Why is carbon a good moderator

Notice how much more probable fission is for slower neutrons. Also note logarithmic scale. Some 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 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 H 2 O does the trick, giving the name to light-water reactors. Neutrons can be parasitically absorbed by fuel as they are slowing down in reactions that compete with fission.

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. There 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 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. 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 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.

Graphite has been a popular moderator in the past, however, one drawback is that it needs to be extremely pure to be effective. So that is what happens here when it when the neutron hits the lead atom, it simply Pontus back with almost exactly the same speed on Dhe. Let does not absorb any NGO tall, whereas carbon. When you ranches the carbon trans for some off its momentum into the carbon and it bounces back with a much smaller velocity on the carbon, takes off its gigantic energy and start vibrating, which heats up the hole.

What could I say? Moderator that we have sense. Carbon is a much better moderator than leg because it's efficiently absorbs Arctic energy.

Nuclear Reactor Lead often is used as a radiation shield. Why is it not a go… Explain the functions of a moderator and a control rod in a nuclear reactor.

What is the difference between a moderator and a control rod in a fission re… Problem Is the mass of an atomic nucleus greater or less …. View Full Video Already have an account? Sri Datta Vikas B. Problem 50 Medium Difficulty Why is carbon better than lead as a moderator in nuclear reactors? Answer Carbon is more effective stimulator of fission reaction than lead. View Answer. Discussion You must be signed in to discuss. Top Physics Educators Elyse G.

Cornell University. Marshall S. University of Washington. Aspen F. University of Sheffield. Jared E. University of Winnipeg.