How easy is it to build a nuclear bomb?

[shadron]

There is no yield limit for a hydrogen bomb, only a maximum yield/weight ratio: 6 megatons/ton.

The best US bombs for which the data is publicly known made it to 4.25 tonne-yield/kg ratio. 600 Mt would be about 141 tonnes.

OTOH, the W-54^WP was placed on a man portable weapon, the Davy Crockett^WP in 1962. According to wiki,

The Mk-54 weighed about 51 lb (23 kg), with a selectable yield equivalent to 10 or 20 tons of TNT (very close to the minimum practical size and yield for a fission warhead). The complete round weighed 76 lb (34.5 kg). It was 31 in. (78.7 cm) long with a diameter of 11 in. (28 cm) at its widest point

It was tested twice in Nevada in June of 1962 (Sunbeam/Little Feller I and II) at 22 and 18 tons equivalent, respectively..

 

[Loss Leader]

France received very little US assistance and did most of their own work.

My research indicates that France got early help from the UK, including double-checking their reactor blueprints and handing over a small sample of plutonium. Frace received help secretly from the US in the 1970s and 80s to improve the safety of their weapons.

Ullman, Richard H. "The Covert French Connection." Foreign Policy 75 (summer): 3-33.