Iran has stated that the purpose of their nuclear program is to produce electricity atomically, and they have no desire for atomic weapons.
Thus, they should want the atomic program that is the least costly, the most efficient, the safest and the least polluting of atomic waste products.
That describes thorium nuclear reactors exactly. The first thorium was developed by the Tennessee Valley Authority in the later 1940s and was recommended to President Truman as the best way to produce electricity by atomic energy.
Thorium, while it is one of the rare earth elements, is more common than uranium and would cost less to mine. Thorium also costs less to refine. Thorium also produces more energy per gram than uranium.
Truman went with uranium because uranium reactions produce plutonium, which is used for atomic weapons, while thorium by products cannot be used to produce atomic weapons.
Nuclear research has established that beryllium oxide is the perfect substance to hold the atomic substance in a nuclear reactor. It is an excellent conductor of heat from the atomic reactions to produce steam that drives the generators.
It has an extremely high melting temperature, which much reduces the chance of a nuclear meltdown. Is also is very strong which allows the rod to be left in the reactor so that almost all atomic substance is consumed; thus higher efficiency and less nuclear by products.
If Iran really wants its stated goals, then they should jump at any offer that would give the best in nuclear reactor designs combined with at cost thorium in beryllium-oxide rods.
They would have no further need for those centrifuges in underground bunkers, the uranium that they have already produced, nor atomic weapons.
If they would refuse such an offer, then their real goal of their atomic program is to produce atomic weapons, and sanctions really would not stop them.
That would mean that their nuclear facilities would have to be destroyed if one was to stop Iran from developing atomic weapons, and the sooner done, the less chance of their development of such weapons.