The answer is that at 20 percent enrichment a country has already expended 97 percent of the effort needed to get to weapons-grade material.
The table below shows what is involved to reach weapons-grade.
Natural uranium contains only 0.7 percent U-235, the type of uranium needed for weapons. Most of natural uranium is U-238, which is useless for weapons purposes.
“Enriching” uranium involves getting rid of excess U-238 and concentrating the amount of U-235. A centrifuge spins at great speed to spin off the heavier U-238 atoms and leave the lighter weight U-235 atoms.
The first line of the table below shows 1,000 atoms of natural uranium split between 7 atoms of U-235 and 993 atoms of U-238.
At its Natanz plant, Iran’s centrifuges spin for long periods to get rid of 800 atoms of U-238. The result is 200 atoms remaining of which the original seven U-235 atoms are now 3.5 percent of what remains.
At the new Fordo plant, centrifuges are taking that 3.5 percent enriched uranium and spinning some more in order to get rid of another 164 atoms of U-238. That leaves just 36 remaining atoms of which the original seven are U235 or just short of 20 percent enrichment.
The last line shows that only 28 more atoms of U-238 need to be spun-off in order to reach 90 percent enrichment or weapons-grade uranium.
Eighty percent of the effort needed to reach weapons-grade was expended just going from natural uranium with 0.7 percent U-235 to 3.5 percent enrichment, which is the concentration needed for a plant generating electricity.
Another 17 percent of the effort to reach weapons-grade is expended in the second step to reach 19.75 percent concentration the concentration required for the small Tehran reactor that makes medical isotopes.
At that 19.75 percent concentration, the centrifuges have already spun off 97 percent of all the useless U-238 atoms. Only another very small effort amounting to just 3 percent of all the effort needed is required to go the remainder of the distance and produce weapons-grade material of 90 percent U-235.
The fear is that Iran will stock up on a large volume of 19.75 percent enriched uranium and then breakout or rush to spin off U-238 atoms to suddenly produce a stockpile of weapons-grade uranium.
Olli Heinonen, who used to be the deputy chief of the International Atomic Energy Agency (IAEA) in charge of monitoring Iran’s operations, wrote last week in Foreign Policy magazine that Iran is no longer stockpiling 3.5 percent enriched uranium to use for some future power generating plant. Instead, he said, all the 3.5 percent uranium is now going into the centrifuges that are producing 19.75 percent uranium.
Heinonen said that if Iran used its current type of centrifuges and started with 3.5 percent enriched uranium, it would take about six months to produce enough weapons-grade material for one bomb. Starting with uranium already enriched to 19.75 percent, it would take only about four weeks to get enough weapons-grade material for one bomb. And another four weeks for a second bomb. And another four weeks…….
This is what has people concerned.
All of this assumes the Islamic Republic sticks with the 8,000 centrifuges it now has installed. But the Natanz plant alone is built with a hall having a capacity for a total of 54,000 centrifuges.