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... ^_^

Reactivity II

WARNING!!! ... Today topic is slightly nuclear science tuned. Therefore it might be a bit complex, but we are ready to be received comments and questions. =)

Recalling back, reactivity is define as fractional change in neutron population per generation. But today we are more into reactivity coefficient.

In general, reactivity coefficient is governed by 4 other important coefficients which are:

1) Moderator temperature coefficient

2) Fuel temperature coefficient

3) Pressure temperature coefficient

4) Void temperature coefficient

Moderator temperature coefficient, are found to be more effective for light moderators rather than heavy moderators. This is because, light moderator can remove more energy than heavy moderators. Do note that moderator can be in 2 conditions which is under moderated or over moderated. Under moderated condition will help the nuclear system to have self-regulating effect. This means that the reactivity and power produced by the nuclear power plant is under control automatically due to the nature of the system. This will be a relief to all nuclear engineers. On the other hand, over moderated is very dangerous. This is because, over moderated condition will caused the reactor to have higher reactivity, and this will lead to more power being produced. With the high amount of power being produced, the temperature of the core will significantly increase and causes the core in the nuclear reactor to be melted down. Hence, for every nuclear reactor, under moderated condition is always the best choice. But, how do we know if the system is under moderated or over regulated? This can be done by calculating the moderator-to-fuel-ratio, which is a ratio of moderator and fuel.

More explanation regarding moderator will be given in future, I promise. This is because, we need to fulfill certain criteria for our blog on a daily basis and i am rushing out of time.

Fuel temperature coefficient is rather hard to be explained without the graphs, but in short, it can be defined as the change in reactivity per degree change in fuel temperature. This means that there is an increased tendency of the fuel to absorb neutron as the core temperature begins to increase. This is because, an increase in the core temperature will causes the power level and the control chain reaction to be increased. Hence, it will contribute to higher reactivity.

Pressure temperature coefficient is usually negligible in reactors. This is because, most nuclear reactors uses sub-cooled liquid which means that the liquid density is nearly constant and does not change significantly within the nuclear power plant operating pressure.

Void temperature coefficient, is the change in reactivity per percent change in void volume. Hence, it means that void temperature coefficient is the result of the formation of the steam voids in the moderate. An example is, in theory air bubbles formation in most pipe (closed system) calculation is neglected if and only if the value is small enough. However in reality, air bubbles really does exists and it does contribute to a very minor extend such that its existence can be neglected.

That's all for now ....

P/S Sorry for no cool pictures were being uploaded, but it will ... tomorrow on the fun facts label!!