From my understanding nuclear fission takes place in two different scenarios and correlates with the speed of a neutron. In terms of material, we have fissionable material which undergo nuclear fission after attaining a fast neutron or a relatively slow/low energy neutron. We also have fissile material which will only undergo fission when capturing a slow/low energy neutron. Nuclear fission in general happens when we start chucking neutrons at a heavy nucleus to cause it to become unstable and split into two. Massive amounts of energy and free neutrons are a byproduct of this reaction.
Suppose I was to create a field that messed around with the required ranges to induce nuclear fission in both fissile and fissionable material such that the speeds never line up appropriately for the heavy nucleus we are throwing free neutrons at (i.e. neutrons are either too fast, or way to slow in a range based on the nucleus material type). Maybe I start violating the Pauli Exclusion Principle to mess with the critical energy required to start a fission reaction, or maybe I just use pure handwavium to get the field up and running. Either way, I am messing with neutrons such that when in the field they cannot induce a nuclear fission reaction based on their speed.
What would the unforeseen consequences of this be? Would reality cease to be, or would we have a region with no nuclear fission possible with everything else fine and dandy. For example by messing with the speed and energy of free neutrons, would ionizing radiation from neutrons be possible, would radioactive decay even be possible, would it be possible to create isotopes etc.