The European Union donated 5.5 million potassium iodide tablets to Ukraine this week amid fears that reported shelling and other military activity near the Russian-occupied Zaporizhzhia nuclear power plant could spur a nuclear disaster.

A statement from the European Commission on Tuesday announced the donation, describing it as a "preventative safety measure to increase the level of protection" around the plant.

"No nuclear power plant should ever be used as a war theater," the bloc's commissioner for crisis management, Janez Lenarčič, said in the statement. "It is unacceptable that civilian lives are put in danger. All military action around the Zaporizhzhia nuclear power plant must stop immediately...We will continue to be on the lookout and stand ready to act, because preparedness saves lives."

The Zaporizhzhia plant was captured by Russian forces in the early days of Russia's invasion but is still operated by Ukrainians. Ukraine and Russia have been trading accusations of shelling near the plant in recent weeks. Operator Energoatom said Thursday that one of the plant's two operational reactors had been shut down due to Russian shelling, Reuters reported but since confirmed on Telegram that it is back online.

A mission from the International Atomic Energy Agency that CNBC confirmed arrived at the plant on Friday is aiming to "help ensure nuclear safety and security" there.

Now, Ukraine's efforts to prepare nearby civilians for potential nuclear fallout at Zaporizhzhia include distributing potassium iodide pills. According to the U.S. Centers for Disease Control and Prevention (CDC), potassium iodide (KI) is a type of iodine that is not radioactive but can help prevent radioactive iodine from being absorbed by the thyroid. Radioactive iodine can be released into the environment in some radiation emergencies, such as nuclear power plant accidents, and enter the body via breathing or eating.

The thyroid absorbing high levels of radioactive iodine can increase the risk of thyroid cancer in infants, children and young adults "many years after exposure," the CDC said. In order for potassium iodide to work against the potentially harmful effects of radioactive iodine, it must be taken before or shortly after exposure.

"When a person takes the right amount of KI at the right time, it can help block the thyroid from absorbing radioactive iodine," the CDC explained. "This happens because the thyroid has already absorbed the KI, and there is no room to absorb the radioactive iodine. Think of filling a jar with blue marbles. If you then pour green marbles over the jar, there will not be room and they will just spill out."

The CDC said that potassium iodide can still cause harmful health effects and should be taken only at the direction of a health care provider, a public health official or an emergency response official. There are also limits on how much it can safeguard civilians in the event of a nuclear accident.

Potassium iodide may not offer 100 percent protection from radioactive iodine, the CDC said. That's because it's most effective when taken shortly before or right after exposure, and its effectiveness might also depend on how much radioactive iodine enters the body, as well as how quickly it is absorbed.

Taking potassium iodide is recommended only for people under age 40 and for those who are pregnant or breastfeeding, according to the CDC.

Potassium iodide protects only the thyroid, not other parts of the body, and does not protect against other types of radiation.

Newsweek reached out to the defense ministries of Russia and Ukraine for comment.

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