United States Nuclear Regulatory Commission - Protecting People and the Environment

IE Bulletin 76-07 and IE Circular 76-01 - Crane Hoist Control - Circuit Modifications


                                JUL 27 1976 

                          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 


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 

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.) 


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. 

                                        (Regional Director) 

IE Circular 76:01

IE Circular 76-01                                     DATE: 



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: 


1.   Determine and report to this office within 90 days the following 

     (a)  Have you made, or do you plan to make modifications to the hoist 
          control for your installed cranes similar to the described 

     (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 

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). 

Licensee Event Report dated 6/10/76

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) 


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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