Which radioisotopes are released into the environment by nuclear explosion?

From the bomb itself there will be released a lot of unused plutonium because the reaction is not one hundred percent efficient. Usually there will also be a lot of uranium, if it is used as the tamper in the fusion stage. From the reacted plutonium and uranium will be released a lot of radioactive daughter isotopes. Other parts of the bomb will become radioactive due to neutron activation from the fission and/or fusion process depending on the design.

If the fireball touches the ground (eg in a groundburst), a great amount of radioisotopes will be generated by neutron activation of the soil. This tends to get draughted up into the cloud and returns to Earth as "fallout".

Neutron irradiation of seawater will result in radioactive sodium. Nasty stuff. Many other elements will also capture neutrons and become radioactive.

Cheers!

Strontium 90 and a number of others.