Second, since the deal would permit Iran to keep only a small amount of enriched uranium in the gaseous form used in centrifuges, Mr. Obama assumes that a dash for the bomb would start mainly from unenriched uranium, thereby lengthening the breakout time. But the deal would appear to also permit Iran to keep large amounts of enriched uranium in solid form (as opposed to gas), which could be reconverted to gas within weeks, thus providing a substantial head-start to producing weapons-grade uranium.

Third, Mr. Obama’s argument assumes that Iran would require 59 pounds of weapons-grade uranium to make an atomic bomb. In reality, nuclear weapons can be made from much smaller amounts of uranium (as experts assume North Korea does in its rudimentary arsenal). A 1995 study by the Natural Resources Defense Council concluded that even a “low technical capability” nuclear weapon could produce an explosion with a force approaching that of the Hiroshima bomb — using just 29 pounds of weapons-grade uranium.

Based on such realistic assumptions, Iran’s breakout time under the pending deal actually would be around three months, while its current breakout time is a little under two months. Thus, the deal would increase the breakout time by just over a month, too little to matter. Mr. Obama’s main argument for the agreement — extending Iran’s breakout time — turns out to be effectively worthless.