The operator of Japan’s crippled nuclear plant says 10 workers have been exposed to small amounts of radiation while conducting clean up activities.

Tokyo Electric Power Co. said it is still investigating how the workers were contaminated on Monday at the Fukushima Dai-ichi nuclear power plant, but that it may have been from radioactive dust.

It says small amounts of radiation were found on the workers’ faces and hair.

A powerful earthquake and tsunami destroyed power and cooling systems at the plant in March 2011, causing the release of large amounts of radiation. Makeshift equipment is being used to cool the reactors, and the plant has been plagued by blackouts and leaks of contaminated water.

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