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Information Notice No. 88-63, Supplement 1: High Radiation Hazards from Irradiated Incore Detectors and Cables
UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF NUCLEAR REACTOR REGULATION WASHINGTON, D.C. 20555 October 5, 1990 NRC INFORMATION NOTICE 88-63, SUPPLEMENT 1: HIGH RADIATION HAZARDS FROM IRRADIATED INCORE DETECTORS AND CABLES Addressees: All holders of operating licenses or construction permits for nuclear power reactors, research reactors, and test reactors. Purpose: 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 cables. 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 storage. 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 unshielded 9010010016 . 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 mrem. 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 R/hr. 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 Brunswick 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. Discussion: 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 .
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