Nuclear plant operators in Japan say it will be difficult to install anti-terrorism facilities at their plants within the timeframe set by the government.
They informed the Nuclear Regulation Authority of the expected delays at a meeting on Wednesday.
Regulations introduced after the 2011 Fukushima Daiichi accident require operators to build standby control rooms to retain control of reactors in the event of a terrorist attack or a plane crash.
Operators are required to construct such facilities within five years after a nuclear plant clears the requirements to go back online.
The Sendai plant in southwestern Japan was the first nuclear power station in the country to go back online under the new regulations.
Kyushu Electric Power Company has less than one year left before the deadline to build an emergency response center for its Sendai plant. If the utility fails to complete the facilities in time, it may have to suspend the plant's operations.
Kansai Electric and Shikoku Electric predict it will take five-and-a-half to seven-and-a-half years to build their emergency response centers due to the scale of the construction work.
Nuclear Regulation Authority Chairman Toyoshi Fuketa criticized the operators, saying they were too optimistic. He said the regulator will discuss how to deal with this matter.