Kozloduy nuclear plant's fifth reactor could run until 2049 and its sixth until 2051, but neither date is final, the head of the Nuclear Regulatory Agency, Tsanko Bachiyski, told reporters.

The two reactors will keep running as long as they keep proving they are safe, he said. He said the plan to extend their working life is already paying off. Based on the reactors' current condition, and once all needed work is done, reactor five could run until 2049 and reactor six until 2051.

The final date depends on the condition of the equipment, which is checked in a periodic safety review that follows world standards. The review happens every ten years, and plant operators must prove the reactors are safe, Bachiyski said. The agency can also ask for more frequent checks if it decides that's needed.

Results from the ten-year review of reactor five are due by the end of this year, and for reactor six a year later, the agency chief said.

"If we find any faults, we can change the deadline and shorten it. If there are faults, we draw up a plan of steps to fix them, which can include swapping out equipment, technical fixes, or studies that cover certain safety issues," Bachiyski explained.

He added that the checks also look at how the reactors are run — especially how often they are shut down and restarted — and how that wears on the equipment. All these checks together should show the real level of safety for the years ahead, he said.