In April 1997, a diver entered the spent fuel pool and refueling cavity at Calvert Cliffs Unit 2 in Maryland to inspect and repair malfunctioning fuel transfer equipment.
The fuel transfer equipment was located in the south end of the spent fuel pool. During the fourth dive, the diver left the previously surveyed and approved work location at the south end of the spent fuel pool, and swam into an unapproved, unsurveyed area in the north end of the pool where freshly discharged irradiated fuel assemblies were stored. In doing so, the diver entered areas of significantly higher radiation fields, where he received an unplanned radiation exposure. It was estimated that the diver’s right knuckles may have entered radiation fields of 155 to 310 rem per hour and his right arm may have entered radiation fields ranging from 45 to 90 rem per hour. Federal limits restrict radiation exposure for workers to 5 rem per year.
Elaborate controls had been provided so that the diver would not enter high radiation fields. The diver was equipped with a tether. However, the individual on the other end of the tether did not question the excessive amount of rope he let out as the diver traversed the nearly 40 feet to the north end of the pool. During his prior three dives, the diver was continuously monitored by a television camera. That protection was abandoned for the fourth dive. A radiation protection technician was stationed on the refueling floor to visually monitor the diver’s progress. However, the technician became ‘distracted’ from this sole task and did not observe the diver’s trip to the north end of the spent fuel pool.
Prior to entering the pool, the diver had been briefed by radiation protection personnel. He was shown a map with radiation levels around the fuel transfer equipment in the south end of the pool. There were no radiation readings shown for the north end of the pool. The diver took this to indicate that the north end of the pool was safe. In reality, the readings in that end of the pool had not been shown because the radiation protection personnel only knew about the work to be done in the south end. They did not know the diver also had work to do in the north end, so they had not bothered to record the radiation readings in that end of the pool.
After the diver emerged from the pool, there was a discussion on the refueling floor about his unplanned radiation exposure. Before his dosimetry could be processed to determine the extent of his exposure and reasons for the multiple breakdowns identified and corrected, the diver was sent into the pool fifth time.
Although detailed dose assessments for the diver indicated that no apparent radiation exposure in excess of NRC limits had occurred, the NRC levied a fine of $176,000 on Calvert Cliffs’ owner for the breakdown in radiological controls.
Fission Stories #18 talked about a radiation overexposure event involving a diver at Indian Point fifteen years prior to this event. Both events involved failures of multiple barriers erected to protect workers from excessive radiation exposures.
Multiple barriers can be a boost or boon to safety. When multiple barriers are independent and highly reliable, safety is enhanced. When deficient barriers are tolerated because other barriers are available, safety is eroded. Multiple unreliable barriers may be better than a single unreliable barrier, multiple reliable barriers are best.
“Fission Stories” is a weekly feature by Dave Lochbaum. For more information on nuclear power safety, see the nuclear safety section of UCS’s website and our interactive map, the Nuclear Power Information Tracker.