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Heavy water works as a neutron moderator because it is a collection of relatively light nuclei (deuterium and oxygen) that naturally exists in a concentrated form and is not made particularly radioactive as a result of being exposed to neutrons.
Explanation:
The purpose of a neutron moderator is to slow down neutrons. Fast neutrons need to enter the material, lose kinetic energy and then leave.
In order for that to happen you need a few things things.
1. Neutrons need to hit atoms. Since Neutrons are not charged, they only hit the nucleus of atoms. The "neutron cross section" of a material means the size of the nucleus as it appears to a neutron. Thus, one wishes a moderator to be made of materials with large neutron cross sections.
2. Neutrons need to lose energy when they hit the moderator. If a small object (like a neutron) hits a large object (like an iron nucleus) it just bounces off and loses little energy. Conversely, if a large object hits a small object, the large object loses little energy. To lose the most energy, you need neutrons to hit nuclei that are near in mass, i.e.
the moderator should be made of very light nuclei packed close together.
3. To lose energy, neutrons have to bounce off the target and not react with the target. Many nuclei absorb neutrons and sometimes they just keep the neutron and sometimes they become radioactive. Either way, it is bad news. So, one wants nuclei in the moderator that do not absorb or react with the neutrons.
Protons in the target are not good because of item (3), they absorb neutrons and make deuterium.
Deuterium is only twice the mass of a neutron, so that works for item (2) and they don't absorb, so good for (3) and it turns out, they are not so bad for the cross section, item (1). The real problem is that pure deuterium is like hydrogen, a gas, and compressing it to a lot of them are close together is hard. Helium gas has exactly the same problem, hard to compress enough.
Of the other light nuclei, they almost all get radioactive except for beryllium, carbon and oxygen. Carbon was one of the first moderators and requires special care so that it does not catch fire. Beryllium is also used, but it is also expensive. Oxygen, being heavier starts to lose attractiveness, but it has the wonderful property that it can combine with deuterium to make heavy water.
Thus, heavy water works well because one gets both a dense collection of deuterium nuclei and oxygen nuclei and both are relatively resistant to becoming radioactive or absorbing neutrons. In addition, it does not require any special treatment for storage and sits around it barrels for years waiting to be used.
What else can I help you with?
What defines the process in a nuclear reactor that reduces the speed of neutrons so they can be captured by the reactor fuel in order to continue the chain reaction?
You did not provide the list of "the following". However, the answer to the question is moderation. Moderation is the process whereby the neutron is slowed down in order to facilitate its subsequent capture by the nuclei of the fuel.
How can nuclear fission be slowed?
Nuclear fission can be slowed by inserting control rods, such as boron or cadmium, into the reactor core. These control rods absorb neutrons, reducing the number available to initiate fission reactions and thus slowing down the rate of fission in the reactor.
A neutron is an unstable particle outside a nucleus but to start nuclear fission requires the slowing down of a neutron. How is nuclear fusion carried out in the case of uranium?
The neutrons released from Uranium are fast neutrons. In a reactor they are slowed down by a moderator. The moderator could be water, heavy water, graphite, among others. When the neutron is slowed down, it is more likely to create fission.This is what happens with the U-235. The U-238 does not fission, but it does transmute through a series of neutron absorption and beta decay etc. into plutonium which does fission also.
Is moderation of neutrons always used to slow nuclear fission?
No, moderation of neutrons is not always used to slow nuclear fission. In some types of nuclear reactors, such as fast breeder reactors, fast neutrons are intentionally not moderated to slow down the fission process. These reactors operate using fast neutrons to sustain a chain reaction. However, in most commercial nuclear reactors, moderation of neutrons is employed to slow down the fission process and maintain a controlled chain reaction.
What is the elementary particle for neutron?
The elementary particle for a neutron is called a quark. Neutrons are made up of three quarks: two Down quarks and one Up quark.
What is the slowing down of neutrons to increase the probability of their absorption by atoms in uranium called?
neutron moderation
What is the slowing down of neutrons to increase the probability of their absorption by the Atoms in uranium called?
neutron moderation
What is the slowing down of neutrons to increase their probability of absorption by the atoms in uranium 235 called?
Neutron moderation
What defines the process in a nuclear reactor that reduces the speed of neutrons so they can be captured by the reactor fuel in order to continue the chain reaction?
You did not provide the list of "the following". However, the answer to the question is moderation. Moderation is the process whereby the neutron is slowed down in order to facilitate its subsequent capture by the nuclei of the fuel.
In neutron moderation neutrons are slowed down so that?
they will more likely be absorbed by more atoms in uranium-235
What does neutron moderation counteract?
the minimal chance that newly formed neutrons of a fussion reaction are absorbed by moore atoms
Does graphite slow neutrons down?
Yes, graphite can slow down neutrons through a process called moderation. Graphite has a lower neutron absorption cross-section compared to other materials, allowing neutrons to penetrate and interact with the carbon atoms, effectively slowing down the neutrons.
What is neutron moderation used to counteract against?
Neutron moderation is used to counteract against the high speed (fast) neutrons produced during nuclear fission. By slowing down the neutrons through moderation, they are more likely to cause further fission reactions in nuclear reactors, sustaining the chain reaction.
Slows down neutrons in a nuclear reactor?
In a nuclear reactor, materials such as water or graphite are commonly used as neutron moderators to slow down fast-moving neutrons to speeds where they are more likely to induce fission in uranium or plutonium atoms. Slowed down neutrons are termed thermal neutrons and are key to sustaining a nuclear chain reaction.
What does thermal indicates in thermal neutrons?
A thermal neutron has much less energy / velocity than a fast neutron. As a result, it has a much larger neutron absorption cross section, making it easier for it to be absorbed by certain nuclei and subsequently initiate fission. The fast neutrons that result from fission are slowed down, i.e. moderated, usually by water, in order to become thermal neutrons and to sustain the fission chain reaction.
How can nuclear fission be slowed?
Nuclear fission can be slowed by inserting control rods, such as boron or cadmium, into the reactor core. These control rods absorb neutrons, reducing the number available to initiate fission reactions and thus slowing down the rate of fission in the reactor.
How can the rate of reaction be slowed down in a nuclear fission reactor?
By inserting the control rods which absorb neutrons using boron, cadmium, or other material with a large neutron capture crosssection. If the reactor should begin to run out of control the SCRAM system will suddenly insert large amounts of neutron absorbing material, instantly stopping the neutron chain reaction.
