MILNET: Nuclear Weapon Detonation

This is intended to be a brief explanation of how a nuclear weapon is detonated. It is based upon interviews but also consists of conjecture and supposition as the author has never been cleared to know anything classified about the actual physical construction of nuclear weapons.

However the physics are pretty widely known. Key to the nuclear detonation is critical mass. Placing two pieces of subcritical mass together will generate heat and deadly radiation. However this will not create a nuclear explosion and unless your are using your hands to put the sub-critical masses together, the danger is minimal.

What is necessary for a nuclear detonation is to simultaneously (we are talking VERY simultaneously...nuclear triggers are devices which make the events occur within fractions of a second akin to computer speeds) compress sub critical pieces together in just the right configuration. We are trying to lodge loose neutrons which in turn bump other neutrons, and so on, creating the chain reaction. Dropping two pieces of critical mass from the top of a ten story building will not make this work. The pieces must be brought together in a certain form, then a huge compression of the entire mass must take place very quickly after that.

What happens otherwise depends on what goes wrong in the effort. For instance, take an unarmed bomb falling from a plane accidentally. The case of the bomb will absorb energy from the impact with the ground. The high explosive surrounding the core will explode in uneven sequence, that is the stuff that hits first will probably go off first (if at all...a number of bomb accidents have shown that the HE just decides not to go bang at all!). This uneven explosive force will instantly unshape the carefully shaped core area and prevent the attainment of critical mass and make the necessary huge levels of compression at just the right moment impossible.

Moreover, many nuclear weapons rely on physical movement of a sub critical path into the core. When this is the case, even a "mounted" sub critical mass is in the wrong position to detonate. This is not like the little toy bombs the baby boomer generation may have played with...a little striker pops the top of a cap when the little plastic bomb hits the ground. No, just banging the bomb around does not make the masses clump together and then it wants to go off. On the contrary, the one subcritical mass must be positioned precisely in position in relation to the other subcritical mass AND the triggers must detonate the surrounding mass of explosive AND compress exactly at the right moment to ensure a chain reaction takes place.

Any one of these precise events must not fail or there will be no nuclear explosion.

Fizzles A fizzle is a case where low order nuclear reaction takes place. A weapon designed improperly or damaged by physical stress may, when triggered appropriately (dropping won't make it happen), begin the process of a chain reaction. But as the nuclear events begin to occur to sustain that chain reaction, improper physical positioning introduces inappropriate elements into the cycle. For instance, a moderator used to reflect neutrons back into the core, may itself have been forcefully injected into the core by a physical event (bomb hitting the ground). This kills the reaction before it ever develops into a sustained chain reaction. Misshaped cores, misdirected compression explosions, and many other events cause the reaction to fizzle out, not sustaining itself long enough to get the massive compression and temperature needed to sustain a chain reaction.

The fizzle is quite dangerous, however, not in terms of mass destruction perhaps, but dangerous never-the-less. High energy neutrons escape from the fizzle almost by definition (it is their escape that ceases the chain reaction). Not a big bang, but a very deadly burst of radiation. A fizzle is used by fiction writer Tom Clancey, in the book (and now a movie) The Sum of All Fears, describes the explosion of an improperly built terrorist weapon...a thermonuclear weapon that denegrates into a fizzled fission explosion ...

Tom Clancey's book (as opposed to the movie) is a fizzle because the construction crew was murdered before they could add the key ingrediant, a tritium "squirt" that would help eliminate chemical/atomic impurities and also boost the yield of the weapon. Because the weapon in Tom Clancey's book is very old and its nuclear material has degraded, it is a rather large fizzle (destroys a footbal stadium). In the movie, they go ahead and use a "full up" nuclear explosion, pretty much levelling Baltimore's central district around the football stadium.

Our definition of a fizzle is NOT the fizzle in Tom Clancey's book. Clancey's weapon is a full fledged, triggered weapon, intact and for all intents and purposes, a well designed and fully functional weapon.

Our definition of a fizzle is a damaged weapon such as would be found after dropping off an aircraft. Dangerous to those immediately around it due to radiation, but not certainly the city killer alarmists might proclaim. In fact, a fizzle of this kind would not go bang at all. No flash of light. Quite possibly no one would know an event occurred unless of course it fell in your lap, the HE went bang to create the fizzle.

The fizzle would most likely generate lots of heat, emit dangerous radiation and if near combustible material (jet fuel for instance) could start a fire. The fire of course would burn the remaining HE and an (another) conventional explosion could occur. But this has to be emphasized, it is a conventional bang that would toss around bomb casing and core material...dangerous from a radiological contact and shrapnel viewpoint, but again not a white ball of fire that takes out the crash site and all the country side for 10 miles.

High Explosive (HE)

Nuclear weapons accidents indicate that HE content of larger bombs is extremely dangerous, leaving craters of 25 feet wide and 10s of feet deep when exploded. A blast like that could certainly take down a building. But this is also doable with a fertilizer bomb like that used at Oklahoma City.

Fire is actually the most dangerous element of a broken nuclear weapon. Smoke from the combustion of unexploded HE, mixed with bomb casing material and core material can create a deadly toxic and radioactive cloud (not fallout but smoke particles dosed with actual unranium or plutonium is extremely toxic do to their basic nuclear radioactivity -- think of holding Plutonium 239 in your hand). This is a severe danger, but again not even in the same scale as the nuclear detonation.

Would I want a nuke accidentally falling in my backyard? Nope. But I also don't want four 2,000 pounders from an A-10 snuck in there either. The HE in a good sized nuke can run upwards to 4 tons. Not exactly my idea of pet food.