[Who is John Galt?]

"Look up the “Tsar” device. The Soviets built the thing for a yield of 100 megatons, then got nervous and scaled the yield back by half... The Tsar test results showed that, yeah, it could take out an entire coastal city out to the far suburbs."

Exactly - the project is entirely feasible, and much of it involves decades-old technology. Frankly, any piss ant, entry-level nuclear power (i.e., North Korea, etc.) could do something similar (on a much smaller scale), with a fission device and a small cargo ship...

[DNME]

Rookie members of the Nuclear Club could build a deadly low-yield device but it takes a ton of knowledge in metallurgy, electronics, miniaturization, etc., to build a (physically) small yet reliable device. Nukes are NOT easy to detonate because there is an awful that can go wrong.