Note: This story was updated at 12 p.m. July 19.
Four workers received small external radiation doses and a planned outage of the Spallation Neutron Source was started a few days early after an unexpected “pressure transient” in March, according to Oak Ridge National Laboratory.
The low radiation dose levels (less than 25 millirem) were well below the regulatory threshold of 5,000 mrem annual exposure, which was established to protect worker safety, ORNL said in June.
The pressure transient occurred in the SNS mercury loop. When radiation was detected, ORNL staff closed off the affected area and reviewed workers’ dosimeters.
“Readings showed four workers received small external doses, none more than 25 millirem,” ORNL said.
For comparison, a chest x-ray produces a radiation dose of about 6 mrem; a mammogram about 13 mrem, and a head and chest CT scan is 1,100 mrem.
A leak at a flange in the mercury loop was at least partially to blame for the pressure transient at 7:26 a.m. Thursday, March 21, during a normal top-off fill of the mercury loop, according to a report available online.
The pressure transient forced radioactive material/mercury into helium lines contained within unshielded conduit in the experiment hall area, the report said. The unshielded radioactive material in the helium lines caused elevated radiation levels in part of the North Experiment Hall, affecting some experimental areas. Elevated radiation levels persisted for more than nine hours before being identified by a radiological control technician, the report said.
“Upon discovery, affected areas were cleared of personnel, and appropriate radiological area boundaries and postings were put in place,” the report said. “Line management was promptly notified. Dosimetry was collected from personnel in the affected area and promptly analyzed. A maximum whole-boy dose of 24 millirem (mrem) was recorded for an experimenter working in the affected area. Two other experimenters received a whole-body dose of approximately 20 mrem, and another experimenter received a shallow dose equivalent of 19 mrem. Dosimetry was collected the next day from 12 other individuals who had been in the affected area of the building that day, and no other whole-body dose was recorded.”
ORNL said the staff members are qualified as radiological workers according to U.S. Department of Energy criteria. That means that, based on their job assignments, they are considered likely to receive more than 100 mrem per year under typical working conditions, ORNL said.
“The average external dose for ORNL radiological workers who receive positive results on their dosimeters is about 70 mrem per year,” ORNL said.
A scheduled outage at SNS was moved up to March 28 from April 1 because of the mercury loop event. There are typically three planned outages a year at SNS during normal operations. ORNL said one outage had been planned from April 1 to May 8 to replace the mercury target and perform routine maintenance on the radio frequency quadrupole, which is one of the first accelerating elements that provides power to the accelerator beam.
SNS resumed normal operations at 1.4 megawatts on Sunday, June 23.
The occurrence report available online said the cause of the pressure transient was the injection of pressurized helium into the mercury loop.
“This outcome was possible due to an unrecognized low mercury level in the storage tank,” the report said. “This was due to a leak at a flange in the mercury loop. In addition, the installed storage tank mercury level device had been inoperable since before initial SNS operation. An ancillary sensor, the collection basin leak detector, is designed to identify the presence of mercury or water in the collection basin. The leak sensor did not perform as expected and only indicated the presence of water in the collection basin…Until this event, a significant leak out of the mercury loop had not occurred in the operating history of the SNS. Note that mercury leaking out of the system is contained in a collection basin per the system design.”
ORNL said the SNS target is part of the mercury loop system, and it is contained inside the target service bay. The target service bay is designed to collect any mercury that leaves the system. When the system is “topped off,” mercury is added back into the system so no mercury is lost, ORNL said.
The storage tank mercury levels are measured using alternative methods: Instead of using a single probe to measure the mercury tank directly, multiple indications are used to indirectly compute the volume of mercury in the storage tank, ORNL said.
 National Council on Radiation Protection and Measurements. Sources and magnitude of occupational and public exposures from nuclear medicine procedures. Bethesda, MD: National Council on Radiation Protection and Measurements; NCRP Report 124; 1996.
More information will be added as it becomes available.
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