The secretariat of the Nuclear Regulation Authority may start preliminary safety checks on offline reactors next spring, paving the way for some to be restarted by summer, sources said Thursday.

The NRA plans to adopt new safety standards for nuclear plants next July in response to the Fukushima No. 1 meltdown disaster that started in March 2011, while 48 of the nation's 50 commercial reactors remain offline amid safety concerns. The two in operation, started back up in July, are running under tentative standards.

To make safety inspections under the new standards efficient, the secretariat plans to launch preliminary checks next spring when an outline of the new standards becomes available, the sources said.

A bill was passed earlier this year to require all reactors from next July to meet safety standards based on the latest knowledge, before being allowed to restart.

If they fail to meet the standards, they will have to undergo improvements, including modifications as well as construction of additional facilities.

Given the time needed to build new facilities, the NRA has indicated that all necessary improvements would not necessarily have to be completed before the designated reactors are allowed to restart.

The authority, while accepting responsibility for the safety of reactors, added that the government the must be the party that decides which reactors are restarted.