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^[1].

[Read more…]