It would have next to zero impact on any present day nuclear fission technology
To oversimplify, in order for a neutron to cause a fissile atom to fission, it needs to be whizzing past the nucleus of that atom close enough to be captured via quantum mechanical processes. We measure the probability of capture by a value called "cross section" where the higher the cross section of an isotope, the more likely it will capture the neutron. Here is the fission cross section for selected actinides:
As you can see, the slower the neutron, the more likely it is for the nucleus to capture it and cause fission. An analogy is to imagine throwing a ping-pong ball (the neutron) at a slightly sticky bowling ball (the uranium nucleus). If you throw the ping-pong ball fast, it is more likely to bounce off, but if you gently toss it, it is more likely to stick.
Neutrons from fission are generally produced with energies around a couple of MeV. Because they are unlikely to interact with fuel atoms at this energy, we design reactors such that the neutrons will scatter off light atoms (usually hydrogen in water or carbon in graphite), hence slowing down enough to the point where they are likely to cause fission. These slowed neutrons generally have less than a couple of hundredths of an eV, well below the 100 keV limit of the field you propose. This is easily achieved with a few tonnes of water or graphite.
Some reactors are designed to use faster neutrons, perhaps in the 200-500 keV region. However, the change in cross section by slowing these down to 100 keV is pretty negligible. Consequently even nuclear reactors (or weapons) that rely on fast neutron fission probably won't be affected much.
In summary, if you brought your "neutron slowing field" into any modern nuclear powerplant, they probably wouldn't even be able to tell if you had turned it on without sensitive equipment.