ML20070V423

Submits Info Re Inverter Problems,In Response to NRC 830203 Request

ML20070V423
Person / Time
Site:	Calvert Cliffs
Issue date:	02/09/1983
From:	Tiernan J BALTIMORE GAS & ELECTRIC CO.
To:	Clark R Office of Nuclear Reactor Regulation
References
NUDOCS 8302170334
Download: ML20070V423 (3)

Text

s o

BALTIM ORE GAS AND ELECTRIC CHARLES CENTER P. O. BOX 1475

• BALTIMORE, MARYLAND 21203 JOSEPH A. TIERN AN MANAGER NUCLEAR PowCR DEPARTMENT February 9,1983 U.S. Nuclear Regulatory Commission Office of Nuclear Reactor Regulation Washington, DC 20555 ATTENTION: Mr. Robert A. Clark, Chief Operating Reactors Branch #3 Division of Licensing

SUBJECT:

Calvert Cliffs Nuclear Power Plant Unit No. 2; Docket No. 50-318 Inverter Problems Gentlemen:

As requested earlier in a February 3,1983, conference call between Messrs. R. A. Clark and 3. A. Tiernan, the following information is forwarded:

QUESTION:

Please describe the modifications made on Unit 2 to date.

ANSWER:

The first modification made to Unit 2 was to replace the one-time (normal blow) 20 amp load distribution panel fuses with form 101 (fast blow),12 and 15 amp fuses on the vital AC instrument buses #21 and #22. After completing this modification, the current limiting function for #21 and #22 Inverter was removed. These actions were based on manufacturer's recommendations, BG&E engineering evaluations, and field testing results.

The manufacturer of the inverter assured t s that removal of the current limiter would not substantially degrade protection of the equipment, if recommended fusing was provided.

QUESTION:

@f Please provide your understanding of the problem. I 8302170334 830209 PDR ADOCK 05000318 p PDR

Mr. R. A. Clark February 9,1983 Page 2 ANSWER:

Our assessment of the problems we have had to date is summarized as follows: First, the fusing on the vital AC instrument buses was not consistent with the recommendations of the inverter manufacturer. The fuses were above the recommended current rating and of the wrong type (blow rating). Secondly, the problem may have been compounded by an intermittent problem (i.e., a short or ground in a load supplied by the vital AC instrument bus and/or a problem in the inverter current limiter itself). If a load had an internal short, and the fusing was improper, this short may result in an overcurrent condition on the inverter supplying the bus. If this current surge is of sufficient magnitude to cause the current limiter to operate, a drop in voltage on the AC side of the inverter would result. This decrease in voltage would cause a decrease in supply voltage to the ESFAS system. The characteristics of the bistables in the ESFAS system are such that they could assume an actuated state most of the time. By actuating, we mean most or all logic modules trip, but the ESFAS equipment does not operate. If the current limiter function results in a tripping of the DC input fuse on the inverter, only an actuation would normally result (the loss of power to ESFAS would cause the relays to the equipment to be deenergized, and the Loss of Coolant incident Sequencer would also lose power). The only equipment we have observed to operate under a loss of power condition are the safety injection header isolation valves, and on one event, the letdown isolation valves shut. The safety injection valves moved only slightly off their shut seat. We are presently investigating the precise cause of such valve movement. If, however, the voltage is restored, the equipment would perform its design function; or initiate. By initiating, we mean most or all logic modules trip, and the ESFAS equipment operates.

using the fast-blow (form 101) fuses, and removing the current limiter feature, the fault should be isolated before the inverter DC fuse blows. This should additionally reduce the probability of a reduction in voltage on the vital AC instrument bus and essentially eliminate the condition where power is automatically restored in an uncontrolled t

manner. Since type 101 fuses are now utilized, the location of any short or ground should l be readily identified in the future, with no effect on the voltage supplied to the related i ESFAS actuation cabinet. This understanding has been additionally supported by field testing results.

Numbers 23 and 24 vital AC instrument buses do not supply power to the ESFAS Logic Cabinet, therefore, they were not our immediate concern. However, since the sensor modules for the ESFAS system are also supplied by these buses, we will investigate the same modifications for these buses.

QUESTION:

Please provide a commitment to review Unit I to determine if similar modifications should be made on Unit I and a schedule to complete these modifications, if determined to be appropriate.

o Mr. R. A. Clark February 9,1983 Page 3 ANSWER:

During a forced outage from February 5,1983, to February 6,1983, the current limiters for #11 and #12 Inverters were removed and the equivalent fuses for #11 and #12 vital AC instrument buses were changed out. Number 13 and 14 instrument buses will be reviewed for similar modifications, to determine if they are appropriate.

QUESTION:

Express in your letter especially the review you have made which indicates your conclusions that there is no interaction that would defeat both of your safety features actuation channels and their trains.

ANSWER:

We have reviewed the design of the ESFAS system and have concluded that both ESFAS logic trains and associated equipment are independent, and that no interaction would defeat both of these trains. We would be pleased to discuss this further at the Friday, February 11,1983, meeting.

Should you have any further questions on this matter, please do not hesitate to contact us.

Sincerely yours, iC l

- lbt4222L-Manager - Nuclear Power JAT/LES/sjb cc: 3. A. Biddison, Esquire

! G. F. Trowbridge, Esquire D. H. Jaffe, NRC

! R. E. Architzel, NRC

_.