A magnitude-9 earthquake and tsunami crippled the Fukushima Daiichi nuclear power station in 2011. Operator Tokyo Electric Power said it uncovered "high densities" of radioactive material in groundwater observation sites at the facility.
Tepco said it was "continuing to tackle this problem" but has so far been unable to explain the increase in detected levels of radioactive elements.
Four Japanese utility companies said Monday they applied to nuclear regulators to determine if their 10 reactors meet safety requirements. New rules require operators to enact tough counter-terrorism measures, emergency procedures and leak-protection systems.
The Platts news service, the energy arm of publisher McGraw Hill, said the applications to the Nuclear Regulatory Authority are the first step in a process meant to restart the nuclear energy sector in Japan.
Japan started using more natural gas after the 2011 disaster to compensate for the loss of nuclear power. Two of the nation's 50 nuclear reactors are in service and those are scheduled to close for routine maintenance in September.