A total of 157 US residents who were supporting Fukushima victims at the time filed the class action suit in a California district court earlier this month against the utility known as Tepco and a US company.
A massive tsunami triggered by a 9.0-magnitude earthquake smashed into Tepco's Fukushima Daiichi power plant on Japan's northeast coast on March 11, 2011.
The giant waves overwhelmed reactor cooling systems and sent three into meltdown, spewing radiation over a wide area in the world's most serious nuclear accident since Chernobyl in 1986.
The plaintiffs, who joined aid efforts along with US troops shortly after the disaster, claim they were exposed to radiation because of the improper design, construction and maintenance of the plant.
They are seeking $5 billion to cover the cost of medical tests and treatment needed to recover from the disaster, Tepco said in a statement.
It was the second multi-plaintiff suit filed against the utility in a US court following one by more than 200 individuals in 2013.
In Japan, more than 10,000 people who fled their homes over radiation fears have filed various group lawsuits against the government and the firm.
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