in reacter U-235 fission is due to slow neutrons because in reacter the probability of fission from fast neutron is approximatly zero.

Helium-3 has a high cross section for thermal neutron absorption. For fast neutrons you must either thermalize the neutrons for use with boron or helium-3 based proportional counters or use a fast fission chamber based on fast fission of uranium. It's worth noting that a uranium based fast fission detector is really only useful in extraordinarily high neutron fluxes like that of a reactor.

No, moderation of neutrons is not always used to slow nuclear fission. In some types of nuclear reactors, such as fast breeder reactors, fast neutrons are intentionally not moderated to slow down the fission process. These reactors operate using fast neutrons to sustain a chain reaction. However, in most commercial nuclear reactors, moderation of neutrons is employed to slow down the fission process and maintain a controlled chain reaction.

Nuclear fission