Because it is not yet up and running, the Arak heavy-water reactor has remained in the background of the nuclear controversy. But it looms larger every day. The reason: once Arak goes online, the option of destroying Iran’s nuclear program with air strikes becomes moot. The reactor is essentially invulnerable to military attack, because bombing one risks a catastrophic release of radioactivity. In the words of Israel’s last chief of military intelligence, Amos Yadlin, who piloted one of the F-16A’s that cratered Iraq’s Osirak heavy-water reactor in 1981 before it was due to become operational: “Whoever considers attacking an active reactor is willing to invite another Chernobyl, and no one wants to do that.”

That reality is the reason why some experts are drawing attention to a peculiar notice filed by Iran’s nuclear agency to the International Atomic Energy Agency (IAEA) in May. Iran told the U.N. agency that, as it readies the Arak plant for operation, it intends to do a practice run: instead of inserting real fuel rods filled with uranium into the reactor’s core, where nuclear fission occurs, they would insert inert “dummy” fuel rods. And instead of pumping heavy water into the reactor to moderate the nuclear reaction and absorb the thermal energy being released, Iran said it plans to use “light water,” just ordinary H2O…

“Anything above that is hard to achieve and testing the system with light water would leave a residual atmosphere of H2O that would degrade the heavy water when it is added,” writes one U.S. specialist of heavy-water reactors, who has worked with the Institute for Science and International Security (ISIS), a Washington, D.C.–based think tank, and who shared his assessment on condition he not be identified further. In other words, rather than save time, using ordinary water would delay the project for the weeks required to clean the system thoroughly enough to assure no trace of H2O remained; it wouldn’t take much to dilute the heavy water below 99.75%.