UNDISCLOSED LOCATION, Ukraine - When a Russian attack plunged a Ukrainian thermal power plant into darkness on March 29, 51-year-old Ihor did not have time to think.

He grabbed a flashlight and made his way through the dust-filled control room to save remains of the system as the walls of the station fell, calling out to see if the other essential staff had survived the blast.

"We are scared, like all normal people would be, but this is our work," said Ihor, who has been at the plant for 23 years.

Russia began a second major assault on Ukraine's energy system last month, devastating at least eight power plants and several dozen substations.

Kyiv says Russia used more than 150 missiles and 240 attack drones in a single week from March 22 - cutting off electricity, heating and even running water to 2 million Ukrainians, according to a parliamentary estimate.

The intensity of the attacks, which have also targeted solar and hydro-electric power facilities, forced Kyiv to import power and sparked fears about the resilience of an energy system that was hobbled by a Russian air campaign in the war's first winter.

Russia has said the energy system is a legitimate military target and described last month's attacks as "revenge strikes" to punish Ukraine for attacking Russian border regions.

A complete collapse of the system that could cut off electricity and water supplies to towns and cities is unlikely for now, the head of national grid company Ukrenergo Volodymyr Kudrytskyi told Reuters last week.

Avoiding energy system collapse depends largely on rapid repairs of facilities like the one visited by Reuters on Monday, where people in protective suits and hard hats worked in a vast hall filled with metal and concrete dislodged and twisted by an air strike.

"To produce in the winter, we need to repair the building construction, the roof," said Andriy whose family has worked at the plant for generations. "[The equipment] will freeze otherwise."

RACE TO REPAIR

The plant asked Reuters not to disclose its location and the last names of its employees for security reasons. A single unit there could power some 10-15 small towns, operators say, but a March 22 attack halted its energy production for the first time, with recent strikes damaging almost all of its equipment.

The plant's private operator DTEK has said its stations, which meet about a quarter of Ukraine's energy needs, lost 80% of their capacity in the attacks. The company told Reuters it hopes to restore at least 50% of the losses in the next four months, with total costs estimated at $230 million.

Three nuclear power plants provide most of Ukraine's electricity even after Russian troops seized and occupied the six-reactor Zaporizhzhia facility, Europe's largest nuclear plant, at the start of the invasion.

But the damage to Ukraine's thermal and hydro-electric generation facilities is likely to make it harder to navigate ebbs and flows of demand, energy officials say.

And spare parts are hard to come by.

"This equipment is not produced by any plants on Ukraine's territory anymore, especially since most of it came from the Soviet Union," Andriy said of the plant that started functioning in late 20th century. "We are doing everything in our power, and beyond, to find replacements."

PATRIOTS, PATRIOTS, PATRIOTS

Protecting energy facilities and other vital infrastructure in a country the size of France while also defending the front is a major challenge.

"Confidence that this situation will not be repeated again tomorrow is the most important thing at the moment for us and for the essential staff who can't leave their workplace regardless of missile attacks,” said Andriy.

President Volodymyr Zelenskiy and other senior Kyiv officials appeal to their allies and partners almost daily to supply Ukraine with more air defences.

This year's Russian high-precision missile attacks on power generation assets have dealt damage that takes longer to repair than the strikes on transmission systems last year, said DTEK's spokesman, who declined to be named.

Ihor put it starkly: "They know exactly what they are targeting," he said.

Zelenskiy said Ukraine could cope using stockpiles for the moment, but was already making difficult choices about what to protect. His plea for 25 Patriot air defence systems on Saturday followed months of Republic resistance to the passage of a major U.S. military aid package in Congress.

At the power plant, repair work goes on around the clock despite the imminent threats. Another employee, Oleh, said the fact that Ukrainians had not given up kept him and others going.

"The boys on the front lines are defending our country," he said, "and we are fighting here as much as we can."

  - Reuters