TOKYO (AP) — Japan’s planned 40-year cap on nuclear power plants could be extended up to 20 years, but exemptions will be rare, the government said Wednesday.

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.