In a decision that will set a precedent for Japan's rapidly aging nuclear reactors, Kansai Electric Power Co. must soon choose whether to restart reactors 1 and 2 at its Takahama plant in Fukui Prefecture and operate them beyond the 40-year threshold, the first time a Japanese utility has faced such a dilemma.

Under new government guidelines adopted in 2012, 40 years is the maximum limit, in principle, on how long the nation's reactors are allowed to operate.

After that, they are supposed to be decommissioned, a process that can take decades. However, if a utility decides to continue to use them, it can apply for a one-time-only extension of 20 years if it meets a series of additional safety tests the government describes as "stringent."