There are several main concerns.

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.

Ukrainian officials have accused Russia of hiding dozens of military vehicles with an unknown amount of munitions on the premises of at least two reactors. If a fire were to break out at the power transformers and the electric network was 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 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.