Probably enriched uranium. If they had plutonium I doubt it would be only 30kt.
Nobody has exploded a U235 bomb (Little Boy type) since the 1950s (AFAIK)
https://en.wikipedia.org/wiki/Gun-type_fission_weapon#Comparison_with_the_implosion_method
All bombs since the 1950s are implosion type using Pu.
The reason is because U235 bombs are just too unstable and dangerous to handle.