The excessive heat may be blamed for damaging a lightning device at the Zion nuclear power plant in the far northern suburb this afternoon, according to Exelon Nuclear, the plant's owner.

A lightning arrester, a protective device for limiting surge voltages, "blew off" but there were no injuries, said Craig Nesbit, a spokesman for the energy company.

The arrester, which was possibly damaged by the heat, did not cause an explosion or a fire, he said.

Although the nuclear reactor at the site was shut down in 1998, power still flows through the plant and the incident did not affect service.

Traces of radioactive tritium were discovered in groundwater at the shuttered plant last June.

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