TOKYO, July 21 (Reuters) - Japan's health ministry said it would investigate reports that workers at the stricken Fukushima nuclear power plant were urged by a subcontractor to place lead around radiation detection devices in order to stay under a safety threshold for exposure.

The Asahi Shimbun newspaper reported on Saturday that an executive from Build-Up, a subcontrator to plant owner Tokyo Electric Power, told workers to cover the devices called dosimeters when working in high-radiation areas.

Dosimeters can be worn as badges or carried as devices around the size of a smartphone to detect radiation.

Nine workers wore the lead plates around the devices once after the executive's plea, Public broadcaster NHK said, citing the subcontractor's president.

Japanese law has set an annual radiation exposure safety threshold of 50 millisieverts for nuclear plant workers during normal operations.

But a massive earthquake and tsunami that crippled the Fukushima plant in March 2011 led to a breach of containment structures that released radiation, keeping large areas around the plant off limits more than a year later.

A Tokyo Electric Power spokesman told Reuters on Saturday the company was aware from a separate contractor that Build-Up made the lead shields, but that they were never used at the nuclear plant.

Build-Up could not be reached for comment.

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  • Greenpeace activists in protective suits cross a road during an anti-nuclear protest commemorating last year's earthquake and tsunami in Japan that set off Fukushima nuclear crisis, in Jakarta, Indonesia, Monday, March 5, 2012. (AP)

  • Greenpeace activists in protective suits hold umbrellas during an anti-nuclear protest commemorating last year's earthquake and tsunami in Japan that set off Fukushima nuclear crisis, at the office of Indonesian Ministry of Research and Technology in Jakarta, Indonesia, Monday, March 5, 2012. (AP)

  • Two men take a break at the smoking area in an evacuation center built in an empty high school in Kazo, Saitama Prefecture, for the people who fled Futaba, the town nearest to the tsunami-crippled Fukushima Dai-ichi nuclear plant, Tuesday, March 6, 2012. About 570 people, who had no choice but to abandon their houses, have lived in the high school for nearly one year after the plant began spewing radiation. (AP)

  • Masaharu Yamada, 58, hangs his laundry at an evacuation center built in an empty high school in Kazo, Saitama Prefecture, for the people who fled Futaba, the town nearest to the tsunami-crippled Fukushima Dai-ichi nuclear plant, Tuesday, March 6, 2012. (AP)

  • Women push carts of bottled water at an evacuation center built in an empty high school in Kazo, Saitama Prefecture, for the people who fled Futaba, the town nearest to the tsunami-crippled Fukushima Dai-ichi nuclear plant, Tuesday, March 6, 2012. (AP)

  • A woman fills a thermos flask with hot water at an evacuation center built in an empty high school in Kazo, Saitama Prefecture, for the people who fled Futaba, the town nearest to the tsunami-crippled Fukushima Dai-ichi nuclear plant, Tuesday, March 6, 2012. (AP)

  • Satomi Kuroki, a 13-year-old junior high school student, receives a whole-body counter radiation check as doctor Tatsuo Hanai looks on at Minimisoma City General Hospital in Minamisoma, just outside the 20-kilometer (12.5 miles) evacuation zone around the tsunami-crippled Fukushima Dai-ichi nuclear plant, Fukushima Prefecture, Japan, Wednesday, March 7, 2012. (AP)

  • Factory worker Joji Kumagaya tests a Geiger counter in Otama Village, Fukushima Prefecture, Japan, Wednesday, March 7, 2012. The factory produces reasonable Geiger counters named "Geiger Fukushima" for people in Fukushima nearly one year after a tsunami-hit nuclear power plant began spewing radiation. Japan marks one year since the disaster on March 11. (AP)

  • Municipal worker Masahiko Watanabe checks his PC monitor as he conducts a test for radiation levels on vegetables in Otama Village, Fukushima prefecture, Japan, March. 7, 2012. Radiation is still leaking from the now-closed Fukushima Dai-ichi nuclear plant, though at a slower pace than it did in the weeks after the March 11 earthquake and tsunami. Some experts say the risks are quite low outside the 20-kilometer (12-mile) no-go zone, and people can take steps to protect themselves, such as limiting intake of locally grown food, not lingering in radiation "hot spots" such as around gutters and foliage, and periodically living outside the area. (AP)