Certainly some of the red-hot radioactive waste with short half-lives has to be doing something.
A poster up the line noted that when the Japanese point to a problem they're probably trying to hide a worse problem. No doubt we will all find out what that is over the next few months.
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.