Ukrainian engineers at the country’s largest nuclear power plant are relying on its one active reactor to run the station’s critical safety and cooling systems — a precarious stop-gap measure that reflects the desperation of the moment, the head of the Ukrainian energy company said on Thursday.

Petro Kotin, the head of the Ukrainian national energy company, Energoatom, said in an interview that while the emergency measure was known to the engineers and outlined in technical documents, it has never been attempted for more than a few hours — and has already gone on for days.

The Zaporizhzhia Nuclear Power Plant has been using the measure since it was disconnected from external power on Monday, leaving diesel generators as the last fail-safe. Reconnecting to external power requires spare parts, Kotin said, and workers are racing to bring them from Ukrainian-controlled territory, crossing the front line to reach the Russian-controlled station.

Even so, Kotin said the “degradation” of the facility continued to grow “worse and worse and worse”. Inspectors from the UN nuclear watchdog visited the plant last week and called for the creation of a safe zone around the facility.