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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.
Wiki User
∙ 11y ago
Wiki User
∙ 14y agoOnly a neutron absorber such as a control rod containing boron or some other strong neutron absorber, or possibly a change in temperature in the reactor. Most reactors are designed so that an increase in temperature will have a negative effect on reactivity, this is called a negative reactivity coefficient and is desirable for reactor control.
Wiki User
∙ 14y agoActually, it is not technically necessary to slow the neutrons down to continue the chain reaction. Nevertheless, they are slowed down to control the reaction in nuclear reactors.
Slowing them down is done by giving them things to collide with. These are called moderators, and often consist of graphite rods, heavy water (deuterium oxide), or light water (ordinary H2O).
They slow down because the collisions absorb kinetic energy from them, so when they bounce off, they are less energetic and, thus, slower, and colder
Wiki User
∙ 14y agoSlowing down neutrons is called moderation of neutrons. The medium is called moderator. Moderator examples are water and graphite.
Decreasing the number of slow-moving neutrons is called absorption. Absorption process includes either capture or fission processes.
Wiki User
∙ 14y agoThe use of a moderator. Boron, beryllium, and heavy water are commonly used.
Wiki User
∙ 10y agoControl rods are used inorder to slow down the fast moving neutrons , in a fission reactio which occurs in a nuclear reactor .eg of control rod , boron and cadmium
Wiki User
∙ 15y agothe moderator
anon
control rods (apex)
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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.
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.
How many neutrons are produced during a typical uranium fission reaction and what is the role of these neutrons in a nuclear chain reaction?
A typical uranium fission event produces 2 to 3 neutrons. These neutrons are moderated (slowed down) and go on to initiate the fission of more uranium. On average, in a controlled reaction that is maintained at normal criticality (KEffective = 1), each fission creates exactly one neutron that is used to produce another fission.
Neutron activation is done most easily at a nuclear reactor facility. Why can't a lamp or an X-ray tube be used to add neutrons to an object's nuclei?
A lamp or an X-ray tube cannot be used to "add neutrons" to other nuclei because lamps and X-ray tubes are not neutron sources. Neutron activation is generally something we do in an operating nuclear reactor. In the core of the reactor, there is a high neutron flux. Many, many neutrons are being released in the fissions that are going on in the nuclear core. Materials that are to be activated are lowered through ports and brought down into the neutron flux. Activation occurs. Lamps or X-rays do not produce neutrons, and cannot be used in neutron activation activities. No neutrons means no neutron activation.
In neutron moderation neutrons are slowed down so that?
they will more likely be absorbed by more atoms in uranium-235
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 the probability of their absorption by 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.
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.
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.
Does graphite slow neutrons down?
The neutrons simply collide with graphite atoms and get slowed by impacts. Some get absorbed but not too many, and it is important to have very pure graphite to avoid absorption by other elements in the graphite as impurities.
Why must neutrons in a chain reaction be controlled?
Neutrons in a nuclear fission chain reaction must be controlled for two reasons... First, they must be moderated, or "slowed down", to exactly the right level of energy required to sustain the reaction. This is because, initially, the neutrons are too fast to sustain the fission reaction. They must be slowed down, but not too much, otherwise the reaction will stop. Second, they must be controlled. You want the reaction to proceed at an orderly pace, at a constant rate. To do this, you need, on a statistical average, exactly one neutron to go on to fission one atom to produce one parcel of binding energy release and one neutron, to repeat without multiplying or dividing. This is what we call KEffective = 1, where the rate of reaction does not change. Moderation and control. In tight balance. Easily upset. Fortunately, when upset, the tendency is to shutdown. That is engineering safety.
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.
What is neutron absorption?
Neutron absorption is the process wherein an atomic nucleus will absorb a neutron. Many different atomic nuclei will do this, and different nuclei will present a larger of smaller target for the neutron, as you might have guessed. This is the neutron absorption cross section for the material, and it varies as the material does and as the kinetic energy of the neutron does, as well. You may have figured out that there are many different resulting products or outcomes that can be seen from neutron absorption. It is neutron absorption that powers up a chain reaction, so let's look at that aspect of this phenomenon. In a nuclear reactor, some spontaneous fissions will release neutrons, and these neutrons will, if the control rods are pulled out sufficiently, begin a chain reaction. The nuclear fuel, usually either uranium or plutonium, will absorb a neutron (after some slowing down of that neutron), and they'll fission as a result. These fissions will release more neutrons, which will be absorbed and will create more fissions, which will release more neutrons, etc. A neutron released from a fission event will have a high kinetic energy; it will be moving very quickly. It might be absorbed, but will have a higher probability of being absorbed if it is slowed down, or thermalized. The moderator in a reactor, usually water, does this slowing down of the neutrons. The slower neutrons have a much higher probability of being absorbed and continuing the chain. Fission by neutron absorption is the mechanism by which a nuclear chain reaction is maintained in a nuclear reactor.
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.
