Tokyo Electric Power Co. said Friday that 1,973 workers at the Fukushima No. 1 nuclear plant have estimated thyroid radiation doses exceeding 100 millisieverts and are therefore at higher risk of getting thyroid cancer.

The workers will be allowed to undergo annual ultrasonic thyroid examinations free of charge, the utility said.

Tepco has given health checks to 19,592 workers — 3,290 Tepco employees and 16,302 employees of its partner firms.

However, only 522 workers had their radiation doses checked and reported to the World Health Organization. Last February, the WHO said thyroid radiation doses in 178 of them surpassed the threshold of 100 millisieverts.

For the remainder of the workers, Tepco estimated their radioactive iodine doses based on their radioactive cesium intake.

Tepco also said Friday that no steam was seen in the reactor 3 building.

Employees saw vapor through a monitoring camera Thursday. It was rising from near the central area of the top floor of the reactor building, which was severely damaged by a hydrogen explosion early in the crisis.

As of Thursday evening, the temperature readings on reactor 3 pretty much stayed the same as before the steam was found, as have radiation figures around it.

But when Tepco checked the same area at 7:55 a.m. Friday, it did not find any steam, according to the utility.

Workers can’t access the floor because the radiation level is too high and work to remove the rubble is done by remote control. Operations were suspended when the steam was observed.

Tepco suspects that rain Wednesday and Thursday may have found its way to the lid of the reactor’s primary containment vessel and evaporated due to the warmth of the container.

The Nuclear Regulation Authority said in a press release Friday that it has not detected any significant changes in either the temperature of the primary containment vessel or in radiation levels at the plant.

The NRA also said it has ordered Tepco to thoroughly investigate the incident, noting the utility’s explanation is insufficient.

“We don’t believe this could lead to a serious situation, but any abnormalities should be quickly reported to us,” said NRA senior official Hideka Morimoto.