Bulletin 76-07: Crane Hoist Control - Circuit Modifications

IE Bulletin 76-07                      DATE: July 27, 1976 

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 Bulletin 76-07                    -2-                DATE:  July 27, 1976 

ACTION TO BE TAKEN BY LICENSEE: 

1.   Determine and report to this office within 20 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