The Cernavoda plant, which draws all of its cooling water from the Danube, has been threatened by the river's sharply reduced water levels caused by drought.
Soldiers detonated the rock in a branch of the Danube near the village of Izvoarele, about 50km upstream from the plant.
The rock had become exposed as water levels fell.
Its fragments will be transported by barges over the next two days and sunk at a designated location to raise the water level and thereby divert more water towards the plant in eastern Romania, authorities said.
Defence Minister Radu Miruta described the detonation as a success, saying officials hoped the operation would ultimately provide enough water to keep the plant operating.
He added that the project would also improve navigation on the Danube, noting that the military had unsuccessfully tried to remove the rock for 30 years.
If the partial diversion of the river fails, the plant will have to be completely shut down by Thursday at the latest, Miruta said.
One of Cernavoda's two reactors was already taken offline last week because of insufficient cooling water.
Authorities had also considered shutting down the second reactor but instead opted to redirect more water from the Danube.
The river is currently carrying only about one-third of its average seasonal flow, and water levels are expected to fall further in the coming days.
The Cernavoda nuclear power plant generates around 10 million megawatt-hours of electricity a year, supplying about 20 per cent of Romania's electricity demand.
