The Office for Nuclear Regulation (ONR) said a radiation emergency was "no longer reasonably foreseeable" at the Trawsfynydd plant, Gwynedd.
It has now lifted a near one-mile emergency planning area around the site in the event of an incident.
Decommissioning began in 1995, two years after it ceased operating.
An ONR report said owner Magnox were due to begin a long term care and maintenance programme from 2016.
It plans to allow remaining structures to be "kept in a passively safe and secure state for a great number of years in order to allow for radiation levels to naturally decay over time".
Gwynedd council and Magnox still have other legal duties and emergency arrangements in place to protect the public, said the regulator.
Regulators say there are "no longer hazards" at a nuclear power plant requiring an emergency buffer zone as the site's decommissioning continues.