However, not all substances are suitable for this slowing down process. Many materials will absorb the neutrons while they are slowing, and others will barely react with neutrons at all. For this reason water is used as the slowing medium or moderator in many reactors, the RCF included. The fuel in the RPI Reactor Critical Facility cannot become critical unless it is submerged in water. As mentioned above, the reactor power depends on what the neutron population is doing from one neutron generation to the next.
Many factors effect the neutron population.
As mentioned before, many substances absorb neutrons removing them from the reactor. Many other neutrons simply leave the reactor core and are lost. The key is to control the number of neutrons so that those available to produce fissions balance those that are lost. We also want to be able to adjust power, that is, increase the number of neutrons available for fission to increase power, or reduce their number to lower power. This is accomplished with control rods.