Why modern nukes aren't as clean as people think

In a recent appearance on Real Time with Bill Maher, the astrophysicist Neil deGrasse Tyson commented that “modern nukes don’t have the radiation problem”. I wanted to share my thoughts on why I believe Mr. Tyson is right about the science, but misunderstands how modern nuclear weapons work.

Nuclear bombs have many destructive features. The one I’m going to talk about here, is nuclear fallout. Fallout is produced when left-over radioactive particles (which are produced by a nuclear blast) fall back down to earth where they emit their DNA-altering, radioactive death rays. It’s this radioactive fallout which keeps an area un-usable for days, months, or even years after a nuclear explosion.

There are two types of nuclear reactions (not bombs, but reactions – the difference is important) that produce the massive amounts of energy released by nuclear weapons: fission reactions and fusion reactions.

Fission reactions produce their energy by splitting atoms of heavier elements (typically Uranium-235) apart. The parts into which those uranium atoms are split (called nuclear fragments) are highly radioactive. When a fission bomb explodes, it’s the dispersion of these tiny radioactive particles which we call "fallout".

Fusion reactions produce energy by fusing two lighter atoms together (originally hydrogen isotopes were used, hence the name "Hydrogen Bomb"). Since this process binds atoms together, as opposed to a fission reaction which pries them apart, it doesn’t create any of those nasty radioactive fragments. Also, the fused atoms (created by the fusion reaction) are more stable and therefore don't release the same amount of radioactivity.

So far, so good: Fission reactions bad. Fusion reactions good.

The problem is that there is no such thing as a "pure" fusion bomb (now we are talking bombs, and not reactions). That’s because, at least at the moment, humans haven't figured out how to create a fusion bomb without using a fission reaction as a trigger. Essentially, at the heart of every environmentally friendly fusion bomb sits a dirty little fission secret.

But it doesn’t end here, unfortunately.

Scientists soon discovered that if they housed a fusion reaction in a casing (called a tamper) of Uranium-238, they could improve the efficiency of the fusion reaction. Using U238 to boost the efficiency for a fusion reaction also had another added benefit: a second (yes… second!) fission reaction.

Under normal circumstances, U238 can’t be used for fission reactions which is why fission-only bombs use U235. However, if you bombard U238 with enough energy you can get a fission chain reaction going. Guess what produces a lot of energy… fusion reactions! Not only does adding U238 make the fusion reaction more efficient but it also acts as fuel for a secondary fission reaction once the fusion reaction gains in strength.

The majority of modern nuclear weapons with a yield of more than 50 kilotons (small by contemporary standards) use this three-stage approach:

1.??????A “small” fission explosion of U235 as a trigger

2.??????A much larger fusion explosion

3.??????A secondary (and much larger) fission explosion coming from the U238 tamper

All of this means that fusion bombs generate at least as much fallout as fission-only bombs. Also, because fusion bombs make a much larger fireball, it can disperse that fallout over a much wider area. That’s because the bigger kaboom of a fusion bomb allows it to propel those radioactive particles (generated by the fission reaction) to much higher altitudes.

The conclusion: fusion reactions might be clean(er) than fission reactions, but the same cannot be said about fusions bombs. Not yet in anyway.

Have a safe weekend.