Around 2,000 people who have worked at Japan's wrecked Fukushima nuclear plant face a heightened risk of thyroid cancer, its operator said Friday.

Tokyo Electric Power (TEPCO) said 1,973 people -- around 10 percent of those employed in emergency crews involved in the clean-up since the meltdowns -- were believed to have been exposed to enough radiation to cause potential problems.

The figure is a 10-fold increase on TEPCO's previous estimate of the number of possible thyroid cancer victims and comes after the utility was told its figures were too conservative.

Each worker in this group was exposed to at least 100 millisieverts of radiation, projections show.

Although little is known about the exact health effects of radiation on the human body, the level is considered by doctors to be a possible threshold for increased cancer risk.

The Fukushima Daiichi nuclear plant became the site of the worst nuclear disaster in a generation after the massive tsunami of March 2011 destroyed its cooling systems.

The plant's reactors went through meltdowns that caused explosions in the buildings housing them, spewing radioactive materials into the air, sea and soil.

Tens of thousands of people were forced from their homes in a large area around the plant, where crews continue to clear debris and cool the reactors.

The fragility of the wrecked plant was brought into sharp relief again Thursday with the discovery of steam in the roofless building around Reactor 3.

TEPCO said Friday it still did not know exactly where the steam was coming from, although readings showed it was no more radioactive than expected and suggested it could have been accumulated rainwater.

The huge utility, which has faced frequent criticism for downplaying dangers and not being forthcoming about problems at the site, revised its method of estimating the level of radiation exposure among workers earlier this month.

TEPCO reported to the World Health Organization in December that only 178 workers at the plant were believed to have received radiation doses to their thyroid glands above 100 millisieverts.

Japan's health ministry voiced concern that the criteria the company used in its estimates of exposure for its own workers as well as for those employed by contractors were too narrow, and called on the utility to re-evaluate its methods.

There were also errors in calculations and differences of interpretation.

Not all of the approximately 20,000 workers have actually been tested. The numbers have been arrived at by extrapolating the results of tests that have been carried out.

All 1,973 workers now deemed to be at increased risk of thyroid cancer are eligible for an annual thyroid checkup and other health services paid for by the company.

TEPCO has already informed those affected about the health and monitoring programmes.

Tens of thousands of people were forced from their homes by the threat of radiation in the immediate aftermath of the disaster, with many still unable to return.

While the natural disaster claimed more than 18,000 lives, no one is officially recorded as having died as a direct result of the radiation released by the disaster.