Physics · nuclei · NEET
They do opposite jobs. The moderator SLOWS neutrons down (it does not remove them) so more neutrons become slow enough to fission U-235. The control rod ABSORBS (removes) neutrons from the reactor to keep the number of fissions steady. Moderator = heavy water or graphite. Control rod = cadmium or boron.
U-235 splits most easily when hit by a SLOW (thermal) neutron. Fresh neutrons from fission are very fast (about 10^7 m/s). NCERT shows they must be slowed to about 10^3 m/s so they have a high chance of fissioning U-235. Fast neutrons mostly miss or scatter without causing fission.
By elastic collisions with LIGHT nuclei. A neutron loses most of its kinetic energy when it hits a nucleus of similar mass (like deuterium in heavy water, or carbon in graphite). Heavy nuclei just bounce the neutron back with little energy loss, so heavy materials make poor moderators.
k is the average number of neutrons from one fission that go on to cause the next fission. If k = 1 the reaction is steady (critical) and power is constant. If k greater than 1 it grows (supercritical). If k less than 1 it dies out. Control rods are pushed in or pulled out to hold k = 1.
Not always. The moderator slows neutrons; the coolant carries heat away to make steam. In many reactors heavy water or ordinary water does BOTH jobs, which is why students mix them up. But the roles are different: slowing neutrons vs removing heat.
Heavy water (D2O) and graphite (carbon) are the common moderators, because their light nuclei slow neutrons well through elastic collisions. Ordinary water is also used in some reactors.
Cadmium and boron are used, because they strongly absorb neutrons. Pushing the rods deeper removes more neutrons and slows the reaction; pulling them out speeds it up.
Uranium enriched in the isotope U-235. A slow neutron makes U-235 fission, releasing energy and 2 to 3 more neutrons that continue the chain reaction.
Critical means the multiplication factor k = 1, so each fission causes exactly one more fission on average. The chain reaction is self-sustaining and the power output stays constant.
A reactor uses a CONTROLLED chain reaction (k held at 1 by control rods). An atomic bomb is an UNCONTROLLED chain reaction (k much greater than 1) that releases all its energy at once.
