The shutdown has removed a huge element of risk. The Zaporizhzhia Nuclear Power Plant is a pressurized water reactor. These reactors need constant cooling, and the cooling pumps are gigantic, powerful electricity-guzzling machines.

Cold shutdown is the state in which you do not need to constantly run the primary cooling pumps at the same level to circulate the cooling water in the primary cooling loop. Now, at least if the plant loses offsite power, the operators won’t have to worry about trying to cool an operating reactor with cranky diesel generators.

And by shutting down all the reactors, the plant operators have been relieved of a considerable amount of their workload monitoring the reactors amid the ongoing uncertainties around the site. This substantially reduced the potential for human error.

The operators’ jobs are likely to be much less demanding and stressful now than before. However, they still need to constantly monitor the status of the shutdown reactors and the spent fuel pools.

The plant still needs a reliable source of electricity to cool the six huge spent fuel pools that are inside the containment structures and to remove residual heat from the shutdown reactors. The cooling pumps for the spent fuel pools need much less electricity than the cooling pumps on the reactor’s primary and secondary loops, and the spent fuel cooling system could tolerate a brief electricity outage.

One more important factor is that the spent fuel storage racks in the spent fuel pools at the Zaporizhzhia Nuclear Power Plant were compacted to increase capacity, according to a 2017 Ukrainian government report to the IAEA. The greater number and more compacted the stored spent fuel rods, the more heat they generate and so more power is needed to cool them.

There is also a dry spent fuel storage facility at the plant. Dry spent fuel storage involves packing spent fuel rods into massive cylinders, or casks, which require no water or other coolants. The casks are designed to keep the fuel rods contained for at least 50 years. However, the casks are not under the containment structures at the plant, and though they were designed to withstand being crashed into by an airliner, it’s not clear whether artillery shelling and aerial bombardment, particularly repeated attacks, could crack open the casks and release radiation into the grounds of the plant.

The closest analogy to this scenario could be a terrorist attack that, according to a seminal study by the National Research Council, could breach a dry cask and potentially result in the release of radioactive material from the spent fuel. This could happen through the dispersion of fuel particles or fragments or the dispersion of radioactive aerosols. This would be similar to the detonation of a “dirty bomb,” which, depending on wind direction and dispersion radius, could result in radioactive contamination. This in turn could cause serious problems for access to and work in the plant.

Rafael Mariano Grossi, the head of the IAEA, briefed the U.N. Security Council on May 30, 2023, about the situation at the Zaporizhzhia plant. He called on Russia and Ukraine to ensure that the conflict does not put the plant at risk. Grossi has been to the Security Council several times. A week before the dam failed, he said it was the most important briefing that he had given to the council. To date, there has been no draft resolution from the Security Council.

This situation is rapidly evolving. And if something happens and there is a radiation release, it’s going to spread around the world.

This is an updated version of an article originally published on Sept. 13, 2022. The article has been updated to include news of the destruction of a dam downriver from the nuclear power plant and the IAEA’s report to the U.N. Security Council about reducing the risk that combat poses to the plant.

This article is republished from The Conversation, an independent nonprofit news site dedicated to sharing ideas from academic experts. The Conversation is trustworthy news from experts, from an independent nonprofit. Try our free newsletters.

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Najmedin Meshkati received research funding from the U.S. Nuclear Regulatory Commission in the mid-1990s.