0
Control rods have a large neutron absorption cross section. That means that they slow down, absorb, and effectively stop neutrons. In order to sustain a fission reaction, you need a certain number of neutrons of a certain energy interacting with fissile material at a certain rate so that everything is in equilibrium.
When you insert the control rods, you bring the core geometry to a sub-critical state, what we call KEffective < 1, and the fission reaction stops.
Wiki User
∙ 13y ago
Add your answer:
Earn +20 pts
Is it possible to stop a fission reaction at a nuclear power plant?
Yes. You insert the control rods, deprive the reactor of the moderating effects of water, inject boron, etc.
How is the rate of a fission reaction controlled within a nuclear reactor?
Using control rods that obsorb neutrons, and can be gradualy raised or lowered into the core. In emergencies, "neutron poisons" are used, which almost instantly stop most fission within the core.
What control the chain reaction in a nuclear reactor?
The reason a nuclear chain reaction occurs is that one of the by products of a single reaction: the neutrons can start reactions of their own. So how do you stop a chain reaction? Stop these neutrons from causing any more reactions. This is done in nuclear reactors by placing control rods into the reactor core. These control rods are made of elements that can absorb the high energy neutrons without reacting themselves. Therefore, by moving these rods in and out of the reactor, the rate of reaction in a nuclear reactor can be easily controlled.
How is a nuclear reaction controlled?
Nuclear fission reactions usually depend on neutrons released by fission, which cause more fission in a chain reaction. The neutrons are in what is termed a neutron flux, which really means just that there are a lot of them there. If the flux is insufficient, the chain reaction stops. If it is too dense, the reaction overheats, which can lead to meltdown. Clearly, a way to control this is necessary. The control rods are made of materials that absorb neutrons. By adjusting them, the density of the neutron flux can be increased and the fuel made hotter, or it can be decreased, and the fuel made cooler. Adjusting them means changing their exposure to the fuel, for example, by changing how far they go into the fuel. They can be used to stop the chain reaction by inserting all of them all the way.
What are fuel rods?
Nuclear fuel rodsFuel rods are long, cylindrical rods or long, flat plates containing fissile material (usually uranium) in a nuclear reactor core. The uranium is in rods or plates so it can be bundled into a fuel element and fixed in place to make up the core. The spacing and arrangement of the rods and, beyond that, the bundles themselves, is the key to reactor geometry.Other rods in the reactor called control rods are also moved into channels in the reactor core, but these have the opposite effect. They contain neutron-absorbing materials that can moderate (or stop) the fission reaction that takes place when a critical mass is achieved. Control rods are pulled to start the reactor up, and are put back in to shut the reactor down.
Does a chain reaction involve fission?
Yes, a chain reaction is all fission, just out-of-control fission. Usually, fission creates 2 neutrons per decay, but it is controlled by the fact that lots of neutrons get absorbed by U-238, which doesn't fission, unlike U-235, which does, and by the control rods, which also absorb lots of neutrons. But if these fail to contain the outbreak of neutrons, and the fail safes (which usually just drop the control rods totally into the reactor, stopping any chain reaction) fail, then an exponentially accelerating chain fission reaction can start, and once it starts, it's pretty much impossible to stop.
Is it possible to stop a fission reaction at a nuclear power plant?
Yes. You insert the control rods, deprive the reactor of the moderating effects of water, inject boron, etc.
How is the rate of a fission reaction controlled within a nuclear reactor?
Using control rods that obsorb neutrons, and can be gradualy raised or lowered into the core. In emergencies, "neutron poisons" are used, which almost instantly stop most fission within the core.
Do nuclear isotopes stop decaying when control rods are down?
No, it is not correct; only a nuclear chain reaction can be stopped with control rods.
Why did the control rods not work in japan nuclear facility?
They did work perfectly to stop the fission chain reaction, the problem is the residual heat from the radioactive fission products, which still require cooling. They couldn't be cooled because the water from the tsunami had diluted the diesel fuel that powers the backup cooler for the plant.
What control the chain reaction in a nuclear reactor?
The reason a nuclear chain reaction occurs is that one of the by products of a single reaction: the neutrons can start reactions of their own. So how do you stop a chain reaction? Stop these neutrons from causing any more reactions. This is done in nuclear reactors by placing control rods into the reactor core. These control rods are made of elements that can absorb the high energy neutrons without reacting themselves. Therefore, by moving these rods in and out of the reactor, the rate of reaction in a nuclear reactor can be easily controlled.
How is a nuclear reaction controlled?
Nuclear fission reactions usually depend on neutrons released by fission, which cause more fission in a chain reaction. The neutrons are in what is termed a neutron flux, which really means just that there are a lot of them there. If the flux is insufficient, the chain reaction stops. If it is too dense, the reaction overheats, which can lead to meltdown. Clearly, a way to control this is necessary. The control rods are made of materials that absorb neutrons. By adjusting them, the density of the neutron flux can be increased and the fuel made hotter, or it can be decreased, and the fuel made cooler. Adjusting them means changing their exposure to the fuel, for example, by changing how far they go into the fuel. They can be used to stop the chain reaction by inserting all of them all the way.
What is done to keep the nuclear reactor under control?
control rods act like brakes to slow the neutron chain reaction rate in normal operation. the SCRAM system acts in emergencies to completely bring the neutron chain reaction to an instant stop. even with the reactor stopped, the cooling system must operate to prevent overheating from the radioactive decay of the built up fission products.
What are fuel rods?
Nuclear fuel rodsFuel rods are long, cylindrical rods or long, flat plates containing fissile material (usually uranium) in a nuclear reactor core. The uranium is in rods or plates so it can be bundled into a fuel element and fixed in place to make up the core. The spacing and arrangement of the rods and, beyond that, the bundles themselves, is the key to reactor geometry.Other rods in the reactor called control rods are also moved into channels in the reactor core, but these have the opposite effect. They contain neutron-absorbing materials that can moderate (or stop) the fission reaction that takes place when a critical mass is achieved. Control rods are pulled to start the reactor up, and are put back in to shut the reactor down.
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.
Why you use moderators in a nuclear reaction chamber during a fission chain reaction?
A moderator in a fission chain reaction is a system (usually water) that slows neutrons down (decreases their energy) to the point where they can interact with fissile material, causing the fission reaction to be self sustaining. This is necessary because, without the moderator, the neutrons emitted from fission have too much energy to cause subsequent fission. The design of the moderator is such that it provides automatic control of the reaction. As it heats up, the moderation effect decreases, causing the reaction to decrease. Conversely, as it cools down, the moderation effect increases, causing the reaction to increase. In the event that the moderator fails, such as when a depressurization event causes the water to flash to steam, the loss of moderation causes the fission reaction to stop.
How does a control rod work?
Control rods need to contain material that is a strong absorber of neutrons, so that rapid shutdown can be achieved when necessary. Two such materials are Cadmium and Boron. Both have been used in different types of reactor. The elements may be used as alloys or as chemical compounds, and will be encased in stainless steel to avoid corrosion in the reactor.
