TOKYO — Japan says it will soon require atomic reactors to be shut down after 40 years of use to improve safety following the nuclear crisis set off by last year's tsunami.

Concern about aging reactors has been growing because the three units at the Fukushima Daiichi plant in northeastern Japan that went into meltdown following the tsunami in March were built starting in 1967. Among other reactors at least 40 years old are those at the Tsuruga and Mihama plants in central Japan, which were built starting in 1970.

Many more of the 54 reactors in Japan will reach the 40-year mark in the near future, though some were built only a few years ago.

The government said Friday that it plans to introduce legislation in the coming months to require reactors to stop running after 40 years. Japanese media reported that the law may include loopholes to allow some old nuclear reactors to keep running if their safety is confirmed with tests.

The proposal could be similar to the law in the U.S., which grants 40-year licenses and allows 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 additional license extensions that could push the plants to operate for 80 years or even 100.

Japan does not currently have a limit on years of operation. It had planned to expand nuclear power before the meltdown, but has since ordered reactors undergoing routine inspections to undergo new tests and get community approval before they can be restarted. The new restrictions mean that only six Japanese reactors are currently running.

The Asahi newspaper reported Saturday that Japan is likely to face a power shortage if it carries out the 40-year rule, which barring loopholes would force 18 more reactors to shut down by 2020, and another 18 by 2030.

The government has already decided to scrap six reactors at Fukushima Daiichi, where backup generators, some of them in basements, were destroyed by the March 11 tsunami — setting off the world's worst nuclear crisis since Chernobyl. The government has said it will take 40 years to fully decommission the plant. It is unclear whether the age of the reactors was related to the nuclear crisis.