[Curious Cat (OP)]

 .

[Bored chemist]

Because it was a "gun type" nuke and cordite is good for that sort of thing.

https://en.wikipedia.org/wiki/Gun-type_fission_weapon

[evan_au]

It was hot burning, and they wanted to maximise the heat?

Once the explosive nuclear chain reaction starts, mere chemicals can't hope to compete with the heat.

[Bored chemist]

Thanks, BC. Are U saying that Yanks also used Cordite, for artillery!?

Wiki tells me it was Canadian.
I suspect Cordite  may have become a
https://en.wikipedia.org/wiki/Generic_trademark
like Hoover.

[alancalverd]

The value of cordite as a propellant is that it is a fast "low" explosive, unlike its constituent nitroglycerine which is a "high" explosive. That is to say that the flame rate does not exceed the speed of sound inside the charge so the product gases are released at a smoothly increasing rate, accelerating the missile very predictably without shattering it (or the gun barrel).   

Apparently it is a British invention

A United Kingdom government committee, known as the "Explosives Committee", chaired by Sir Frederick Abel, monitored foreign developments in explosives and obtained samples of Poudre B and Ballistite; neither of these smokeless powders was recommended for adoption by the Explosives Committee.[citation needed]

Abel, Sir James Dewar and W Kellner, who was also on the committee, developed and jointly patented (Nos 5,614 and 11,664 in the names of Abel and Dewar) in 1889 a new ballistite-like propellant consisting of 58% nitroglycerine, by weight, 37% guncotton (nitrocellulose) and 5% petroleum jelly. Using acetone as a solvent, it was extruded as spaghetti-like rods initially called "cord powder" or "the Committee's modification of Ballistite", but this was swiftly abbreviated to "Cordite".

[alancalverd]

Practically everyone.

[alancalverd]

I thought that nitrocellulose had been generally replaced by cordite after WWI for reasons of stability and consistency. Certainly the case in civilised countries (UK) and probably in the colonies (USA).

[alancalverd]

Cordite was a British invention but widely available as it was manufactured in quantity in Canada. If I was designing a one-off bomb based on wholly new physics, I'd want the least number of engineering unknowns, so a stable, off-the-shelf propellant that has been around for 50 years is a good start. Remember a fair number of Commonwealth citizens were involved at Los Alamos and if you can turn the gun design over to a guy who has done it a hundred times before, let him choose the chemistry he is familiar with, for the best chance of it working first time.