Nuclear Fusion: Radioactive Decay?

[A M]

Radioactive decay modes always release energy;
but why can't nuclear fusion of light elements be a mode of radioactive decay?
I guess because although such processes are exothermic, we need an inaccessible fairly high amount of energy to overcome the electrostatic repulsion barrier.

But now, I'm facing two more challenging questions:
What exactly is the universally agreed upon definition of radioactive decay?
If nuclear fusion is not a decay mode because we need energy as input for such processes, don't we also need energy for splitting a nucleus in [fission] decay modes? (Like SF, CD, α, p.)

[Note for mentors: These are not my OP questions, they're only some of my questions by which it is better to start discussion. I'll ask the rest during the discussion ]
Edition: fusion is a decay? what a silly idea I had!

 

[Dr.AbeNikIanEdL]

I would say a decay is a $1\to n$ process. Of course this is just a label that has nothing to do with physics, so I am not sure what the question is actually about.

 

[A M]

I would say a decay is a 1→n1→n1\to n process.

What do you mean?

 

[Dr.AbeNikIanEdL]

I mean a process where 1 particle decays into several (usually 2). That is what the word decay means to me. Fusion starts with two particles (at least).

 

[A M]

Yes, I know it's 'decay' after all.
According to nuclear stability, a nucleus is stable if it can't go or hasn't gone under radioactive decay.
It means that stability is not the ability of a nucleus not to change into other nuclei?

 

[Dr.AbeNikIanEdL]

Sure, “this particle is stable” does usually not mean “this particle can not undergo interactions” if that is your question.

 

[A M]

Iron-56 & nickle-62 have the highest binding energies per nucleon; meaning that if they go under a nuclear reaction changing them to other nuclei, the process is endothermic and thus very unlikely.
So they are usually known as the most stable nuclei.
But according to what you said, how can binding energy per nucleon be a parameter upon which stability of stable nuclei depends?

 

[Dr.AbeNikIanEdL]

I am having the feeling we are just discussing nomenclature here. Is there an actual physics question this is building up to? Do you want to as about the use of the word stability in a particular context? Then it would be nice to provide some reference.