Officials said Sunday that authorities at Pennsylvania’s Three Mile Island nuclear plant were investigating what caused a weekend radiation leak that resulted in 150 workers being sent home.

According to a statement from Exelon Nuclear, which operates the Three Mile Island plant near Middletown, Pennsylvania, an airborne radiological contamination alarm sounded about 4 p.m. Saturday in the Unit 1 containment building. It added that the unit had been shut down since October 26 for refueling, maintenance, and steam generator replacement.

The company said, “A monitor at the temporary opening cut into the containment building wall to allow the new steam generators to be moved inside showed a slight increase in a reading and then returned to normal. Two other monitors displayed normal readings.”

Three Mile Island was the scene of the worst US nuclear accident, a partial meltdown in 1979 that resulted in the plant’s second reactor being shut down permanently.

(article and photo source: CNN)