The last reactor still producing energy at Europe’s largest nuclear power plant, in southern Ukraine, was put into a “cold shutdown” — a state in which it no longer generates electricity — as a safety precaution after the destruction of a nearby dam threatened the facility’s water supply, Ukrainian energy officials said.
Five of the six reactors at the Zaporizhzhia Nuclear Power Plant have been fully shut down since September, but one — Reactor 5 — was kept in what engineers call a “hot state” to help supply energy within the plant.
Like the other reactors, Reactor 5 was finally switched over to cold shutdown on Thursday, the state nuclear company, Energoatom, said in a statement. That means control rods have been inserted into the reactor core to stop the nuclear fission reaction and generation of heat and pressure.
“Cold shutdown is the safest state of operation of a nuclear power unit under such conditions,” the company said, citing the destruction of the Kakhovka dam and the rapid loss of water from the dam’s reservoir.
The Zaporizhzhia plant has been occupied by Russian forces since the war’s early stages, but is still run by Ukrainian staff.
Water continued to drain from the Kakhovka Reservoir after an explosion caused its dam to collapse, leaving less available water to cool the reactors. On Saturday morning, the water level in the Nikopol area, which sits on the opposite side of the Dnipro River from the plant, had fallen more than three feet over the previous day, according to the state hydropower utility, Ukrhydroenergo.
Even though the plant had used the reservoir water for cooling, the head of Ukraine’s state-owned nuclear company said the loss of the reservoir poses no immediate risk of a meltdown.
That’s because the Zaporizhzhia Nuclear Power Plant was designed with a backup cooling pond on the grounds to let engineers shut down its six reactors even in the event the Kakhovka dam were to break and the reservoir to drain, as is now happening.
“There are design conditions which were calculated for this event,” Petro Kotin, the president of Energoatom, said earlier this week.
Even though the six reactors are now in cold shutdown, they still require water to circulate in their cores to dissipate residual heat from nuclear reactions. Each reactor also needs water for a cooling pond for spent fuel.
Supplying water at the plant now, and perhaps for years to come, will depend on maintaining water levels in the site’s cooling pond, which used to be fed by the reservoir.