Romania has detonated a rock outcrop in the drought-stricken Danube River to redirect crucial cooling water to its sole operational nuclear reactor.

The blast, carried out by the Romanian navy, sent a column of water skyward on Monday.

Extreme heat and prolonged drought have depleted river levels across Europe this summer, jeopardising water supplies, tourism, and freight.

Both Romania and its neighbour Hungary are increasingly dependent on costly power imports as their nuclear facilities operate at reduced capacity amid soaring electricity demand driven by the heatwave.

The Danube, which is Europe’s second-longest river, serves as a vital coolant source for both countries' atomic plants.

Hungarian authorities, who had previously warned of a potential shutdown for their Paks nuclear plant as early as Sunday, now anticipate it can maintain its current diminished output until Monday or Tuesday, offering a brief respite for the national power grid.

The 2-gigawatt Paks facility, which is typically responsible for approximately half of Hungary's electricity, was operating at just over 10 per cent of its capacity on Monday, due to the Danube's unprecedented low levels.

"Today, and maybe tomorrow the last turbine producing 240 megawatts can keep working," Hungary's Prime Minister Peter Magyar said.

"This is very good news, as the complete shutdown of the plant can be avoided on one of the days with the highest consumption."

Further downstream, Romanian state-owned nuclear power producer Nuclearelectrica, which normally generates a fifth of the country's electricity needs, had to shut down one of its two reactors early last week.

Experts said that the controlled explosion would clear the way for authorities to build a temporary dam which would send more water to the river channel where the remaining Cernavoda reactor sits.

The work to redirect more water to the reactor would finish on Wednesday, the navy said.

"Any day past Wednesday or Thursday that the nuclear power plant functions is a gain," Romanian Defence Minister Radu Miruta said on Monday.

"A day of it functioning is three times more efficient than the costs of this operation."

Romanian car-making plants run by Renault-owned Dacia and Ford have temporarily halted production until 19 August, Prime Minister Ilie Bolojan said on Monday, voluntarily reducing power consumption by some 200 MW.

Mr Magyar earlier said that voluntary cuts in power usage by Hungarian households and companies, made after a government appeal, had reduced demand by 700 MW on Sunday, significantly easing pressure on the grid.

Adding to voluntary cuts by other companies, pharma company Richter said on Monday that it would reduce its electricity consumption by more than 50 per cent in the next three weeks, and offered a daily 4 MW of electricity produced by its solar energy park.