The issue isn't whether it is weapons grade as you can make one that is larger and bulkier with 20% enriched uranium... They would have to take it there and drop it off then leave and set it off as 20% would be a massively large weapon, 60% would be smaller than that but still much larger than the weapon with 90% enrichment and would have to have a backwater gun-type trigger, like "Little Boy" dropped on Nagasaki (though that was 80% Enriched).
The bulkier device would have delivery issues and would not be as efficient, but it would not be impossible nor would it be guaranteed to be fizzled... the gun-type fission is also less efficient and would produce a less "explosive" action, but not necessarily "fizzled".
Plutonium is created in Reactors and does not need to be enriched to be weapons grade, it is separated by chemicals, at least one of Iran's nuclear reactors does produce plutonium, which could then be separated and made into a weapon as well. The second nuclear device we dropped in Japan (Fat Man) used just 6.2 kg of Plutonium as opposed to the 64kg of uranium in the one dropped on Hiroshima and it used an implosion type trigger device.