Moderator, Coolant & Control Rod : Part I

As we always mentioned in most of our posting, do you all really know and understand  what is actually the moderator, coolant and control rods  is all  about???

       Well, today i would like to share more about the informations regarding moderator used in nuclear reactor.  As discussed in Reactivity II,  we knowing that the neutrons produced from the fission reaction having very high speeds which also called as fast neutrons. Most of the power reactors in the world, are thermal reactors. Which uses thermal neutrons to cause fissions. For the reactor to operate, the fast neutrons released during fission must be slowed to thermal energies before they will cause another fission. By doing so,  thermal neutrons are capable of sustaining a nuclear chain reaction involving uranium-235.

       The moderators  used to slow down the high speed neutrons flying all around the reactor core.  What wrong with the neutrons if they moving in a very high speeds?  If a neutron is moving too fast, and thus is at a high-energy state, it passes right through the 235U nucleus. It must be slowed down in order to be captured by the nucleus and to induce fission without absorbing them. The most common moderator is water, but sometimes it can be another material.

       To achieve the results we wanted moderators must functioning effectively by  thermalize the neutrons in as few collisions as possible over a short distance. Not to absorb too many of the neutrons. Fast neutrons lose their energy mainly by elastic collisions with other nuclei. Elastic scattering with light nuclei is more effective than elastic scattering with heavy nuclei. Based on experiments, it takes an average of 18 collisions to thermalize a neutron in pure hydrogen but 2172 collisions to thermalize the same neutron in U-238. Therefore, only light nuclei are suitable as moderators.The second point is low absorption. Boron-10 could thermalize a neutron in ~90 collisions but, with an absorption cross-section of 3840 barns it would absorb the neutrons it thermalized.

        In fact, light water thermalizes a neutron faster than either heavy water or graphite (higher scattering cross-section coupled with fewer collisions to thermalization). However, light water’s absorption crosssection is 664 times that of heavy water and 195 times that of graphite. Due to light water’s neutron absorption, it is impossible to obtain a self-sustaining chain reaction with natural uranium fuel and a light water moderator. Light water moderated reactors must use 2 to 3% enriched fuel (uranium in which the percentage of U-235 has been increased from 0.7% to 2 or 3%).

         Because of these nuclear considerations and other engineering and economic considerations, only three moderators are suitable for thermal reactors arw  light water (H20), heavy water (D2O) and graphite. Moderators  like water (H₂O), heavy water (D₂O), beryllium metal and oxide, and graphite are inserted in reactors for this reason .

 Here is all about the moderator, coming next is the "Coolant" in reactors... ^_^