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Seconds after the three workers started going into the basement, the dosimeters began ringing loudly and then went silent, a sign the intended limit was exceeded, though the team’s leader said it must be an error. The three workers in the basement waded through the ankle-deep water to check the wall-mounted switchboard and came back up, saying the water felt warm through their rubber boots.

Another team sent in to do other tasks rushed back out without doing any work, ignoring Shinichi’s team, after measuring dangerously high radioactivity in the basement.

But his group stayed, making several more trips into the flooded basement. Two workers wearing short boots got their feet soaked and suffered beta-ray burns which were not life threatening. The three men who stayed there the longest were exposed to about 180 millisieverts — nearly four times the annual safe limit, according to a government report released in July. Shinichi refused to help tie up the dangling cable in the basement because of his short boots, and a colleague wearing long boots volunteered to do it instead, saving Shinichi from injury.

TEPCO spokeswoman Mayumi Yoshida said the team leaders later told officials that they decided to stay because they took their mission very seriously and that they might have been too occupied to think carefully about the water. But TEPCO should have thought more carefully given the unpredictable plant conditions, she said.

Shinichi’s radiation exposure from 13 days of working at the plant was just over 20 millisieverts, not considered a serious health risk, though he still worries.

His lawyers, who are representing several nuclear plant workers in other cases, say TEPCO and its top contractor Kandenko illegally sent him and five other men into areas with radioactivity far exceeding the allowable limit without full protection.

‘‘Just sending the workers into the harsh environment and putting them at risk of exposure to dangerously high radiation is a labor safety violation,’’ said Taku Yamazoe, a lawyer representing Shinichi. ‘‘Even if TEPCO didn’t anticipate the consequences of all that water it had pumped in, it clearly lacked consideration for the workers’ safety.’’

Shinichi’s experience was typical of the inadequate protection received by workers laboring in the extremely harsh conditions at the plant, though Yamazoe said the multi-tiered subcontracting system used at nuclear plants can obscure who is directly responsible in case of an accident.

Investigations by the government, parliament and private groups have faulted TEPCO for inept crisis management, inadequate emergency training and miscommunication with authorities.

The parliamentary investigation took TEPCO to task for failing to deal with leaking contaminated water until the two workers suffered beta-ray burns in Unit 3, concluding that the operator was fully aware of the consequences of massive spraying and pumping of water into the reactors and spent fuel pools from the very beginning.

Shinichi said that when he finished work at the nuclear plant each day, he would take off his clothes before entering his home to minimize the risk of radiation exposure for his 5-year-old son. He would toss the clothes into the washing machine and immediately rush into a bath.

Many other nuclear workers face the same worries, he said.

‘‘I don’t have education, and I'm already over 40. There is little choice,’’ he said. ‘‘I was dumped. I worked hard, sacrificed my family and my child and this is how I ended up.’’