1. armed
2. prearmed
3. travel
4. proximity failsafe
5. interception failsafe
option 1.
The missle is armed when the SAM/AAM intercepts. Depending on location in relation to the US (assuming we are the 'innocent' victims in this hypothetical plot' and the altitude, one of three things can happen:
- high altitude: nuclear fallout from the nuke spreading throughout the atmosphere. Like water vapor, these superheated nuclear materials will travel with the winds across the world, plaguing everyone. However, it will be diluted in the air and thus not as toxic as if it were lower. Necessary missles for this to occur: cross-continental ballistic or atmospheric 'hoppers'
- medium altitude: fallout will follow the nearest world wind pattern. i.e. gulf stream, jet stream, mediteranean, etc. Fallout will be denser, larger materials. actual nuclear rain wll be observed within air currents affected for up to a 2,000km distance from interception point. about 15-35,000feet
- low altitude: high concentration of nuclear fallout. ground will be devasted from force of heatwave. loss of life expected near 90%. radius small, approx. 10km. similar to the tests in nevada on project manhattan (80-foot tower detonated 'fat man'). about 1,000-10,000 feet
(this is why atmospheric testing is banned...it hurts us all)
option 2.
In the prearmed state, the missle or rocket is between its travel and armed states. This is where failsafes and preemptive strike jetisons are checked and armed before the actual warhead is primed. In case of an interception and the failsafes are not completely checked, the warhead will simply be destroyed or redirected in case of failed obliteration. Possible fallout from 'dirty bomb' syndrome in which the payload is detonated from the interception payload, but the nulear material has not reached critical mass to create the full nuclear effect. Dirty bomb is the cheap way of causing nuclear casualities with a nuclear device. Deaths come from nuclear poisoning caused from nuclear fallout. In this case, fallout will not be fine, but a girtty, sometime marble-sized nuclear-contaiminated materials. Fallout reduced to area directly below point of interception with varibale of wind and humidity.
option 3.
travel is when the rocket is heading towards it's target. Depending on the manufacturer, the rocket is not primed or in its preemptive stages (noone wants a nuke going off on takeoff). Interception is generally obliteration due to combined payload of intercpetion device with concentrated fuel from deliverance rocket. If not, fallout similar to option 2 occurs.
option 4.
proximity failsafe can be viewed one of two ways.
The payload can set a proximity failsafe where if the payload is within a certain distance of it's target and it is intercepted, it will still detonate it's charge. This fail will prime the payload to reach critical mass upon a proximity of the target, to a larger extent than what it would have without. It will be traveling a greater distance in primed status.
Another way is less agressive and more passive, reserved. If the payload is within a certain distance of a target and the payload is primed, a remote deactivation can occur within a certain period of time to disarm the payload and self-detonate the package (what you see in movies featuring a concervative American president watching his nuke approach russia...but at the last minute he gets a call saying 'it wasn't the russians' and he presses the abort button)
interception of a proximity failsail with agressive intent will trigger a nuclear explosion similar to in a low-altitude option 1.
self detonation will render the package harmless
option 5.
Intercpetion failsafe is a totally agressive means of preventing interception. The package will be equiped with anti-intercpetion devices such as heatpemitting flares and a massive ammount of tin strips to deploy to confuse tracking radars. There are generally nuclear0equipped cruise missiles or submarine-based rockets. Short distance travelers, but pre-primed at launch. interception at any point between launch and target acquisition wil result in low-alititude Option 1.
hope this helps. Never thought military knoweldge would come to use in a computer forum

you can view these on the CIA declassifieds spread out on the net. bunch more variables and option, forgot them though...very, well, "if this and this and this happened, and this and this were in play, then this option is possible"