Further to TSM's post a couple of thoughts. Firstly the Nimrod bomb bay is protected by fire extinguishant on certain occasions.
"Regarding the bomb bay fire extinguisher issue. The Nimrod has a 'trooping role' and in order to be used in that manner 6 extended range fuel tanks would be fixed in the bomb bay. With this fit 10 fire extinguishers would also be fitted. However, in the normal fit we do not have either the extended range tanks or the fire extinguishers fitted. That means that day to day there is no fire protection in the bomb bay. However, the bomb bay fire drill calls for all stores to be jettisoned, and to my knowledge there has only been one actual bomb bay fire in the life of the ac. This was caused by an electrical fault causing a flare to ignite when power was supplied to the weapons carrier."
I am sure it doesn't take the brains of a rocket scientist to work out how many would be required for an empty bomb bay.
Secondly, when the Nimrod was originally designed nobody expected it to do AAR. So, if AAR is the weakest link and it is not possible to safely protect the bomb bay then bloody well stop doing AAR and stop letting the likes of AOC2Gp overrule safety decisions.
Furthermore, if hot air is the likely reason for the ignition, (The theory has now been explained to me), TSM is quite correct in that some fire extinguishant may well habe bought the boys enough time to get the aircraft on the deck.
Finally, OBIGGS is a one shot system if it fails to put the fire out in the duration of the nitrogen charge, then the fire could come back. PROBIGGS is a constant nitrogen producing system. But guess what, it costs money.