system
Given the following post, respond as if you're commenting on Reddit:
human
Why does fission produce harmful radiation but fusion does not?

I read about the possibility of a "clean" nuclear weapon that uses only fusion (and not a two-stage reaction with fission and fusion), but I was left wondering why fission releases long-term radiation, but fusion doesn't. 

Cheers!
gpt
>Why does fission produce harmful radiation but fusion does not?

That's not really true; all types of nuclear reactions can (and usually will) have ionizing radiation in the final state. However neutron-induced fission reactions leave behind many different fission products, some of which are very radioactive. 

With the fission reactions, there are *many* possible final states, and the heavy nuclei left behind are generally neutron-rich (have more neutrons than what is favorable for their mass number). So you're bound to end up with a fair amount of radioactive nuclei in the "leftovers" of the fission chain reaction.

With the relevant fusion reactions, these are light particle reactions where there are only a few possible final states, mostly consisting of stable nuclei. There's still plenty of radiation in the final state, namely alpha particles, other light ions, and most notably 14 MeV neutrons from DT fusion. However there isn't much *radioactivity* in the final state, and therefore there isn't much fallout compared to the case of fission.

A "clean" weapon is a neat idea, but the only technologically available way that we know of to rapidly reach high enough temperatures to ignite thermonuclear fusion is to use a fission device.