A SUBCONTRACTOR at Japan's crippled Fukushima nuclear plant told workers to lie about possible high radiation exposure in an apparent effort to keep its contract, according to media reports in the country yesterday.

The Asahi Shimbun newspaper along with other outlets ran stories detailing how an executive from Build-Up, a subcontractor to plant owner Tokyo Electric Power (Tepco), told workers to cover the devices, called dosimeters, when working in high-radiation areas.

Dosimeters can be worn as badges or carried to detect radiation.

Nine workers wore the lead plates around the devices on at least one occasion after the executive's plea, according to Japan's public broadcaster NHK, which cited 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 9.0-magnitude 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.

Several workers at Build-Up told the Asahi that a senior official from the firm who served as their on-site supervisor said in December that he used a lead casing and urged them to do the same.

Without faking the exposure level, the executive told the workers they would quickly reach the legally permissible annual exposure limit, according to the Asahi.

The workers had a recording of their meeting, the newspaper said.

"Unless we hide it with lead, exposure will max out and we cannot work," the executive was heard saying in the recording. Some workers refused to wear it and left the company.

The workers were hired for about four months to March to insulate pipes at a water treatment facility.

A Tepco spokesman said yesterday the company was aware from a separate contractor that Build-Up made the lead shields, but that they were never used at the plant.

Build-Up was not available for comment and Japan's health ministry said it would investigate.

In a scathing report this month, an investigative panel appointed by parliament concluded that the Fukushima disaster was preventable and resulted from "collusion" among Tepco, its regulators and the government.

The latest allegations come just days after the new head of Tepco said he was baffled by fierce criticism of the firm where he has worked nearly 40 years, and hoped to rebuild public trust.

Naomi Hirose, 59, has taken over Tepco's key post at a time when anti-nuclear protests are growing, ruling-party lawmakers are bolting over a decision to restart idled reactors despite safety fears and the government is nationalising the struggling utility.

"You can destroy trust in a single day but to rebuild it takes time," Hirose told a news conference on Thursday.

Japan's nuclear issue has taken centre-stage in political debate ahead of parliamentary elections.

Last week, 100,000 anti-nuclear protesters took to the streets in Tokyo, adding to pressure on the government which is already under fire over its handling of nuclear policy issues.