For a Pressurized Water Nuclear Reactor, Uranium is enriched to achieve 5% U-235 and 95% U-238. U-235 occurs at 1% in nature and is what makes the process happen. The mixture is formed into pellets. The pellets are placed into long tubes which are then arranged with spacing for cooling water channels into what are called fuel assemblies. 

      The fuel assemblies are shipped by truck to the reactor site. They're not highly radioactive. There are some spontaneous fissions but the resulting Neutrons are fast Neutrons. Fast Neutrons just bounce off the U-235 nuclei.           

      It is not until the fuel assemblies are installed into the reactor vessel and water is added that things start to happen. Boron control rods that absorb Neutrons stop the reaction from starting. The control rods are slowly withdrawn to expose a portion of the reactor core to itself. Spontaneous fission neutrons enter the water channels striking Protons in the water, slowing them down. (Not unlike a queue ball striking a billiard ball where the queue stops and the billiard ball moves on.)

     The slowed neutron then migrates through the water channel into adjacent fuel where it encounters a U-235 atom, is “absorbed” and the U-235 subsequently fissions. The fission rate proceeds and a criticality factor = 1 is maintained. That is, one fission creates one more fission, no more, no less. The Boron control rods are periodically withdrawn to expose new U-235 as the initial exposure starts to burn out. The water has two functions;

     1. It provides cooling.

     2. It is the moderator that controls the fission rate. Neutron's have to be slowed down because fast Neutrons just bounce off the U-235 nuclei. Too much fission and the water heats up, becomes less dense and less Neutrons are slowed down decreasing fission rate. Cooler water, more fission occurs. The water moderator maintains the criticality factor = 1. The Reactor is in a sense, Self Moderating.

      In addition, some of the U-238 absorbs some of the slowed neutrons. It becomes U-239 which decays by Beta to become Neptunium 239 (Np-239) which again decays by Beta to become best friend, Plutonium 239. In effect, the reactor manufactures more fuel for itself. In fact, Fast Breeder Reactors 

      So, the big question is; 

Why do the Neutrons have to be slowed down?

(If you liked anything up to this point, You’re going to love this.)

     The answer is; Time........

     Given that a "Free" Neutron decays to an Electron and a Proton in about 14 minutes, it's obviously a particle that contains both Positive and Negative components.

     A slowed Neutron has the very small amount of time required to become polarized in order to attach itself to the surface of the U-235 and subsequently be absorbed to cause fission to occur. Observe;

    Bombs are different. It is desirable to go from a criticality factor of .9 to 2 in less than 10 generations. Introducing Tritium into a Pu-239 pit just prior to detonation provides an abundance of stationary neutrons for fission. 
Also a Be-4 reflector slows and returns first generation detonation Neutrons to the pit giving us a most efficient 5 kiloton yield from a less than non reflected critical mass.