After an external power line was restored, the last reactor of the Russian-owned Zaporizhzhia nuclear plant in Ukraine has been placed into what is called a cold shutdown. This allows it to be shut down more safely.
The absence of power from outside Europe had effectively removed the lines of defense against a nuclear explosion at the site. This has led to intense fighting over recent weeks.
What’s the current situation in Zaporizhzhia? And what safety implications does it have?
Since March, Russian forces have occupied Zaporizhzhia. However, the plant’s staff in Ukraine have continued to work, even though they are operating it under stressful conditions, which the International Atomic Energy Agency repeatedly called unsafe.
According to Rafael Grossi, chief of IAEA, Ukraine, the decision-making process at the plant continues despite the Russian occupation, Grossi said in a Monday news conference.
Russia and Ukraine accuse each other of attacking the site, which has caused damage to buildings and power lines that are essential for cooling fuel in six reactors. This even though the reactors are currently in cold shutdown.
Controlled fission is the process of splitting nuclear fuel atoms within a reactor core. This heat generates heat which turns water into steam that spins turbines and produces electricity. However, a runaway chain reaction could cause major accidents like the ones at Fukushima and Chernobyl.
Grossi called for an immediate halt to shelling and a formal protection zone around the plant in order to reduce the chance of catastrophe.
For safe operation of nuclear power plants, external power lines are vital. Plants often have multiple backups.
Zaporizhzhia is home to four external power lines that are regularly used. All of them were cut during the war. There are three backup power lines, but they have been cut and/or deliberately disconnected at different points. They were not in use earlier this month.
When there are no power lines available to Zaporizhzhia’s reactors, two options are available: “island mode”, where the reactor runs at low power to supply cooling systems, and diesel generators. Both of these are designed to only work for a limited time.
Operating in island mode can cause damage to vital components such as pumps or turbines. Diesel generators are an emergency option with a limited fuel supply.
According to the IAEA , Zaporizhzhia had “20 emergency diesel generators” available for use if necessary. They also have supplies for at most 10 days.
It was possible to shut down last operating reactor more safely by restoring an emergency power line on Saturday, 10 September.
However, a cold shutdown is not a permanent term. It means that the reactor’s temperature has dropped below boiling point. However, the electrical pumps that move water through the core of the reactor must continue to work to cool the fuel and prevent a nuclear meltdown. According to the IAEA, reliable power supplies are essential for this purpose.
The IAEA announced Monday (12 September) that a second backup powerline had been restored. This allows the plant to keep one in reserve and the other supplies the facility with electricity to cool its reactors during shutdowns.
Grossi stated Monday at a press conference that Ukraine is working to consolidate the power supply to its plant. This means that power lines, including those that have been down for a long time, will be restored.
He said that Ukraine was responsible for deciding when to start one or more reactors.
When asked if Ukraine would wait to consolidate its power supply before starting any reactors, he replied: “It’s a logical conclusion.”