Japan currently does not have a limit on the operational lifespan of reactors, and the government had hinted when it announced the cap that extensions were a possibility. The proposed legislation requiring plants to shutter after 40 years is part of the government's campaign to improve safety following the nuclear crisis set off by the March 11 earthquake and tsunami.
Concern about aging reactors has grown because three of those at the tsunami-hit plant were built starting in the late 1960s and many more of Japan's 54 reactors will reach the 40-year mark in coming years.
Chief Cabinet Secretary Osamu Fujimura said the government still plans to stick to the 40-year cap in principle. He said exemptions would be rare, with each reactor only allowed a maximum of one. He said to qualify a reactor would have to meet strict safety requirements.
The Cabinet is set to approve the bill by the end of January before submitting legislation to parliament for further debate, he said. It's aiming to enact the cap by the end of March.
The proposed legislation is similar to regulations in the U.S., which grant 40-year licenses and allow for 20-year extensions. Such renewals have been granted to 66 of 104 U.S. nuclear reactors. That process has been so routine that many in the industry are already planning for extensions that could push the plants to operate for decades longer.
If the 40-year-rule is applied, 36 reactors would have to close by 2030, the Asahi newspaper reported.
Since the meltdowns at the Fukushima Dai-ichi plant, Japan has ordered reactors across the country to undergo new "stress tests" and get community approval before they can be restarted.
On Wednesday, Japan's nuclear officials moved a step closer to restart two of more than 40 nuclear reactors that are offline — most of them for regular inspections.
The Nuclear and Industrial Safety Agency presented a preliminary ruling on two nuclear reactors at the Oi power plant in western Japan, telling a panel of experts that operator Kansai Electric Power Co. had properly carried out stress tests. It was still unclear if the community would approve.
The operator said the tests found the two reactors had a safety margin of 1.8 times the strength of an anticipated quake, and four times the height of an anticipated tsunami.
The meeting was delayed for several hours as activists stormed into a conference room demanding they be allowed to observe the proceedings in the same room, not on a TV monitor downstairs.
The stress tests are similar to those used in France and other European countries, where they conduct a simulation designed to assess if the plants could weather extreme events such as earthquakes, tsunamis, storms and other disasters.
Some experts and concerned residents in Japan say the tests have no clear criteria, rendering them meaningless. They also say disasters often occur in a string of events, and evaluation by computer simulation on a single event is not realistic.
University of Tokyo metallic material scientist Hiromitsu Ino, who is on the panel, said the way stress tests are designed is not adequate even though an attempt to find vulnerable spots to improve safety is good.
"The problem is that stress tests are not comprehensive. They only look at certain areas, and it's not appropriate to determine safety based on an evaluation on limited areas."
Japan is currently reviewing its future energy policy and plans to announce one this summer. Fujimura also said that Japan is trying to be less reliant to nuclear energy.
"If you limit an operational lifespan at 40 years, obviously the number of nuclear power plants would decrease," he said. "We are still aiming to reduce reliance on nuclear energy, but it's a goal that we cannot be achieved overnight."