Workers may have been exposed to radioactivity at the Bruce nuclear power station on the eastern shore of Lake Huron in late November, during work on a reactor that was shut down in 1995, according to a report posted to the company's website.

The document says a routine test of air quality on Nov. 26 detected alpha contamination at the Bruce A Unit 1 reactor. According to an information update on the incident posted to the Canadian Nuclear Safety Commission's website, alpha contamination can be hazardous if inhaled or ingested, "which may have been the case here."

According to the CNSC, preliminary tests have concluded that workers at the Ontario plant were likely not exposed to dangerous levels of radiation.

"The contamination in question was fully contained within Unit 1 and there is no risk to the public or the environment," the CNSC said.

According to a regulatory filing from the CNSC obtained by the Globe and Mail, as many as 217 workers may have been affected.

Bruce Power's incident report said all affected individuals have been placed "on removal from further radiation work until additional information regarding potential exposure is obtained." Access to the Unit 1 vault was restricted to clean up.

"A root cause investigation into the event is underway to identify the magnitude and cause of this event and to identify actions to prevent a recurrence," the report said.

Steve Cannon, a spokesperson for Bruce Power, told The Canadian Press that preliminary tests on workers believed to have faced the highest exposure found levels "not even approaching the regulatory limit."

According to Cannon, the workers were inside a reactor vault that had been empty for 15 years, and officials did not expect to find alpha contamination during the air quality tests. Alpha radiation is often linked to defective cladding on reactor fuel bundles, Cannon said.

The CNSC will discuss the incident at a board meeting on Thursday.

Bruce Power is Canada's first private nuclear generating company that provides Ontario with about 20 per cent of its electricity. The company is currently refurbishing two Bruce A reactors, Unit 1 and Unit 2. In addition to improvements to the Unit 3 and Unit 4 reactors, the total cost of the refurbishment now stands at a projected $5.25 billion.