Work paused at Three Mile Island

Monday, 23 November 2009

Refuelling and steam generator replacement was disturbed on 21 November by the sounding of an airborne radiation alarm at Three Mile Island 1.

 

The containment building of Three Mile Island 1, showing  a temporary opening for steam generator replacemen

About 150 workers went home early and checked for contamination after one of three detectors registered heightened levels of radioactivity. Plant owner Exelon said the levels remained below regulatory limits, that one worker was exposed to 40 mrem (0.40 mSv), less than 2% of Exelon's self-imposed the annual occupational dose limit of 2000 millirem (20 mSv). The US Nuclear Regulatory Commission insists on nuclear licensees ensuring an average annual radiation dose of below 5000 millirem (50 mSv) for nuclear industry workers, and this level has not been exceeded since 1989.

"To put the levels in perspective," the NRC said, "the average American receives about 300 millirem (3 mSv) of radiation exposure each year from natural sources."

 

The alarm was one placed near a temporary opening in the containment building, made to allow the large steam generators to be moved inside, but a radiological survey showed the contamination was confined to internal surfaces only. The NRC is working with Exelon to determine the exact source of the radiation, thought to be tiny particles dislodged from pipework under maintenance by a change in air pressure.