EDF stressed that none of its plants had safety weaknesses.
The investment will be spent on “a number of steps that can further improve these strong safety standards.”
EDF Energy Chief Executive Vincent de Rivaz said: “March 11 is the anniversary of an earthquake and tsunami which killed thousands of people.
“At this time, our thoughts are with those who were affected. We are fortunate in this country not to have seismic activity on the same scale.
“Nonetheless it is vital to learn the lessons from the nuclear event caused by the tsunami.
“Looking back, a year on, it is clear nuclear in the UK is even safer because of what has been done.
“We have put in place a range of measures which further strengthen our safety. It will cost us money. But we will be even safer.
“These enhancements, and others, will be implemented over the next few years, at a total cost of around £200m, in addition to the £300m that we already spend each year on maintenance.”