While individual nuclear reactions can release significant energy at the atomic scale, you need a lot of them occurring in the same place over a short time period to get significant energy production at the macroscopic level. This was the basis of chain reaction theory developed by Meitner, Fermi and others in the 1930s. As discussed above, there is no credible evidence that uncontrolled criticality has occurred in the damaged cores.
If you're alluding to quantum effects, things like tunneling and so forth, you really need to do a calculation that relates to the probability of those things happening with the particles that are interacting. That quantity, called the expectation value, is vanishingly small for classical particles interacting at energies in the tens or hundreds of MeV range.
Aside from that, I must admit I don't know what you mean by "windows of opportunity".
Certainly some of the red-hot radioactive waste with short half-lives has to be doing something.
It was. It was decaying, releasing energy, some of which showed up eventually as thermal energy which produced other effects, fuel damage, hydrogen evolution, etc. All of this is well-known and understood from decades (going back to the 1950s) of studying the likely evolution of LWR core accidents. The thermal energy released in decay heat, while producing chemical effects and changes in the state of materials, was insufficient to approach the unification energy for the electroweak force, certainly insufficient to initiate even random fusion events. If you don't have the energy, those things simply don't happen.
Then, finally, the rogue waves were found, and they weren't as rogue as once thought ~ in fact, they are quite predictable ~ today we have computers analyzing satellite images to see if they are out and about.
What a change.
Now, keep that mind as we revisit your statement about ".... That quantity, called the expectation value, is vanishingly small for classical particles ....." ~ no doubt you've had to think about this in the past so what is the explanation for classical wave forms? (understanding that all those particles are themselves waves of some kind, and in various manners linked together).
Can we change the functions that create the columbe barrier in virtually any wave form? Or are we simply stuck with condensed states of matter as suggested by Widom and Larsen?
Seems to me that if we can sink a large ship with simple wave functions we should be able to stuff a proton or two inside a hydrogen nucleus, right?