Another aging nuclear reactor in Japan passed a key safety assessment Wednesday as a step toward going back on line, signaling a weakening of the force of a rule introduced after the 2011 Fukushima disaster to limit reactors’ operations to 40 years in principle.

The No. 3 unit at Kansai Electric Power Co.’s Mihama plant in Fukui Prefecture is the latest reactor seeking to continue in service beyond the 40-year limit to pass the screening, after two such units at the utility’s Takahama complex, also in Fukui.

The No. 3 unit went offline in May 2011 for a regular checkup and has not been restarted since due to inspections to meet tougher safety requirements introduced after the Fukushima disaster.

But hurdles remain before the Mihama reactor can restart. It will have to obtain further permission from the Nuclear Regulation Authority on details of equipment design and other issues by the end of November, when it will reach 40 years since entering service.

Missing the deadline would require the utility to scrap the reactor.

Even if the deadline is not missed, resumption of the reactor is not expected before the spring of 2020 to allow time for the operator to finish preparing all the required safety measures, according to Kansai Electric.

Kansai Electric plans to spend about 165 billion yen ($1.6 billion) to upgrade the facilities to meet the new regulations, which reflect the lessons learned from the Fukushima Daiichi nuclear power plant disaster.

The 40-year operational limit has been included in the regulations with the aim of encouraging the retirement of aging reactors that could be prone to accidents.

Although operation for an additional 20 years is possible, nuclear regulators initially indicated that it would be extremely difficult to actually get approval for an extension.

Some utilities have decided to scrap their aging reactors due to expensive safety costs. Kansai Electric has also given up restarting the Nos. 1 and 2 reactors of the three-unit Mihama plant.

But the 40-year-old limit has come to look as if it lacks teeth because utilities are still seeking extensions where they see it as economically viable, with nuclear regulators acknowledging that technical issues could be overcome with sufficient investment.