Consider a hypothetical process that speeds up radioactive decay a little, e.g., alpha decay. For the sake of argument, suppose a strong magnetic field, not hooked up to a control system, could do this. Radioactive decay is a statistical phenomenon, and (in our hypothetical scenario) you've just turned the dial a little.
Here is a non-hypothetical example in the other direction: "Some Experiments on the Decrease of Tritium Radioactivity:"
Along the lines you described, if increasing or decreasing radioactivity could only be accomplished with a feedback loop, then I suppose Reifenschweiler would have seen the reaction stop dead. Assuming his results are valid, that is not what happened.
Moderating a fission reaction by slowing down neutrons with graphite or water (moderator) or quenching it with boron are not feedback mechanisms. In operation they act more like a rheostat. On the other hand, neutrons from one reaction triggering another reaction is a feedback loop. It can go out of control, as everyone knows.
I don't think this would result in the instabilities you describe. That suggests to me that if the feedback loop is slow enough to respond, there wouldn't necessarily be instability when the inducing process (a magnetic field in our case) is hooked up to a control system.
Getting back to cold fusion, there is a feedback loop from heat. However this is very different from direct feedback in a chain reaction. In my book, p. 103, I wrote:
Most researchers think that a runaway reaction or explosion [from cold fusion] is impossible for three reasons:
- Cold fusion only works with an intact metal lattice.
- It ramps up relatively slowly, so it would destroy the lattice before it could increase to high levels.
- It is not a chain reaction. In a uranium fission chain reaction, one event directly triggers two or more others, and the reaction can increase exponentially over a very short time (80 generations in 1 microsecond).
Cold fusion can raise the temperature of the metal, and this higher temperature often causes more cold fusion activity. This is called positive feedback. A wood fire works the same way: the heat from an open flame rapidly vaporizes and ignites more fuel, accelerating the fire. But neither a fire nor cold fusion is a chain reaction in the same sense fission is.