What is a moderator?By Nick Touran, Ph.D.The moderator of a nuclear reactor is a substance that slows neutrons down. In traditional nuclear reactors, the moderator is the same thing as the coolant: it’s water! When fast neutrons strike the hydrogen atoms in H2O, they slow down a lot (like a billiard ball striking another). There are other good moderators like graphite, beryllium, and more. Show
Why moderate?When an incoming neutron causes the nucleus of an atom to split, other neutrons are released at very high speed. If at least one (on average) of these neutrons can be made to cause split another fuel atom, a sustained chain reaction is possible. As it happens, fuel atoms (like uranium) are more likely to absorb a neutron whizzing by if it’s going slow (see Figure 1). So, many reactor designs use moderators to make a chain reaction easier to attain.  Figure 1 The probability splitting a U235 nucleus vs. the energy of the incoming neutron. Notice how much more probable fission is for slower neutrons. Also note logarithmic scale. Choosing a moderator materialSome materials are better at slowing down neutrons than others. Conservation of energy and momentum laws explain that a neutron (mass 1) cannot slow down much after a collision with a heavy nucleus, like a fuel atom (mass 235). However, in a collision with Hydrogen (mass 1), a neutron can in fact slow down very far. Imagine a ping-pong ball bouncing off a bowling ball, as opposed to the same ping-pong ball colliding with another ping-pong ball. Clearly, the ping-pong ball will do a better job of slowing things down. So what material has a lot of light elements like hydrogen in it and will make a good moderator? Good old H2O does the trick, giving the name to light-water reactors. Why have separate moderator and fuel lumps?You could mix your moderator and fuel together, but it’s much better to keep them separate. Neutrons can be parasitically absorbed by fuel as they are slowing down in reactions that compete with fission. So if you can have a fast neutron move into the moderator where it will slow down "in peace," i.e. where there are no strong neutron absorbers, and then have it re-enter the fuel as a slow neutron where the probability of fission is extremely high, then it will be easier to make your chain reaction work. This fact was required in the first reactor, CP-1, which used natural uranium because there was no enriched uranium back in the early 1940s. What is a Thermal vs. Fast reactor?Thermal neutrons have moderators that allow many neutrons to slow down to thermal energies (i.e. the speed that atoms are vibrating in the surrounding materials due to their temperature) whereas fast reactors don’t have a moderator and their neutrons stay at high energies (i.e. they move fast). See Also
© Whatisnuclear.com 2007-2022 | CC-BY-NC Navigation menuNeutron moderators are a type of material in a nuclear reactor that work to slow down the fast neutrons (produced by splitting atoms in fissile compounds like uranium-235), to make them more effective in the fission chain reaction. This slowing or moderation of the neutrons allows them to be more easily absorbed by fissile nuclei, creating more fission events (see Figure 1). Materials used for moderation need to a very specific set of properties. First, a moderator cannot absorb neutrons itself. This means that the moderator should have a low neutron absorption cross-section. However, the moderator should be able to slow down neutrons to an acceptable speed. Thus, in an ideal moderator the neutron scattering cross-section is high.[1] This neutron scattering is a measure of how likely a neutron will interact with an atom of the moderator. If the collisions between neutrons and nuclei are elastic collisions, it implies that the closer in size the nucleus of an atom is to a neutron, the more the neutron will be slowed. For this reason, lighter elements tend to be more efficient moderators.[1] The table below shows that common moderators have a low neutron absorption cross-section but a comparatively large neutron scattering cross-section. Figure 1. When a slow neutron collides with a fissile material like Uranium-235, it produces fast neutrons. The moderator will then slow these fast neutrons, and produce more slow neutrons to continue the nuclear chain reaction. When this process is repeated the fissile events are doubled each time. [2] Properties of Common Neutron Moderators [3]
Types of Moderating MaterialsThere are several different types of moderating materials, and each have places where they are used more effectively. Typically-used moderator materials include heavy water, light water, and graphite. The relative properties of these materials are compared below. The moderators vary in terms of their moderating abilities, as well as in their costs. Light waterLight water (no different than regular water) is used in many reactors because it contains large amounts of hydrogen. Hydrogen works well as a neutron moderator because its mass is almost identical to that of a neutron. This means that one collision will significantly reduce the speed of the neutron because of the laws of conservation of energy and momentum.[1] In addition, light water is abundant and fairly inexpensive. One drawback is that hydrogen has a relatively high neutron absorption cross-section because of its ability to form deuterium. Thus light water can only be used as a moderator along with enriched fuels. Reactors that use light water are known as light water reactors and include the pressurized water reactor (PWR), the boiling water reactor (BWR), and the supercritical water cooled reactor (SCWR). Heavy waterHeavy water is used in reactors because its benefits are similar to light water, but since it contains deuterium atoms, its neutron absorption cross section is much lower.[1] The main disadvantage to the use of heavy water is its high cost of production, as it is made using the Girder-Sulfide process. Reactors that use heavy water include the CANDU designs and the pressurized heavy water reactor. GraphiteGraphite has been a popular moderator in the past, however, one drawback is that it needs to be extremely pure to be effective. Graphite can be made artificially using boron electrodes, however, since boron is a very good neutron absorber— a small amount of contamination will make the graphite an ineffective moderator.[1] One benefit of graphite is that even at the high purity that is necessary for graphite to perform well, it is available at a fairly low price.[4] In addition, graphite is a good moderator as it is thermally stable and conducts heat well. However, at high temperatures the graphite can react with oxygen and carbon dioxide in the reactor and this decreases its effectiveness. Another potential issue with using graphite as a moderator is its ability to oxidize in the presence of air, and its low strength and density which could cause it to change dimensions in the reactor.[4] Reactors that use graphite moderator include the RBMK, pebble bed reactors, and the magnox reactor. AlternativesNuclear reactors can be either thermal or fast. Currently, almost all operating reactors are thermal and thus require a moderator to slow down fast neutrons to the thermal level so that nuclear fission can continue.[5] However, in fast reactors a moderator is not needed, and the neutrons within it move much more quickly. Fast reactors are beneficial as they enhance the sustainability of nuclear power. This is because they have the ability to get more neutrons out of their fuel, can transform nuclear waste into products that decay more quickly, and they respond better to potentially catastrophic equipment failures. However, they are more expensive, and they can overheat fairly easily.[5] For Further Reading
References
What is the purpose of a moderator?A discussion moderator or debate moderator is a person whose role is to act as a neutral participant in a debate or discussion, holds participants to time limits and tries to keep them from straying off the topic of the questions being raised in the debate.
What does a moderator do in a nuclear reactor GCSE?The moderator and control rods together control the rate of reaction in the core of the nuclear reactor. Most nuclear reactors use water as a moderator, which can also act as a coolant, although some do use graphite rods.
|
What DO moderators DO in a nuclear reactor
direito autoral © 2024 cumeu Inc.
