The European Union and the United States in recent days have called for a demilitarized zone to be established immediately around the Zaporizhzhia Nuclear Power Plant in southern Ukraine, as shelling at the plant raises the risk of a nuclear accident.

While such plants are designed to withstand a range of risk — from a plane crashing into the facility to natural disasters — no operating nuclear power plant has ever been in the middle of active fighting, and this one was not designed with the threat of cruise missiles in mind.

The concrete shell of the site’s six reactors offer strong protection, as was the case when the No. 1 reactor was struck in March, officials say. More worrying is the chance that a power transformer is hit by shelling, raising the risk of a fire.

If a fire were to break out at the power transformers and the electric network were taken offline, that could cause a breakdown of the plant’s cooling system and lead to a catastrophic meltdown, said Edwin Lyman, a nuclear power expert at the Union of Concerned Scientists, a private group in Cambridge, Mass.

He noted that the loss of coolant during the Fukushima accident in Japan in 2011 had resulted in three reactors undergoing some degree of core meltdown.

If the cooling is interrupted, Dr. Lyman said, the nuclear fuel could become hot enough to melt in a matter of hours. Eventually, it could melt through the steel reactor vessel and even the outer containment structure, releasing radioactive material.