Is Flagpole spherical? Do you have a diagram?
I'm not aware of any diagrams that I'd really believe. According to Lorna Arnold in
Britain and the H-bomb the British secondaries were all spherical because the British believed the calculations were easier for a spherical primary (he US believed things were easier for a cylindrical primary, which is why they went that route).
Are the primaries based on an improved Red Beard? Can the primary design be passed onto the French for them to manufacture?
I'm not sure is the answer to that one. I know they're Plutonium fuelled because there were major safety concerns relating to
Burgee due to a possible reaction between Tritium and Plutonium if the weapon wasn't fired within a short period of time after the gas was introduced. I suspect that the mass of plutonium inside the weapon was rather lower which implies a very different design -
Burgee was 25kT using gaseous Tritium boosting. Red Beard was a 15/25 kT weapon using no boosting, and which used a composite Pu/HEU core to get around predetonation problems they were having at the time with all-Pu cores. I **think** this implies that
Burgee was a relatively small Pu-only core which used Tritium boosting to improve efficiency and get the yield up to Red Beard size.
What's an immune primary?
Not much information available on this (it was very highly classified by the British at the time, because it gave them a massive fright about whether their deterrent would work at all), but essentially if a nuclear bomb goes off near another one it can potentially disable it through radiation effects even if there is no physical damage. Worse, this effect is particularly strong in Plutonium-fuelled weapons such as the British were concentrating on. Boosting the weapons helps deal with this, both because it improves the efficiency of the reaction and because it allows you to use less fissile material making the weapon less vulnerable to predetonation. As a result of this fright Aldermaston seems to have spent as much time trying to design an immune primary as it was on an actual H-bomb at the time. A number of options were tried including solid and gaseous boosting plus three-stage bombs which used radiation compression from a small primary to ignite a second fission primary, which in turn would be used to ignite a secondary. Such a weapon appears to have been tested at American behest in the
Halliard test.
This also feeds into ABM systems - if the enemy doesn't have an immune primary (and at the time what evidence was available suggested that the Soviets didn't) then a nuclear-armed ABM system didn't even have to get close enough to the missile to damage it to disable it - the radiation effect of a nuclear ABM warhead at quite some distance would be enough to prevent it detonating on arrival. Hence the military pushing ABM systems so hard despite the apparent "hitting a bullet with a bullet" problems - in fact at the time they didn't need to be anywhere close and one big warhead could potentially take out swarms of warheads.