United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 88-63, Supplement 1: High Radiation Hazards from Irradiated Incore Detectors and Cables

                                UNITED STATES
                           WASHINGTON, D.C. 20555

                               October 5, 1990

                                             IRRADIATED INCORE DETECTORS 
                                             AND CABLES 


All holders of operating licenses or construction permits for nuclear power 
reactors, research reactors, and test reactors. 


The NRC issued Information Notice (IN) 88-63 on August 15, 1988, to alert 
addressees to the uncontrolled radiation exposures experienced at Surry Unit 
2, as well as other facilities, because the radiation hazards present during 
work involving irradiated incore detectors and drive cables were not 
adequately evaluated.  This supplement alerts addressees to two similar 
events that have occurred since IN 88-63 was issued.  It is expected that 
recipients will review the information for applicability to their facilities 
and consider actions, as appropriate, to avoid similar problems.  However, 
suggestions contained in this information notice do not constitute NRC 
requirements; therefore, no specific action or written response is required. 

Description of Circumstances: 

Since IN 88-63 was issued, two nuclear facilities have experienced events 
during which employees received unplanned radiation exposures while 
performing work near irradiated incore detectors and their associated drive 

Duane Arnold

On November 11, 1989, instrumentation and control technicians (I&CTs) at 
Duane Arnold manually extracted two intermediate range monitors (B-IRM and 
F-IRM) that had been stuck in the core since October 23 and 24, 
respectively.  The plant was shut down at 5:06 a.m. on November 9, 1989, in 
part, so that this task could be performed.  Monitors B-IRM and F-IRM were 
located approximately 30 and 93 inches, respectively, into the active core.  
The work was to be completed during two drywell entries.  Each IRM was 
manually pulled by its cable until the detector (which was known to be 
highly activated) was positioned in a specially fabricated lead shield 
(pig).  Once in the pig the detector was transferred to the fuel pool for 

The as low as reasonably achievable (ALARA) checklist for this work included 
health physics hold points for handling the shielded IRM detector at 10 R/hr 
at 18 inches.  The checklist noted that the contact dose rate for the 


                                                  IN 88-63, Supplement 1 
                                                  October 5, 1990 
                                                  Page 2 of 4 

B-IRM detector was expected to be 120 R/hr.  The potential radiation hazards 
from the irradiated IRM cables apparently were not considered during the 
pre-job planning or briefings. 

While the B-IRM detector and cable were being pulled, area dose rates 
increased significantly (I&CTs recollected that they were greater than 100 
R/hr) as the detector approached the pig (6 to 8 feet).  The health physics 
technician (HPT) covering this job, believed the increase was due solely to 
the proximity of the activated detector.  The B-IRM detector lodged near the 
exit opening of the pig, resulting in a contact dose rate of 450 R/hr.  The 
IRM cable was cut away from the pig, coiled, and laid aside before the work 
crew left the containment. 

During the review of lessons learned from the B-IRM extraction and the 
pre-job planning for the F-IRM extraction, it appears that the focus was on 
redesigning the lead pig and on the need to place the detector in the pig in 
such a way that the high dose rates previously encountered would be avoided.  
Even though the licensee knew that the dose rate for the shielded B-IRM 
detector was significantly in excess of the estimates in the ALARA checklist 
and that cable activation products resulted in an airborne contamination 
problem, it did not investigate the source of this unexpected radioactivity 
before starting work on the F-IRM.  

At the start of the second drywell entry to extract the F-IRM, an HP helper 
bagged the B-IRM cable, as instructed, and moved it out of the immediate 
work area.  The HP helper remained near the bagged cable, waiting for 
further instructions.  Approximately 2 minutes later an HPT arrived, opened 
the bag, and took a swipe from the cable without conducting a radiation 
survey.  Since a nearby recirculation pump riser was a known source of high 
radiation, the HPT directed the helper to wait in an area where the 
background radiation level was lower.  Approximately 2 minutes later a 
second HPT who had a radiation survey meter entered the area.  Although he 
was not conducting a formal survey, he noted that the radiation background 
level was significantly higher than expected.  As he approached the bagged 
B-IRM cable, the survey meter went off the highest scale at 50 R/hr.  A 
subsequent contact reading on the surface of the bag was 125 R/hr.  An 
investigation by the licensee showed that the dose received by the HP helper 
exceeded the licensee's weekly administrative whole-body dose limit of 300 

After the high dose rates from the B-IRM cable were discovered, work was 
halted on the F-IRM while dose extensions and alarming dosimeters were 
obtained for everyone on the work crew.  Despite the unexpected high dose 
rates found on the B-IRM cable, the licensee apparently did not reevaluate 
the radiological hazards present before allowing re-entry to extract the 
F-IRM, even though the F-IRM was stuck further into the core and was likely 
to have higher dose rates than the B-IRM.  Without performing a radiation 
survey of the cable, an I&CT was allowed to pull the F-IRM cable by hand, 
cut it into segments, and toss the segments into a plastic bag.  The maximum 
contact dose rate on the F-IRM cable segments was later measured at 1000 

For more details concerning this event see NRC Inspection Report 
50-331/90-05, which was issued on March 27, 1990.


                                                  IN 88-63, Supplement 1 
                                                  October 5, 1990 
                                                  Page 3 of 4 


On July 5, 1990, two I&CTs (one vendor and one plant) received unplanned 
radiation exposures while installing new traversing incore probes (TIPs) at 
Brunswick Steam Electric Plant, Unit 1.  This task consisted of installing 
new TIPs into their drive mechanisms and manually cranking them to the end 
of the reactor core guide tubes to calibrate their position indications.  
TIPs A, B, and C were installed without incident.  However, while driving 
the D TIP into the core, the technicians experienced difficulty with the 
clutch mechanism on the drive takeup reel.  

The I&CTs were able to insert the D TIP to the top of the longest guide 
tube, where the detector remained for 3 to 5 minutes while the position 
indication was calibrated.  When this calibration was completed the 
technicians were to retract the TIP to the guide tube indexer and drive it 
into another tube.  At this point the vendor I&CT was operating the drive 
mechanism hand crank and the plant I&CT was positioned to monitor the local 
TIP position indicator which could not be read from the crank.  While the 
TIP was being retracted, the plant I&CT's attention was diverted from the 
position indicator to the troubled clutch with which the technician was 
having difficulty. The vendor I&CT continued to crank the TIP out of the 
core until it entered the drive mechanism housing with a loud sound and set 
off local radiation area monitors.  The vendor I&CT instinctively reacted to 
remove the detector from his work area.  He grabbed the TIP cable (about 7 
inches from the detector) with his left hand, reinserted it into the guide 
tube, and cranked it back to the indexer.  The vendor I&CT had not been 
instructed that it was possible to completely retract the TIP nor what dose 
rates could be expected. 

For more details concerning this event, see NRC Inspection Reports 
50-325/90-25 and 50-324/90-25, which were issued on July 26, 1990. 


During the Duane Arnold event, the I&CT who handled the F-IRM cable and the 
HP helper who handled the B-IRM cable received whole-body doses of 390 mrem 
and 310 mrem respectively.  The maximum extremity dose was less than 4 rem.  
During the Brunswick event, the vendor I&CT received a whole body dose of 
377 mrem, and an extremity dose of just less than 7 rem; the plant I&CT 
received a whole-body dose of 239 mrem.  Although all of these doses were 
well within the regulatory limits, the low doses were fortuitous considering 
the magnitude of the dose rates associated with these irradiated components.  
If the timing of the events in either case had been delayed by even a small 
margin, much more serious doses would have resulted. 

The root causes for these types of exposure events, identified in IN 88-63, 
include the failure to adequately evaluate the radiological hazard, the use 
of inadequate procedures, and the lack of communication among the work 
groups involved.  These same deficiencies were noted during the Duane Arnold 
and Brunswick events.  The inadequate radiological hazard evaluation at 
Duane Arnold was due to a previous experience with low dose rates during an 
IRM extraction.  


                                                  IN 88-63, Supplement 1 
                                                  October 5, 1990 
                                                  Page 4 of 4 

The Duane Arnold staff failed to properly account for the short half-life of 
the major isotope involved and the decay time that the previous task 
provided.  This factor is identical to the major causal factor identified 
for the Surry event in IN 88-63.

During the enforcement deliberations for both of these events, the NRC staff 
noted that neither licensee had adequately reviewed the information in IN 
88-63.  Although the HP and ALARA personnel associated with the IRM 
extraction at Duane Arnold had read the IN, none of them recognized its 
applicability to incore cabling other than the TIP.  The NRC inspector at 
Brunswick noted that a memorandum from the licensee's corporate health 
physicist indicated that IN 88-63 would be reviewed and included in the 
continuing training for HPTs and that the IN would be provided to I&C 
foremen for their staffs' review.  Through interviews, the inspector 
determined that the licensee had not included IN 88-63 in the HP technician 
training nor had it been provided to I&C foremen.  Although information 
notices do not require specific actions by licensees, licensees are expected 
to give them an adequate review.  In both cases discussed in this 
supplement, the licensees failed to take advantage of the information 
provided to prevent similar exposures at their facilities. 

This information notice requires no specific action or written response.  If 
you have any questions about the information in this notice, please contact 
the technical contact listed below or the appropriate NRR project manager. 

                              Charles E. Rossi, Director 
                              Division of Operational Events Assessment 
                              Office of Nuclear Reactor Regulation 

Technical Contact:  Roger L. Pedersen, NRR 
                    (301) 492-3162 

Page Last Reviewed/Updated Friday, May 22, 2015