IE Bulletin 76-07 and IE Circular 76-01 - Crane Hoist Control - Circuit Modifications
CR76001 JUL 27 1976 NUCLEAR REGULATORY COMMISSION OFFICE OF INSPECTION AND ENFORCEMENT WASHINGTON, D.C. 20555 J. P. O'Reilly, Director, Region I N. C. Moseley, Director, Region II J. G. Keppler, Director, Region III E. M. Howard, Director, Region IV R. H. Engelken, Director, Region V IE BULLETIN 76-07 and IE CIRCULAR 76-01 - CRANE HOIST CONTROL - CIRCUIT MODIFICATIONS The subject documents are transmitted for issuance by close of business July 28, 1976. The Bulletin should be issued to all holders of Power Reactor OLs. The Circular should be issued to all holders of Power Reactor CPs. The text is identical except for the response time. Also enclosed are draft copies of transmittal letters. Since the LER enclosure is not appropriate for remote typewriter transmission, text transmission has been forwarded by express mail, scheduled to arrive your office today, with issue date set accordingly. Dudley Thompson, Acting Director Division of Field Operations Office of Inspection and Enforcement Enclosures: 1. Bulletin 76-07 2. Circular 76-01 3. Licensee Event Report dated 6/10/76, sep cov . (Transmittal letter for Circular 76-01 to each applicant for, or holder of a Construction Permit.) Addressee: The enclosed Circular, 76-01 is forwarded to you for information and action. This is the first issue of an expanded system for communication from the Office of Inspection and Enforcement to applicants and licensees, to supplement the issuance of IE Bulletins. Bulletins have been, and will continue to be, limited to subjects considered to be of appropriate significance to require prompt response. Circulars will cover subjects of lesser significance or immediacy for which a longer response time appears appropriate. Circular 76-01 contains the same subject matter as Bulletin 76-07, which is being issued concurrently. If you are also the holder of a NRC Operating License, you will also receive a copy of Bulletin 76-07 which will require a separate response. The only difference between the two documents is the time allowed for response. Future IE Circulars may be addressed to any class of NRC licensees, and may or may not require response. Signature (Regional Director) Enclosure: IE Circular 76:01 . IE Circular 76-01 DATE: CRANE HOIST CONTROL - CIRCUIT MODIFICATIONS DESCRIPTION OF CIRCUMSTANCES: In response to NRC concerns about the potential for, and consequences of, dropping a spent fuel shipping cask or other heavy load, Commonwealth Edison modified the hoist control system for the fuel cask handling cranes at their Dresden Units 2 and 3 and Quad cities Units 1 and 2 to provide additional hoist redundancy and slow speed hoist capability. The original design utilized a General Electric "magspeed" hoist control system. In this system which includes two electro-mechanical brakes in series, spring force holds the brakes engaged while DC solenoids, energized when the hoist motor is energized, disengage the brakes. The modification which added the slow speed hoist capability included installing additional contactors in the brake solenoid power circuit to energize the solenoids when the low speed hoist motor was energized. The original hoist control system design utilized a single Size 2 DC contactor (two contacts in series) in the solenoid circuit. The design modification added a circuit in parallel with the original DC contactor with utilized four AC rated Size 1 single contacts in a series-parallel array to distribute current carrying and interrupting burden. Initial experience with the modified hoist control system at Dresden showed that the circuit interrupting capacity of the series-parallel array was marginal. On several occasions when the low speed motor was stopped in the lowering mode, the solenoid circuit contacts arced resulting in power being supplied to the solenoids long enough so that the load dropped some distance before the brakes engaged. Over travel of as much as 15 inches was reported, but no damage to hoist or load was found. The crane manufacturer's representatives have advised the NRC that the proposed corrective action is to install a single Size 2 DC contactor (two contacts in series) with arc suppressors, the same as originally provided in the General Electric design, in place of the added four AC rated contacts. The original contactor in the normal speed control circuit has shown satisfactory service since initial operation of the plant in 1969. . IE Circular 76-01 -2- DATE: ACTION TO BE TAKEN BY LICENSEE: 1. Determine and report to this office within 90 days the following information: (a) Have you made, or do you plan to make modifications to the hoist control for your installed cranes similar to the described modifications? (b) If such modifications have been made, or are planned, identify changes required in brake power and control circuitry? (c) What steps have been taken or are planned, to provide assurance that brake power contactors are adequate for the service? 2. If modifications are planned, provide the schedule for completion and a brief description of your plans for design review and functional testing. Your response should be submitted to the Director of this Office, with a copy to the Director, Division of Reactor Inspection Programs, Office of Inspection and Enforcement, U. S. Nuclear Regulatory Commission, Washington, D. C. 20555. Approval of NRC requirements for reports concerning possible generic problems has been obtained under 44 U. S. C. 3152 from the U. S. General Accounting Office. (GAO Approval B-180255 (R0072), expires 7/31/77). Enclosure: Licensee Event Report dated 6/10/76 .EVENT DESCRIPTION Malfunction of the drive brakes on the Unit 2/3 reactor building crane special slow-speed hoist caused the Unit 2 reactor vessel head to slip vertically about 15 inches as it was being lowered into place. An attempt to continue lowering resulted in a second 15 inch drop, and the slow-speed drive was taken out of service. Operations were resumed using the normal-speed drive. Since the special slow-speed hoist drive is a new piece of equipment, no data on past performance exists. (50-237/1976-32) CAUSE DESCRIPTION Investigation revealed that the slow-speed hoist brakes, which were designed to engage upon loss of power to a release solenoid, were fully engaging only after a delay of 1-2 seconds. It was further discovered that the contactor for the break release solenoid was undersized, which permitted arcing when the contacts opened. This arcing caused the release solenoid to remain partially energized, and prevented the immediate full engagement of the hoist brakes in the slow-speed mode. The hoist brakes, which are common to both the normal-speed and slow- speed drives, are controlled by two similar circuits, one in each drive system. Each circuit culminates in a single contactor. Since the slow-speed drive had been selected, only the slow-speed circuit contactor was in operation at the time of failure. The normal-speed circuit contactor has proven its reliability; however, representatives of the crane vendor (Whiting Crane Corp.) have been consulted and have agreed that the slow-speed circuit contactor is indeed undersized for its duty requirements. The entire slow-speed hoist drive design is being reviewed for other improper component applications. The slow-speed drive will remain out of service until appropriate modifications have been completed. .
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