Obayashi, Shimizu Corp and Taisei Corp - oversee hundreds of small firms working on government-funded contracts to remove radioactive dirt and debris from nearby villages and farms so evacuees can return home.
Tokyo Electric, widely known as Tepco, says it has been unable to monitor subcontractors fully but has taken steps to limit worker abuses and curb the involvement of organised crime.
"We sign contracts with companies based on the cost needed to carry out a task," Masayuki Ono, a general manager for nuclear power at Tepco, told Reuters. "The companies then hire their own employees taking into account our contract. It's very difficult for us to go in and check their contracts."
The unprecedented Fukushima nuclear clean-up both inside and outside the plant faces a deepening shortage of workers. There are about 25 percent more openings than applicants for jobs in Fukushima prefecture, according to government data.
Raising wages could draw more workers but that has not happened, the data shows. Tepco is under pressure to post a profit in the year to March 2014 under a turnaround plan Japan's top banks recently financed with $5.9 billion in new loans and refinancing. In 2011, in the wake of the disaster, Tepco cut pay for its own workers by 20 percent.
With wages flat and workers scarce, labour brokers have stepped into the gap, recruiting people whose lives have reached a dead end or who have trouble finding a job outside the disaster zone.
The result has been a proliferation of small firms - many unregistered. Some 800 companies are active inside the Fukushima plant and hundreds more are working in the decontamination effort outside its gates, according to Tepco and documents reviewed by Reuters.
Tepco, Asia's largest listed power utility, had long enjoyed close ties to regulators and lax government oversight. That came under harsh scrutiny after a 9.0 magnitude earthquake and a massive tsunami hit the plant on March 11, 2011. The disaster triggered three reactor meltdowns, a series of explosions and a radiation leak that forced 150,000 people to flee nearby villages.
Tepco's hapless efforts since to stabilise the situation have been like someone playing "whack-a-mole", Minister of Economy, Trade and Industry Toshimitsu Motegi has said.
Hayashi is one of an estimated 50,000 workers who have been hired so far to shut down the nuclear plant and decontaminate the towns and villages nearby. Thousands more will have to follow. Some of the workers will