An early warning of Moscow’s intentions came on Feb. 24, when Russian forces seized the decommissioned Chernobyl plant. According to Ukraine’s State Nuclear Regulatory Inspectorate, gamma radiation dose rates recorded the next day in Chernobyl’s exclusion zone exceeded control levels because of “disturbance of the top layer of soil from movement of a large number of heavy military machinery through the exclusion zone.” The IAEA reported those levels weren’t dangerous.

Mr. Putin’s effort to shatter the Ukrainian state dramatically increases the chances of another avoidable disaster. Russian forces captured the territory around Zaporizhzhya on March 2. Poignant images showed power-plant staff and volunteers from Energodar trying courageously, peacefully and unsuccessfully to block the highway leading to the plant. It was sheer luck that the now-extinguished fire didn’t damage critical assets such as storage areas or cause loss of coolant, uncover the core, and increase temperatures to melt the fuel and cause a radioactive release.

Russian forces threaten Ukraine’s other three nuclear plants. Safety upgrades at Ukraine’s nuclear plants provide multiple measures to maintain reactor-core integrity and cope with loss of coolant for fuel rods. But operators have to enter the plant and control key equipment to avoid catastrophic heat increases in the reactor. Cooling water and an ultimate heat sink are necessary. Adequate power must be available in reasonable time for pumps to bring in cooling water and prevent a Fukushima-like accident.