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GPU Nuclear CogmrationNggIg7 Post Office Box 480<

Route 441 SouthMiddletown, Pennsylvania 17057 0191717 944 7621-TELEX 8<4 2386Writer's Direct Dial Number:

April 13, 1989C311-89-2035 !

'

(TAC No. 59151)

U.S. Nuclear Regulatory CommissionAttn: Document Control DeskWashington, D.C. 20555

Dear Sir. ]1

Three Mile Island Nuclear Station, Unit 1 (TMI-1) |'

Operating License No. DPR-50Docket No. 50-289

ATWS Implementation (10 CFR 50.62) i

RAI Meeting - Post Meeting Questions |l

GPU Nuclear Corporation (GPUN) representatives met with NRC staff members on '

February 15, 1989, to discuss ATWS implementation issues. As a result of thesediscussions, the NRC staff requested additional clarification as related to theissue of the number of control rod groups required to be inserted by the ATWSDiverse Scram System (DSS). Enclosure 2 of the Staff's February 27, 1989 1

'

letter presents Post-Meeting Questions on TMI-1 ATWS. These questions are|repeated in GPUN's attached response for convenience.

Also, during the February 15, 1989 meeting, GPUN representatives agreed toprovide a response to the January 3, 1989 - NRC Review of ATWS Implementationletter and its enclosed Request for Additional Information (RAI) in May, 1989.

l The GPUN response, albeit not containing all the typical final design details,will address the RAI questions with what GPUN expects to be the Final DesignPlan in generic terms and with specific information where available.Additional final design details will be available subsequent to the May; 1989transmittal.

! Sincerely,I

b '

h. D. kill

|- Vice President and Director, TMI-1

L GMG/imi

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cc: W. Russell, NRC |

R. Hernan, NRCF. Young, NRC ,t

GPU Nuclear Corporation is a subsidiary of the General Public Utilities Corporation8904190344 890413PDR ADOCK 05000289P PDC

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ATTACHMENT;

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I Response.to Enclosure 2 to the NRC letter dated February 27, 1989. ]

" Post-Meeting Questions on TMI-1 ATWS Systems"

Introduction:' |i

p Following ATWS implementation during TMI-1 refueling outage 9R, fuel enrichment'for Cycle 9 and future cycles will be such that to achieve " hot zero power"upon actuation of Diverse Scram System (DSS), more than control rods Groups 5 - |

6 and 7 must be inserted. It is GPUN's position that a DSS should indeed scramthe reactor and achieve zero reactor power. GPUN will design the TMI-1 DSS todrop all control rod groups (1-7). To accomplish this, electrical power mustbe removed from the control rod drive mechanisms (CRDMs), a function redundantto that provided by the Reactor Trip System (RTS). Figure 1 provides asimplified description of the devices used to interrupt power to the CRDMs for ;

both ATWS-DSS and RTS implementation. j

Question 1:

Provide information regarding the physical and operating characteristics of theWestinghouse circuit breakers being considered for use in the revised ATWS/ DSSdesign'and the. existing General Electric circuit breakers that are currentlyused to interrupt power to the CRDMs at TMI-1. Include in this discussioninformation regarding the differences of the devices used to trip the circuitbreakers and the type and voltage level that is used to operate these' trippingdevices.

Response-

GPLW proposes to use the Westinghouse circuit breakers depicted on the one-linediagram of Figure 1 which are upstream of the Reactor Trip-System. This designis a 2 out of 2 trip logic, which requires the addition of a new shunt tripdevice to the upstream Westinghouse breakers. The new shunt trip is energized-to-trip from the new independent 115 VAC UPS source for ATWS-DSS.

Diversity is the major design criteria to be met by the ATWS DSS. Thediversity of interest is between the Westinghouse breaker and the SCR de gateand also between the Westinghouse. breaker and the General Electric (GE)breaker, as the RTS utilizes the SCR de-gate and the GE breakers to removepower from the CRDMs. The Westinghouse breakers (DSS) and SCR de gate (RTS)are quite diverse in both hardware design and function. There is a general,

I diversity between the Westinghouse and GE breakers from a manufacturingviewpoint and a specific diversity in the manner in which each accomplishes itstrip function. The RTS-GE breakers interrupt 125 VDC to the CRDMs while theDSS Westinghouse breakers interrupt 480 VAC. See Table 1.

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Res'ponse:*

(cont'd)|

The GE breaker utilizes an undervoltage trip coil (120 VAC) and a 125 VDC shunttrip coil controlled from a control relay which is actuated by the loss of thesame 120 VAC. The Westinghouse breaker will utilize a 120 VAC shunt trip coilpowered by the new independent power source for the DSS /AMSAC logic. The GEbreaker (model: AK-2-15) and the Westinghouse breaker (model: DB-25) aredepicted on Figures 2 and 3, also attached. The specific differences ofinterest are the methods of releasing the springs which open the moving arecontacts. The GE breakers utilize rotation of a trip shaft to release theopening spring. The Westinghouse breaker actuates a trip bar in a straightline to release the opening spring.

Question 2,

Provide information on the history that TMI-1 has experienced while operatingthe Westinghouse and General Electric circuit breakers in question. Include inthis discussion information related to the preventive maintenance program thatis being used to maintain these circuit breakers.

Response:

The GE breakers associated with the Reactor Trip System (RTS) have only oneother application at TMI-1. The Westinghouse breakers, both DB-25s and othersizes, are utilized throughout the electric distribution system.

The maintenance history of GE AK Breakers and Westinghouse DB breakers at TMI-1have been reviewed. When examining the maintenance history it is important toconsider the experiance base. The few GE AK breakers at TMI-1 associated withthe RTS receive extensive surveillance testing directed at the trip function.The much larger number of Westinghouse DB breakers are routinely tripped duringnormal plant operations and system testing. Both manufacturer's breakers havetripped reliably using the shunt trip. For comparison with the GE breakers,the machinery history for ten DB-25 breakers was reviewed. These breakers wereselected because they are known to be operated quarterly for ES logic testing.The breaker is tripped using the shunt trip after the auto start. No failuresto trip were found. Inadvertent tripping of DB breakers has occurred on someoccasions due to problems with the electromechanical overcurrent trip devices.This problem was eliminated by replacement of the electromechanical trip devicewith a solid state trip device,which will not affect the desired trip function.

GE breakers at TMI have had several instances of failure to trip using theundervoltage trip. These problems were related to mechanical problems with the"

undervoltage trip device and trip shaft. These problems have been resolvedwith improved lubrication and improved maintenance and testing procedures. Onecase of failure of the shunt trip coil on an AK breaker occurred at TMI. ;

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Responses(cont'd)

NUREG 1000 " Generic Implications of ATWS Events at the Salem Nuclear PowerPlant" documents a number of failures of both DB and AK breakers. Thesefailures relate to the undervoltage trip. We believe tha undervoltage trip ismore prone to failure because it relies on spring fa ce (de-energize to trip)and therefore has less motive force to operate the trip mechanism than the |shunt trip.

Relative to industry experience with Westinghouse DB breakers, review of theNPRD information indicates that there were three incidences of a failure totrip (using shunt trip) of DB breakers at other facilities. Two of thefailures were because of blown fuses and one was due to a loose connection.

Table 1 indicates which plant procedures are used at TMI-1 in the preventativemaintenance program for the subject breakers. These procedures are written toreflect the suggested preventative maintenance methods used by the breakermanufacturers, GE and Westinghouse. The Westinghouse DB breaker inspectionsperformed by PM E-5 include but are not limited to examination of the breakercubicle rails and extensien arms, inspection of the breaker are quenchers,stationary, moving and arcing contacts and main and auxiliary breaker stabs.Wiring is checked for fraying or deterioration. Terminations are checked fortightness and breakage. The breaker internals and insulating surfaces arecleaned and lubrication of the bearing points, sliding surfaces, primary andsecondary disconnects is performed. Electrical checks include measurement ofthe main contact resistance and breaker insulation resistance. A functionaltest is also performed. PM procedure E-5 is currently performed on an 18 monthinterval.

Inspections of the GE model AK breakers performed by PM procedure E-36 includebreaker trip timing checks, undervoltage device pickup and dropout voltagetesting and trip latch roller and latch surface inspections in addition to theE-5 inspection. Numerous checks and inspections of the uv device are alsoperformed and include centering of the uv trip paddle and armature dise pad, uv !armature clearance, air gap and overtravel measurements and uv trip paddleadjustrent. This PM is performed on a yearly interval for the GE AK breakers.

1These procedures are available onsite for further review.'

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f In summary, the shunt trip of both the DB and AK breaker has been reliable at ,

TMI and uhe rest of the industtJ. Use of the shunt trip is a reliable means of |

tripping the breaker and diversity is achieved in design, procedures, powersource, and Jocation of the breakers.

f Question 3

Provide general information on the cost and nanpower requirements if anothermethod of interrupting power to control rods is considered. Use the DavisBesse (All SCRs) reactor trip design or consider the use of a heavy duty linecontactors as possible other design concepts to snterzupt. powee.

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Response.(cont'd)

Alternatives have been considered. The Davis Besse (DB) approach (whichprovides similar diversity to the B&WOG DSS scheme in dropping control rodgroups 5, 6, and 7) is the use of SCR (regulating) power supplies for removingpower to the safety rods, part of the original DB design. The manufacturer ofthe regulating power supplies is no longer in this business. Clearly, thecosts of building 4 new regulating power supplies, (requiring start-up of a newmanufacturing line) to interface with the existing control rod drive system,and the subsequent installation of such supplies would be prohibitive.

The possibility of using heavy duty line contactors as an alternative designconcept to interrupt power to the CRDMs has been evaluated in light of the ATWSrule and found to not meet the requirements because these devices are energizedto close, not energized to trip.

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