Tokyo ( CNN ) -- Japan 's Ministry of Health , Labor and Welfare is investigating a report that workers at the damaged Fukushima Daiichi nuclear power plant were told to use lead covers in order to hide unsafe radiation levels , an official said . The alleged incident happened December 1 , nine months after a major earthquake and tsunami ravaged northern Japan and damaged the plant . `` We 'll firmly deal with the matter once the practice is confirmed to constitute a violation of any law , '' said the ministry official , who could not be named in line with policy . An official with the plant 's operator , TEPCO , said the company received a report of the alleged incident Thursday from subcontractor Tokyo Energy & Systems . The report said a second subcontractor , Build-Up , created the lead covers and ordered workers to use them over their dosimeters , pocket-size devices used to detect high radiation levels . The TEPCO official could also not be named in line with policy . Tokyo Energy & Systems said in its report that the workers never used the covers , the TEPCO official said . Japan 's Asahi Shimbun newspaper , however , reported Saturday that while some workers refused the orders to use the lead covers , nine others did use them for several hours . The newspaper 's report cited plant workers , who described the lead covers as fitting snugly over the dosimeters inside the breast pockets of the workers ' protection suits . TEPCO told CNN it ordered Tokyo Energy & Systems Inc. to conduct an investigation and is awaiting a reply . CNN 's Junko Ogura contributed to this report .
The incident allegedly happened at the damaged Fukushima nuclear plant . The government and plant are investigating the report . Japan : Workers were asked to covers be used to monitor radiation levels .