That's completely unrealistic.
Iran is too huge.
That presentation uses the Bravo test (15 Mt) as an example, which covered a much smaller area with lethal radiation.
I gotta admit that said test was a poor test, though. It was done over an atoll and therefore not able to maximize radiation output because of limited contact of the fireball with the ground.
Nevertheless; Iran is too large.
Even a large quantity of small nuclear weapons amounting to a total of 20 Mt would not even approach 100% lethality.
By the way; the use of nuclear weapons in the Near/Mid East by any regional power would probably lead to its extermination.
The French, British, Pakistani and Indians could very well decide that the survival of no such state with a proven readiness of using nukes against cities (=genocide) could be tolerated.
An immediate domestic overthrow of the government might be the only chance of survival.
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