Bulgaria's Kozloduy nuclear power plant announced Thursday it had switched one of its reactors back on to the electricity grid, after a month of refuelling and repairs.

The 1,000-megawatt reactor number five -- one of only two in operation at Kozloduy -- was shut down for regular maintenance and refuelling on April 21.

Kozloduy's other 1,000-megawatt bloc number six was working at 100 percent capacity, the plant added.

Operations at Kozloduy remained unaffected by a 5.8-magnitude quake that struck early Tuesday 200 kilometres (125 miles) south of the plant, its chief executive Valentin Nikolov told state BNT television at the time.

The reconnection of the refuelled reactor also went smoothly despite a strong aftershock felt across western Bulgaria early Thursday.

Bulgaria's Seismological Institute measured it at 4.0 on the Richter scale, while the US Geological Survey put the jolt's strength at 4.5 and the European-Mediterranean Seismological Centre said it was 4.3.