I’ve heard that cooling systems and piping were sometimes required on nuclear capable aircraft.

I think the Su-24M was an example of this (export models lacked this cooling system).

Possibly. It depends entirely on the weapon design. Not heard of cooling on UK weapons tho’ where the opposite was sometimes the case. Red Beard for example had to be kept warm and within strict temperature limits, esp on the exposed deck of a carrier in Atlantic waters. Usually the problem related to the non-nuclear components, esp the HE implosion system. I believe an electric blanket was issued. Formula 1 cars keep their tyres warm in a similar manner.

SEF key

Actually, the SEF key wasn’t included in the original specs or design. It was very much an afterthought after a guy named Cousins in RAE Weapons Dept spotted a flaw in the arming sequence.

The two flip-out velocity sensors were included to prevent the arming sequence beginning if the bomb rolled of the back of a truck (or similar event). They triggered at approx 120 knots. So a prolonged fall was needed. However, Cousins spotted that they could be triggered by a

PSYCHOPATH TYPE OF INDIVIDUAL

and

… a determined character possessing a working knowledge of the bomb could by a remarkably simple and Health Robinson process activate the environmental [speed] sensor and explode the device. …

The speed sensors could be hot-wired.

The actual original document is uploaded here http://www.nuclear-weapons.info/images/tna-air2-17327note_03-2.jpg signed off by a Squadron Leader Hampton of Operational Requirements 19. An individual not noted for pulling punches.

Subsequent documents disclosed another related issue. Naval versions of WE.177 configured for use as depth bombs had the speed sensors disabled. They were triggered by hydrostatic pistols pre-set to a depth. Loaded on a heliocopter parked on a flight deck, should the weapon break loose, fall onto the deck, and then roll over the side, it could detonate at the preset depth. The Admiral (and all the little fishes) would be very unhappy.

So the SEF key was the chosen solution. It placed a key-break in the circuit between the speed sensors and hydrostat pistols and the weapon’s thermal batteries.

However, the reason it was chosen rather than a PALS system was because a PALS lock would take too long to develop and delay service entry, whereas the SEF lock (while acknowledged as almost worthless to stop a determined psycho) would keep the Whitehall bureaucrats and politicians happy that “something had been done”. When actually, the most effective safeguard was the two-man rule.