A06535, Responds to Concern Re Reactor Trips Attributed to Reactor Pressure Control Sys Noted in Insp Rept 50-245/87-04. Corrective Actions:Licensed Operator Requalification Training Addressed Event & Simulator Exercises Conducted

Responds to Concern Re Reactor Trips Attributed to Reactor Pressure Control Sys Noted in Insp Rept 50-245/87-04. Corrective Actions:Licensed Operator Requalification Training Addressed Event & Simulator Exercises Conducted

ML20235Y197
Person / Time
Site:	Millstone
Issue date:	07/17/1987
From:	Mroczka E NORTHEAST NUCLEAR ENERGY CO., NORTHEAST UTILITIES
To:	Wenzinger E NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION I)
References
A06535, A6535, NUDOCS 8707250148
Download: ML20235Y197 (3)

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P.O. BOX 270 HARTFORD, CONNECTICUT 06141-0270 L L J [,["j,C ] ['j'g * (203) 665-5000 July 17,1987 Docket No. 50-245 A06535 Re: 10CFR2, Appendix C Mr. Edward C. Wenzinger, Chief Projects Branch No. 3 Division of Reactor Projects Region I U.S. Nuclear Regulatory Commission 631 Park Avenue King of Prussia, PA 19406 Gentlemen:

Millstone Nisclear Power Station, Unit No. I Response to I&E Inspection No. 50-245/87-04 In a letter dated April 24,1987,(I) the NRC Staff transmitted the results of Routine Inspection 50-245/87-04. Though no violations were identified, a concern was raised regarding reactor trips attributed to the reactor pressure control system. The NRC Staff requested in the transmittal letter that Northeast Nuclear Energy Company (NNECO) conduct an assessment of the perceived potential problem areas and report our findings and any planned corrective action within 90 days. NNECO is hereby submitting our response.

NNECO has taken prompt corrective actions in response to the concerns identified during the inspection.

Requirement (NRC Request)

In approximately one year, there have occurred three reactor trips at Millstone Unit No. I attributable to the reactor pressure control system. The first two trips were caused by equipment malfunctions in the mechanical pressure regulator (MPR). The most recent trip was caused by operator error when transferring control from the MPR tn the electrical pressure regulator (EPR).

Please review these challenges to safety systems and inform the NRC Staff of your assessment and any planned corrective measures within 90 days of receipt of this letter.

(1) E. C. Wenzinger lener to E. J. Mroczka, " Routine Inspection 50-245/87-04,"

dated April 24, ;787.

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l Mr. Edward C. Wenzinger l <

A06535/Page 2 July 17,1987 Finding On March 22,1987 at 1751 hours0.0203 days <br />0.486 hours <br />0.0029 weeks <br />6.662555e-4 months <br />, while operating at 48 percent power (5300F, 990 psig), a reactor scram and Group I isolation occurred during a transfer from the MPR to the EPR. The event occurred when main steam line pressure decreased below the Group I isolation setpoint of 825 psig as the EPR was placed in service. No safety consequences resulted from this event.

Root Cause The root cause is operator arror. Subsequent invedgation of this event has identified a contributing cease of sensitivity of the instrumentation during a transfer at low reactor pc wer levels. Since the EPR is an integrating type l control system, the internal amplifiers will saturate when the deviation signal I

(actual pressure vs. setpoint) is large. This saturation effect is alleviated only when the deviation signal changes polarity, reversing the direction of the integrating amplifier. Reversal of signal polarity occurs only when the system j l setpoint is decreased below actual pressure. Due to the nature of this system, I i control can be transferred from the MPR to the EPR only by decreasing the EPR j setpoint below that of the MPR. Since the EPR is in a saturated condition prior-to this transfer, there is a time lag between the point where the setpoints are equal and the EPR begins to take control. During this period, it is possible for the operator to continue to decrease the setpoint of' the EPR, thereby overshooting the desired setpoint, without affecting system pressure. The EPR will subsequently reduce main steam pressure to the actual setpoint by opening the turbine control valves. At low reactor power levels, a slight opening of the control valves will create a significant reduction in main steam pressure. A l great deal of operator attention is required during this type of operation in order i to prevent a decrease in main steam pressure below the Group I isolation i setpoint of 825 psig, as occurred during this event.

l Resolution / Corrective Action l Several corrective actions have been undertaken by NNECO subsequent to this event. Specifically, these corrective actions include:

l a) The incorporation of more oetailed guidance within operating procedures. Operations Department Procedure OPS 314, " Turbine Generator" has been revised to provide improved direction to licensed personnel performing a transfer evolution. This revision was the result of efforts conducted by a subcommittee comprised of Operations and I&C personnel.

b) Licensed operator requalification training has addressed this event and reviewed the relationship between the MPR and EPR. This training evolution received both classroom and simulator exercises and was l conducted during the last requalification cycle.

c) Since the March 22, 1987 event, and prior to shut down for refueling, MPR/EPR transfers were being conducted on a weekly basis with the unit at power. Prior to the Marcn 22, 1987 event, this exercise was rarely conducted. Improved operator and staff confidence in the MPR/EPR system has been recognized since the inception of this practice, and plant management intends to continue this practice frequently.

l

. e Mr. Edward C. Wenzinger A06535/Page 3 July 17,1987 l

d) Investigating modifications to aid control room operators during pressure regulation transfers. Completion of the study is expected by September, 1987.

e) The MPR is being overhauled during the 1987 refueling outage which is currently in progress.

A tions to Prevent Recurrence It is anticipated that the above corrective actions will prevent recurrence.

In conclusion, NNECO will continue to monitor operational performance in this area. The corrective actions to date have contributed to significant operational improvements.

If you have any additional questions on this issue, please feel free to contact my staff.

Very truly yours, NORTHEAST NUCLEAR ENERGY COMPANY h MVA/

E. J. Mjr 6~ z~ka ~ //

Senior Vice President cc: Document Control Desk U.S. Nuclear Regulatory Commission Washington, D.C. 20555 W. T. Russell, Region I Administrator M. L. Boyle, NRC Project Manager, Millstene Unit No.1 T. Rebelowski, Resident Inspector, Millstone Unit No. I