Information Notice No. 84-77: Incident Involving Teletherapy Unit (AECL Eldorado-78)

                                                           SSINS No. 6835  
                                                           IN 84-77        

                               UNITED STATES 
                            WASHINGTON, DC 20555 

                              October 26, 1984 



All teletherapy licensees authorized to possess AECL cobalt-60 teletherapy 


This information notice is issued to advise licensees of a component failure
mode that could result in the Co-60 source being exposed without operator 
action as a result of a circuitry defect. It is expected that recipients 
will review their facilities and consider actions, if appropriate, to 
minimize similar problems occurring at their facilities. However, 
suggestions contained in this information notice do not constitute NRC 
requirements; therefore, no specific action or written response is required.

Description of Circumstances: 

A recent incident involving a defective solid-state relay in a circuit board
of an AECL Model Eldorado-78 (9000 curies Co-60) teletherapy unit caused the
source unit to move to the exposed position. 

NRC received notification from one of our licensees regarding this incident 
on July 3, 1984. Apparently, two technologists responsible for the unit were
waiting to treat the next patient. The treatment room door was closed and 
the technologist noticed the source indicator lights were on and the fixed 
monitor, located inside the room, indicated an exposed source configuration.
They tried to return the source to the safe-shielded configuration by 
recycling the unit, but were unsuccessful. The hospital biomedical engineer 
was notified and diagnosed a faulty source driver circuit board with a 
shorted solid-state relay and replaced the entire board with a spare. The 
teletherapy unit was then tested and found to function properly. An AECL 
representative was dispatched from their Philadelphia office on July 5, 
1984, to certify that the unit was in proper condition for the resumption of
patient treatment. An NRC inspector spoke with the AECL representative who 
believed that this was a rare occurrence with these types of units. 

This incident may have generic implications because of the non fail-safe 
design of the relay circuit. Licensees may consult with AECL for their 
specific units to determine whether their units are subject to the same 
failure mode. 

                                                           IN 84-77        
                                                           October 26, 1984 
                                                           Page 2 of 2     

No specific action on written response is required by this information 
notice. If you should have any questions about this matter, please contact 
the Regional Administrator of the appropriate regional office or the 
technical contact in this office. 

                              J. Nelson Grace, Director 
                              Division of Quality Assurance, Safeguards, 
                                and Inspection Programs 
                              Office of Inspection and Enforcement 

Technical Contact:  J. Metzger
                    (301) 492-4947 

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