The temperature of a nuclear explosion won’t cause the nuclei of calcium atoms to come apart into helium and hydrogen nuclei. That process needs even more extreme conditions.

What it will do is excite and largely strip the electrons from those nuclei. That’s what’s causing the behavior of the spectral lines.

The temperature of a nuclear explosion won’t cause the nuclei of calcium atoms to come apart into helium and hydrogen nuclei. That process needs even more extreme conditions, because the transition the other way (toward heavier nuclei) is energetically preferred: fusion liberates energy by colliding lighter nuclei and turning them into heavier ones.

What the high temperature will do is excite and largely strip the electrons from those nuclei. That’s what’s causing the behavior of the spectral lines.