ha.s l Ventilation Fans can controls and plant committed to l be impacted by HELBs hardware changes, as implement by the

! in the Turbine necessary, to protect end of the next i Building. SWGR and equipment from harsh plant refueling l Control Room environments. outage.

Ventilation ducts may also be damaged.

7. HPSI suction valve Reroute HSI-M-51 Maine Yankee has l cable runs through cable. committed to l the MSVH where it complete by the l could potentially be end of the next damaged by a HELB. plant refueling outage. l l

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TABLE 1.3

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IPFFF Identified Enhancament Oooortunities e

Issue Enhancement Disposition i IPEEE SEISMIC I 8. 'RWST has a HCLPF of Increase tank HCLPF Modification

! .21g to .279 by improving complete.

tank achorage.

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9. Based on recent Improve equi Maine Yankee has extension of Seismic anchorages. pment committed to

Margins Program to complete by the

Cold Shutdown (RHR, end of the next Appendix R cooldown) plant refueling various pieces of outage, equipment have

< inadequate HCLPFs.

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10. Two outside Upgrade mounting of Modification

! containment two valves to HCLPFs complete.

i isolation valves of >0.39 have a HCLPF of

.189 i These are 1" i penetrations and the i inside isolation l valve has a HCLPF of

>0.39 l 11. Westinghouse SV Replace relays with Maine Yankee has '

! relays in Diesel GE type NGV relays. committed to

Start Circuits have complete by the

. a low HCLPF. end of the next plant refueling

outage.

4 EXTERNAL FLOODS 5

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12. There are some minor Correct Maine Yankee has inconsistencies in inconsistencies comitted to the external flood complete by the procedures / end of the next documentation plant refueling j outage i 13 The potential exists Provide plant Maine Yankee has for external floods hardware committed to

to enter the plant modt fication. implement by the through the computer end of the next and the Control plant refueling Rooms, outage.

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TABLE 1.3 -

l (Continued) i IPFFF Identified Enhancamant Oooortunities i

Issue Enhancement Disposition IPEEE 14. Flood loadings on Provide scuppers to Maine Yankee has some roofs may relieve / prevent flood committed to '

exceed roof loading. implement by the r capacity. Roofs in end of the 1999 i question include plant refueling l Service Building outage.  !

El. 61 and Service Building El. 39.

15. Vent openings on the Ensure vent openings Maine Yankee has Diesel Generator are adequate to comitted to Building roofs are resist flood impact. implement by the not above the end of the 1999 maximum predicted plant refueling ponding levels on outage.

the roof.

16. Floods could Provide a water-tight Maine Yankee has potentally enter the seal around the committed to seal plant due to affected manholes. the manhole flooding of manholes covers to prevent on the north side of water ingress by the plant, the end of the next refueling outage.

17. Identified FSAR Update FSAR. Maine Yankee has .

inconsistency committed to l dealing with low update the FSAR l river level. by the end of the next refueling outage.

GENERAL

18. EFW pump room Provide additional Modifications ventilation. qualified have recently '

ventilation, been completed.

19. Fires'(and uidance Accident potentially floods) Incorkoratekccident into evere management in the Control Room. Management Guidelines guidelines are Cable Vault, being developed for currently being Protected Cable Tray Maine Yankee. developed.

Room. Protected SWGR and EFW Pump Room have the potential i to cause multiple '

hot shorts or power failures to both in-board and out-board containment isolation valves.

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, TABLE 1.3 l

(Continued)

IPEEE Identified Enhancement Oooortunities

! Issue Enhancement Disposition l l -

l IPEEE 20. Functionally AFW Take actions to Maine Yankee has availability is improve AFW committed to. <

important to availability. j mitigation of

• Track AFW per '

various external the )

i events Maintenance 1 i

Rule. I

• Procedures and )

l the MY risk .

monitor discourage elective maintenance on AFW if an external event challenge is expected.

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located in this area. Loss of these components could cause a plant trip i

due to one or more EFCVs closing.

One channel of nuclear instrumentation. NI-0101A. is contained in this

, area. Loss of this channel would not cause a plant trip or prevent a trip if needed.

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The remaining instrument cables in this area are for Appendix R I monitoring (i.e. the Alternate Shutdown Panel (ASP)) from the EFW Pump l Room. Indication for these instruments is available from the Control i

Room.  ;

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. Auxiliary Feedwater Pump P-25B and its Steam Supply Valves MS-T-163 and MS-P-168 are located in this fire area.

Main Steam Line Excess Flow Check Valves MS-11. MS-22. and MS-33 are located in this area. Failure of these valves to remain open due to a fire would cause a plant trip.

. The decay heat release valve. MS-A-162 is located in this area. Loss of this valve will not have a major effect on the plant since the main steam safety valves would handle steam decay heat removal.

i A brief summary of the effect from the worst case fire in this area is given below.

A fire in this area could cause the main steam line excess flow check valves to shut, the EFW supply lines to be isolated, the main feed pump to trip and the electric main feedwater to be blocked from starting. The Turbine driven auxiliary feedwater pump would also be damaged. To regain control of the EFW supply lines, which were isolated due to false main steam line pressure signals from transmitters which were damaged due to the fire. I the operators would need to bypass this trip from the Control Room. This bypass does exist; and, as a result of the IPEEE process. Maine Yankee hr i reviewed the fire A0P's to ensure that sufficiently detailed g;1 dance exists l to instruct the operators in its use for fires in this area. l This fire can potentially result in a total loss of feedwater, if the operator does not successfully recover EFW operation. Only Feed aad Bleed would be available for decay heat removal. The fire ignition frequency in this area is 2.9E-03/yr. Because of the dependence on operator action to bypass the EFW isolation signal, this area was selected for further detailed analysis which is discussed in Sections 4.8.2.2. and 4.9.

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4.6.10 Purae Air Sunolv Unit (HV-7) Room /Ecuioment Hatch (PURGE-AIR-RM) 4.6.10.1 Descriotion/ Fire Protection Features

The Purge Air Supply Unit Room / Equipment Hatch is located to the west of the Containment Spray Pump Building next to the Containment Building.

The wall common with the Containment Spray Pump Building is constructed of reinforced concrete with a fire rated door. The wall common with the Containment Building is also reinforced concrete with a fire rating. The remaining walls are exterior walls constructed of concrete or exterior steel panels. Exterior walls are not fire rated. The floo" is- concrete on grade with an exterior roof.

The in situ combustibles consist primarily of wood. Transient combustibles are minimal. The combustibles are conservatively estimated to represent a fire loading of 1.1E06 BTUs or 6.03E02 BTUs per square foot.

There is no detection or suppression systems in this area. Manual hose lines from nearby exterior hose houses are available.

4.6.10.2 Qualitative /0uantitative Screen Power and control cables for the RWST outlet to HPSI/ Pump B (HSI-M-51) and the RWST outlet valve to LPSI and CS Pump B are located in this area.

Containment Purge Supply Isolation Valves VP-A-1 and VP-A-2 are also located in this area. Loss of any of these components does not cause a plant trip, loss of reactivity control, core cooling, or RCS Inventory Control.

The most probable nonfire plant initiators that would require the HPSI System would be small LOCAs.

The major impact of a fire in this area would be identical to the loss of HSI-M-51 due to internal failures. The MOV demand failure rate (failure to open) used in the Maine Yankee PRA is 6.95E-03/ demand.

The ignition frequency in this area is 9.5E-04/yr. The assumed unavailability of this valve due to a fire is based upon a three-day LC0 before the plant must be shutdown due to a degraded ECCS System. This results in an unavailability of [9.5E-04/yr

• 3 days) - 7.8E-06. The fire-induced valve unavailability is small compared to the unavailability due to internal failures (6.95E-03). Therefore, this area. quantitatively screened out.

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E l- spurious Steam Generator Low Pressure / Feed Isolation Signal as well as the  !

potential tc spuriou.c(y open PORV(s).  !

From the Maine Yankee PRA (Reference 3),1.5 to 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> are available to

, take this action given: (1) all three main steam lines are isolated; (2) all  !

I main, emergency, and auxiliary feedwater is isolated: (3) one pressurizer PORV

is open: and (4) ECCS is available and actuated. The case of both PORV's open  :

1 (loss of Vital Buses 2 and 3), with the same conditions as discussed above.

I would result in a longer available recovery time. HESGLP assumes 1.5 to 2 l hours is available to take action for any dual loss of vital buses.

l The action was modeled in the PRA as action OPDVBR (recovery from loss of i Dual Vital Buses.) Differences between HESGLP and OPDVBR are negligible. l j Currently. fire response A0Ps in the given area direct the operator to enter l s procedure E-0. Emergency Shutdown From Power or Safety Injection. This l procedure directs the operator determine the status of the steam generators l and if necessary bypass the feed train trip signal to unisolate them. The l procedure for fires in the protected Cable Tray Room does, however, direct the '

operator to open the PORV breakers. l Based on the analysis of this human action. Maine Yankee has reviewed the l following Abnormal Operating Procedures (A0P's): A0P 2-90-2. Plant Shutdown l l Plan for Fire in Containment. Spray Pump Area. Steam / Feed Valve House. I Containment Electrical Penetration in South Elevation 46', was reviewed to l l

ensure that sufficiently detailed guidance exists to respond to a SGLP l isolation in the event of a fire in the Main Steam and Feedwater Valve House.

A0P 2-90-1. Plant Shutdown Plan for Fire in Control Room. Control Room Cable Chase Protected Cable Vault. Protected Cable Tray Room and Protected Switchgear Room was reviewed to ensure that sufficiently detailed guidance l exists to respond to a SGLP trip and isolation (dual loss of vital busses), in l the event of a fire in the Protected Cable Tray Room and the Protected Switchgear Room.

l The' dominant factors affecting evaluation of this human action.are (1)

" Plant Interface and Indications of Conditions." (2) " Training and l 1 Experience." and (3) " Stress." " Plant Interface and Indications of Conditions" l are important because it is critical that the operator'be able to determine if secondary heat sink is not available and when recovery actions are taken, if ,

they have been successful. " Training and Experience" are important because.  !

although this is an action that the operator might see in training, it is not 4-175 Revision 1

normally seen during plant operation. " Stress" is important due to lack of secondary heat sink and the potential to overheat the core resulting in high stress levels for the operator.

Based on the evaluation of all . involved human factors the failure l probability for this action was determined to be 1.7E-02.

Human Action - HECR/ ASP HECR/ ASP describes an operator action to take control of the plant from the Auxiliary Shutdown Panel. This action involves starting the Appendix R Diesel Generator, establishing AFW/EFW control and indication from the Auxiliary Shutdown Panel, and establishing and maintaining secondary heat sink.

This action is performed in the Emergency Feedwater Pump Room.

The need for this action arises because of fires in the Main Control Board (Fire Scenario CR-SBO). resulting in the loss of control from the Control Room.

Based on timing for the PRA action to start the AFW pump, approximately 30 minutes to 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> are available to perform this action. The action is  !

proceduralized in A0P 2-90-1. .

l The dominant factors affecting evaluation of this human action are

" Stress." since the threat to the operators, due to the Control Room fire.  ;

will result in high stress levels. " Training and Experience." since _the operators do not customarily start and load the Appendix R Diesel Generator and " Preceding Actions." since the operator may be distracted by the need to fight the fire, the need to open the PORV breakers, or by attempts to take mitigative actions from the Control Room.

Based on an evaluation of all involved human factors, the mean failure l probability for this action was determined to be 1.4E-02.

Human Action HECR/PORV l

l This action is similar to the HEPORV human action evaluated above.

However. it occurs as the result of a fire in the Main Control Room (Fire Scenario CR-VVSL). The operator experiences additional stress'as the result of the location of the fire, resulting in a higher human error probability.

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, In general, most of the discussion of the HEPORV action above also applies to this action.

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Over one hour is available to perform this action. This action is  !

proceduralized in A0P 2-90-1. l

! The same factors that dominate the HEPORV human action also dominate r HECR/PORVs.

i Based on the evaluation of all involved human factors and a comparison with i action HEPORV evaluated above, the mean failure probability for this action was determined to be 4.9E-02. i Human Action: HELB/PORV

HELB/PORV describes an operator action to disconnect power to MCC-78 and

MCC-8B and, therefore, prevent or mitigate an inadvertent opening of the

j PORVs. This action is similar to the HEPORV and HECR/PORV human actions evaluated above. It occurs as the result of a fire in the Penetration B/MCC i Area (Fire Scenario ELPB-VVSL).

i i The action to disconnect power to MCC-78 and MCC-8B is performed in the

Protected Switchgear Room.

One hour is available to perform this action. The action is proceduralized ,

in A0P 2-90-5. i i

! The same factors that dominate the HEPORV human action also dominate i HEPH/PORV.

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Based on the evaluation of all involved human factors, and a comparison

! with action HEPORV, the mean value for this action is estimated to be 3.9E-02.

i Note: Disconnecting power to MCC-7B and MCC-8B would result in removing

the power to the HPSI suction valves from RWST. If a stuck open I PORV results in a safety injection signal, and the HPSI suction

! valves are closed and depowered, the HPSI pumps could fail due to

loss of suction. Maine Yankee has committed to update AOP 2-90-5 i to open the HPSI suction valves prior to removing power from MCC- l 1 7B and MCC-88.

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Human Action: HEHVAC This scenario involves a human action to install portable ventilation to provide cooling of the Protected Switchgear Room.

The need for this action arises because of fires in the Turbine Building (Fire Scenario TB-HVAC), which can disable Protected Switchgear Room Ventilation.

Fron, the Maine Yankee PRA, 45 minutes are available to accomplish this action. Since the fire is not directly interfering with the operator's ability to install portable ventilation, this action is considered to be essentially identical to PRA action ORCV. Attachment A of A0P 2-90-7 directs the operator to install portable ventilation in areas housing shutdown equipment.

Maine Yankee has committed to increase the amount of dirction provided by this procedure. A0P 2-90-7 will specifically indicate that the Protected Switchgear Room could be affected and the need for temporary measures to maintain cooling.

The dominant factors affecting this human action are " Task Complexity," I since the action involves local installation of portable fans. " Adequacy of Time " since finding the fans, determining where they should be located, and installing them should consume a fair amount of time, and " Training." since this does not appear to be an action where the operators receive a great deal of training.

Based on an evaluation of all involved human factors and a comparison with Maine Yankee PRA human action ORCV, the man value of this action is estimated to be 1.0E-02.

Table 4.9.1 provides a summary of the analysis and evaluation described above. These results are used in the following section to develop complete fire sequence quantifications.

4.9.2 Fire Sequences Evaluation Evaluation and quantification of the Maine Yankee fire sequences used the Maine Yankee PRA model fault trees and event trees to quantify core damage risk. For each fire scenario identified and quantified in Table 4.8.6. the most relevant PRA model initiating event was selected and the models modified as required to represent each fire scenario being analyzed. In other words.

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l Resultant initiating Event: Station Blackout Event Tree (s): Transient Event Tree (similar to the station blackout 1-internal. event). All plant systems were assumed unavailable, except for the auxiliary feedwater turbine driven pump, which could be controlled remotely at the l Alternate Shutdown Panel (ASP) and equipment supported i

by the Appendix R diesel generator. '

Mitigative System Unavailability: 0.147 3

! #ajor Contributors: Unavailability of TO AFW Pump (1.01E-01),

j unavailability of APP. R Diesel Generator l (7.47E-02). and unavailability of EFCV Air System

(8.27E 02). Recovery of AFW Pump was credited at

! 0.741. Recovery of the EFCV Air System was credited i at 0.106.

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! C0F: 4'.1E 06/yr

-Comments: For this scenario, the following recovery assumptions were l i applied:

i- . No credit was taken for recovery of support systems due to significant damage of the area (off-site power or diesel generators).

. No-credit was taken for recovery of the TO AFW Pump when

the Appendix R diesel generators has failed, because of

unavailability of SG. instrumentation. Local manual

{ recovery of the TO AFW Pump was credited when either the j pump has failed or EFCV air has failed.

} 4.9.3 Conclusions i A summary of the plant response to each fire-induced events is given in l Table 4.9.2. In this table, relevant evaluation parameters are given for each ,

f initiator / fire scenario. These include the type of event tree used to analyze l l each given initiator, initiator (fire scenario) frequency, corresponding i backup system unavailability, and resulting core damage frequency. The total

! fire related core damage frequency is 2.8E 05/yr. The last column in

,! Table 4.9.2 gives, for each fire scenario, its percent contribution to the '

I total fire CDF. I i

F Fire risk is an important contributor to the overall plant risk at Maine

! Yankee. The reasons for this conclusion and significance of different  !

contributors are discussed in Section 4.13. 1 nuuse 4-189 i,

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TABLE 4.9.1 Summary of Human Action Evaluation HUMAN FIRE HUMAN ACTION SIGNIFICANT TIME HEAN VALUE OPERATOR ACTION ID SCENARIO DESCRIPTION PRECEDING AVAILABLE GUIDELINES EVENTS FIRE ALL AREAS Diagnose where Fire Several Relatively ADP 2-90-0 DIAGNOSIS LISTED BELOW fire is locat Minutes simple (Estimate) action subsumed in actions below.

HEPORV CABV-VVSL Open PORV Breakers Cable Vault >l Hour 3.9E-02 ADP 2-90-1 in Pen B/MCC Area Fire (Procedural Requirement -

10 Min.)

HESGLP MSFW-LFW Override SG Low Main Steam & 1.5 - 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> 1.7E-02 AOP 2-90-1 l PRCT-L2VB Pressure Feed Feedwater and 2-90-2 l PSWG-L2VB Train Trip Using Valve House direct the l MCB Bypass Fire. operator to l Switches Protected enter E-0. l Cable Tray, or Switchgear Room Fire 4-190 Revision 1

value of 0.147 to 0.074. the CDF will decr' ease 21% to 2.2E-05/yr. If the-reliability of shutdown from ASP decreases by a factor of two to 0.3, the CDF will increase 46% to 4.1E-05/yr. This illustrates the sensitivity of results to the ability to shut down-the plant from outside of the Control Room.

At Maine Yankee, an additional possibility for recovery exists involving shutdown from the Protected Switchgear room. If this action was to be proceduralized and credited, fire CDF would be 2.1E-05,'yr. a 25% decrease from the basecase value.

A major source of uncertainties in the fire sequences lies in the modeling

, assumptions and quantification of human actions / recoveries. Sensitivity to human error probability (HEP) is evaluated by artificaially raising the HEP.

A factor of two higher for all HEPs would result in a fire CDF of 4.2E-05/yr.

an increase of 50%.

Table 4,13.4 summarizes the aforementioned sensitivity results.

In conclusion. it is obvious that the core damage frequency is most sensitive to the fire severity factor for electrical cabinets, human error probability values,and the reliability of alternate shutdown methods.

4.13.3 Plant Recommendations The following enhancements to the plant procedures were recommended and have been committed to by Maine Yankee as a result of the IPEEE-fire effort:

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. A0P 2-90-1, " Plant Shutdown Plan for Fire in Control Room, Control Room Cable Chase. Protected Cable Vault, Protected Cable Tray Room, and Protected Switchgear Room", will be revised to indicate that l there is the potential to hot short open containment penetrations and l that CIV indications may not be reliable. l 4-239 Revision 1

Abnormal Operating Procedure AOP 2-90-5 "P.lant Shutdown Plan for Fire in Reactor - MCC - Elevation 21' and 33' Containment Electrical Panetration Room - North Elevation 46' " will be updated to warn the operator about the possibility that disconnecting power to MCC-78 and MCC 8B would result in removing power to HPSI suction valves from the RWST. If a stuck open PORV results in a safety injection signal, and the HPSI suction valves are closed and depowered, the HPSI pumps could fail due to the loss of suction. The problem can be avoided if the HPSI suction valves are opened prior to removing power from MCC-7B and MCC-088.

. Abnormal Operating Procedure A0P 2-90-7. " Plant Shutdown Plan for Fire in Turbine Hall or Circulating Water Pump House." will specifically indicate that the Protected Switchgear Room could be affected because of a loss of HVAC due to a fire in the Turbine Building and that the requirements of AOP 2-37.SS (SS-1-1) should be l followed, if necessary, to provide temporary measures to maintain l cooling.

. Station operating practice is to isolate the hydrogen supply at the tube trailer after filling the generator and volume control tank (VCT). In order to provide more positive control, the station administrative procedure was updated to reflect this requirement.

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! 8.0 PLANT IMPROVEMENT AND UNIOUE SAFETY FEATURES As a result of the proposed improvements to the design of Maine Yankee with regard to external hazards. and based on the extensive evaluation summarized '

-in this report, no fundamental weakness or vulnerability remains with regard  !

to severe accidents. A summary of the important external evencs' design l features is given in Section 8.1. The process of evaluating potential plant

improvements is described in Section 8.2.

8.1 Uniaue Safety Features A number of features contribute to the level of risk at Maine Yankee.

Features that are more or less unique to Maine Yankee include the following:

Features Specific to External Events (and Spatial Events) l i

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. Cables for both trains of electric power / control pass through a common i Switchgear Room Cable Vault, and Cable Spreading. (Cable Tray) Room.

. The Control Room is located at " ground" elevation with 4" berms. As ,

such, it is potentially subject to Turbine Building floods. .j l

. Ventilation equipment for the Protected SWGR is located in an area of the Service Building which communicates with the Turbine Building. As  !

such. it is potentially subject to Turbine Building HELBs.  !

. The containment structure is very strong.

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. Seismic design has substantial margin. No structures and only a few components have,an estimated HCLPF < 0.39 General Features Which Impact Response to Internal Events and Potentially External Events

. A second, full capacity, standby Main Feedwater System.

r . ECCS operation requires a large number of MOVs to change position. The j recirculation mode of ECCS with Containment Spray feeding HPSI is .

unique.

. Reactor trip breaker, breaker actuation. and trip relay design is a very reliable configuration.

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9 The " fail-safe" steam line break protection design contributes to -

secondary heat removal unavailability.

. RCS loop isolation valves reduce the significance of SGTR and RCP' thermal barrier rupture.

The " Appendix R" diesel generator provides an independent, self-contained ac power source.

The off-site ac power supply and " fast-transfer" design allow degradation of 115 kV power lines without affecting normal plant operation: this may reduce the frequency of loss of off-site power initiating events.

The desi p includes backup emergency diesel generator cooling via firewater.

8.2 Plant Imorovements To reduce risk, several enhancements to the plant or procedures were recommended as a result of the analysis. Following completion of proposed changes, the remaining external event risk is small and is dominated by modeling uncertainties - e.g.. equipment ultimate capacity, transient combustible fire frequencies, etc. Enhancements are summarized below:

Ensure plant. procedures provide adequately detailed guidance in response l to fires. l

. Improve flood protection in the Turbine Building.

. Remove water line in Control Room.

. Leak before break for MSVH.

. Analyze Turbine Building HELBs - make modification as necessary. '

. Reroute cable for ; IPSI suction valve HSI-M-51 outside of MSVH. 4

. Upgrade RWST to a HCLPF of .279 i

. Upgrade HCLPFs of equipment required to reach cold shutdown.

. Upgrade mounting of two outside containment isolation valves to HCLPF of

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As discussed previously. Maine Yankee has implemented or has committed to implement the recommended plant improvements identified in the report.

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TABLE 8.1 -

IPFFF Identified Enhancamant Oooortunities Issue Enhancement Disposition IPEEE FIRES

1. The guidance Add additional Maine Yankee has provided in the fire guidance. committed to procedures for implement l various actions is procedure changes  ;

not as detailed as by the end of the that provided for next refueling similar actions outage.

credited and quantified for the Internal Events PRA.

2. Nonseismically Isolate hydrogen Plant procedures supported hydrogen supply after filling require hydrogen piping. the generator and to be isolated volume control tank when not in use.

(VCT).

INTERNAL FLOODS

3. Turbine Building Install trip Maine Yankee has Floods can circuits, level comitted to potentially affect alarms. SW expansion implement the l the Cable Vault, joint bands changes by the Control Room and weatherstripping, end of the next other important isolation of EDG room refueling outage. 4 areas. drains, and  !

berms / door I modifications.  :

Implement Procedure Controls.

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4. Water line in the Remove water line. Water line has Control Room ceiling been removed.

can affect important equipment. Effects are difficult to analyze, i 8-6 Revision 1

TABLE 8.1 (Continued)

IPEEE Identified Enhancement Oooortunities Issue Enhancement Dispositipn IPEEE HELBs

5. Initiating Event Perform leak before Maine Yankee has Frequency for MSVH break analysis. Take committed to HELBs should be necessary actions to complete by the reduced. A well ensure that piping end of the next designed leak before meets leak before plant refueling break program should break criteria (e.g., outage.

reduce the initiator walkdowns, erosion-frequency by a corrosion factor of 10. examinations etc.)

6. SWGR Room Implement procedural Maine Yankee has Ventilation Fans can controls and plant committed to be impacted by HELBs hardware changes, as implement by the in the Turbine necessary, to protect end of the next Building. SWGR and equipment from harsh plant refueling Control Room environments. outage.

Ventilation ducts may also be damaged.

7. HPSI suction valve Reroute HSI-M-51 Maine Yankee has cable runs through cable. committed to the MSVH where it complete by the .

could potentially be end of the next l damaged by a HELB. plant refueling i outage, l l

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TABLE 8.1 (Continued)

IPEEE Identified Enhancement Donortunities Issue Enhancement Disposition IPEEE SEISMIC l

8. RWST has a HCLPF of Increase tank HCLPF Modification

.21g to .279 by improving complete.

tank achorage.

9. Based on recent Improve equi Maine Yankee has extension of Seismic anchorages. pment committed to Margins Program to complete by the Cold Shutdown (RHR, end of the next Appendix R cooldown) plant refueling various pieces of outage.  ;

equipment have inadequate HCLPFs.

10. Two outside Upgrade mounting of Modification containment two valves to HCLPFs complete.

1 solation valves of >0.39 have a HCLPF of

.189 These are 1" penetrations and the inside isolation valve has a HCLPF of

>0.39

11. Westinghouse SV Replace relays with Maine Yankee has relays in Diesel GE type NGV relays. committed to Start Circuits have complete by the a low HCLPF. end of the next plant refueling outage.

EXTERNAL FLOODS

12. There are some minor Correct Maine Yankee has inconsistencies in inconsistencies committed to the external flood complete by the procedures / end of the next ,

documentation plant refueling )

outage i 13 The potential exists Provide plant Maine Yankee has for external floods hardware committed to to enter the plant modification. implement by the through the computer end of the next and the Control plant refueling Rooms. outage.

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TABLE 8.1 (Continued)

IPEEE Identified Enhancement Oooortunities Issue Enhancement Disposition IPEEE 14. Flood loadings on Provide scuppers to Maine Yankee has  !

some roofs may relieve / prevent flood committed to exceed roof loading. implement by the capacity. Roofs in end of the 1999 question include plant refueling Service Building outage.

El. 61 and Service- I Building El. 39. i

15. Vent openings on the Ensure vent openings Maine Yankee has a Diesel Generator are adequate to committed to Building roofs are resist flood impact. implement by the not above the end of the 1999 i maximum predicted plant refueling I ponding levels on outage. l the roof. l

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16. Floods could Provide a water-tight Maine Yankee has l potentally enter the seal around the committed to seal i plant due to affected manholes. the manhole

, flooding of manholes covers to prevent '

on the north side of water ingress by the plant, the end of the next refueling outage.

17. Identified FSAR Update FSAR. Maine Yankee has inconsistency committed to dealing with low update the FSAR 1

river level. by the end of the next refueling outage GENERAL

18. EFW pump room Provide additional Modifications ventilation. qualified have recently ventilation. been completed.

19. Fires (and Incorporate guidance Accident potentially floods) into Severe Accident management in the Control Room. Management Guidelines guidelines are Cable Vault. being developed for currently being Protected Cable Tray Maine Yankee, developed.

Room. Protected SWGR and EFW Pump Room have the potential to cause multiple hot shorts or power failures to both in-board and out-board containment isolation valves.

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TABLE 8.1 -

(Continued)

IPEFF Identified Enhancement Oooortunities Issue Enhancement Disposition _

IPEEE 20. Functionally AFW Take actions to Maine Yankee has availability is improve AFW comitted to:

important to availability, mitigation of a Track AFW per ,

various external the events. Maintenance Rule.

• Procedures and the MY risk monitor discourage elective maintenance on AFW if an external event challenge is expected.

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