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What does nuclear moderation accomplish in a nuclear reactor?
Moderation slows or reduces the energy of neutrons in a nuclear reactor. By doing this, moderation allows continuation of the chain reaction. Neutrons will only cause more fission events when they have a specific range of energy, but they have too much energy when they are first emitted from their precipitating event, hence the need for moderation. Moderation also regulates the reaction. In the light water moderated reactor, for instance, a common design, water is the moderator. Water is also the heat sink, carrying away the energy of the reaction to make steam which spins turbines and makes electricity. If reactivity were to increase, temperature would also increase, causing an increase in the number of voids in the water. This reduces the effectiveness of the moderator and tends to decrease reactivity. Similarly, if reactivity were to decrease, temperature would decrease, causing voids to decrease, ultimately causing reactivity to increase. Conversely, if the load changes, that will reflect back into the water temperature, causing reactivity to adjust accordingly. It is a self-stabilizing situation. It is also a safety designed system. If there were a sudden loss of heat sink, such as a turbine load rejection, temperature would go up, causing a decrease in reactivity. If there were a steam line break, causing a depressurization incident, the water would flash to steam and the reactor would go instantly subcritical. In both of these scenarios, there would be time to insert the control rods, forcing the reactor further subcritical, and giving the emergency core cooling systems time to startup.
What is the purpose of control rods in nuclear reactors?
A nuclear reactor requires the neutrons released from one reaction to trigger the fission of other nuclei. Control rods are required to absorb some of these neutrons so as to prevent a runaway chain reaction.
How fission is controlled in a nuclear reactor?
All reactors require some form of controllable neutron absorber to accommodate power changes, which cause changes in the concentration of fission product neutron poisons such as Xenon, and to accommodate fuel burn up caused reactivity changes. The reactor must be held at criticality during steady operation and very near it during slow power changes, and moveable control rods are provided for this, usually containing boron which absorbs neutrons strongly. When the reactor is tripped or scrammed the rods drop fully in, and start up requires a slow careful approach to criticality. The following applies to PWR's and is taken from Wikipedia Generally, reactor power can be viewed as following steam (turbine) demand due to the reactivity feedback of the temperature change caused by increased or decreased steam flow. Boron and control rods are used to maintain primary system temperature at the desired point. In order to decrease power, the operator throttles shut turbine inlet valves. This would result in less steam being drawn from the steam generators. This results in the primary loop increasing in temperature. The higher temperature causes the reactor to fission less and decrease in power. The operator could then add boric acid and/or insert control rods to decrease temperature to the desired point. Reactivity adjustment to maintain 100% power as the fuel is burned up in most commercial PWRs is normally achieved by varying the concentration of boric acid dissolved in the primary reactor coolant. Boron readily absorbs neutrons and increasing or decreasing its concentration in the reactor coolant will therefore affect the neutron activity correspondingly. An entire control system involving high pressure pumps (usually called the charging and letdown system) is required to remove water from the high pressure primary loop and re-inject the water back in with differing concentrations of boric acid. The reactor control rods, inserted through the reactor vessel head directly into the fuel bundles, are moved for the following reasons: * To start up the reactor. * To shut down the reactor. * To accommodate short term transients such as changes to load on the turbine. The control rods can also be used: * To compensate for nuclear poison inventory. * To compensate for nuclear fuel depletion. but these effects are more usually accommodated by altering the primary coolant boric acid concentration.
How the first time fission starts in a nuclear power plant?
Insert all the control rods in the unfueled but assembled reactor.Insert a calibrated continuous neutron source in the reactor.Begin inserting fuel rods, stopping periodically to measure neutron flux and calculate multiplication factor. While multiplication factor should rise with each fuel addition it should always stay less than 1 (critical).When you have inserted the design minimum fuel rods for operation, start slowly removing control rods, stopping periodically to measure neutron flux and calculate multiplication factor. If multiplication factor exceeds 1 and becomes slightly supercritical, remove neutron source then the multiplication factor should slowly settle back to 1 (critical).You should now have a fully operational reactor, operating at very low power.The operators then add more fuel and adjust the control rods (inserting/removing) as needed to maintain critical state (multiplication factor = 1) while raising power to full levels.
How do you cycle a key?
To cycle a key in an engine, simply insert it into the ignition, turn it to the "on" position, then turn it back to the "off" position. This can help reset the engine control unit and fix minor issues.
Is it possible to stop a fission reaction at a nuclear power plant?
Yes, it is possible to stop a fission reaction at a nuclear power plant by inserting control rods into the reactor core. These control rods absorb the excess neutrons that drive the chain reaction, effectively slowing down and ultimately stopping the fission process.
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.
How is the rate of the fission reaction regulated in a nuclear reactor?
There are many ways this can be done: (increases/decreases the reaction rate)remove/insert control rods (most common method used)insert/remove fuel rodsmove fuel rods together/apartmove reflector toward/away the coreadd/withdraw moderator (this happens automatically in BWR as boiling creates voids in the water moderator, causing the reactor to regulate itself)etc.
Is there a simple emergency shutdown protocol for nuclear power plants considering the reactors in Japan melting down is there or why isn't there a simple way to shut down the process or reactor?
insert the control rods to stop the fission process
What is the function of control rods in a nucleus reactor?
A control rod in a nuclear reactor absorbs neutrons and limits the chain reaction. It is used as a "setpoint" for operation in that it is pulled out a certain amount to allow the chain reaction to heat the reactor. The control rod can also be inserted to absorb neutrons and shut the reactor down.
How do control rods stop a fission reaction?
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.
What do control rods do in a nuclear reactor?
The control rods include some material that strongly absorbs neutrons-boron is the most common though others like cadmium are also effective. The safety function of the rods is to quickly shut the reactor down should conditions require it. This would most likely be a loss of coolant flow, whether water flow or gas flow, which would cause a rapid rise in fuel temperature, so the best way to kill this quickly is to insert the rods (in a few seconds) so that the reactor power is immediately shut off (except for the decay heat which requires some emergency cooling flow). During steady operation or power changes the rods are used to maintain the reactor just critical, so that the chain reaction is maintained steady or changed at a slow controlled rate. Changes in the reactors reactivity state are caused by variations in neutron absorption by some fission products, notably Xenon 135, which varies in concentration with power level, and by fuel burn up during the period between refuelling outages, so that control rod adjustments are needed to accommodate these changes. If the reactor maintains a steady power level for several days, the xenon reaches an equilibrium level and the rods will move very little from then on, just gradually coming out to offset the fuel burn up.
How do they start a nuclear reactor?
With all control rods inserted all the way, begin inserting fuel rods.When the basic minimum operating number of fuel rods have been inserted, insert a continuous neutron source of known intensity.Measure the neutron flux in the reactor and calculate the reactor's neutron multiplication factor to determine how close you are to design predicted critical.Begin withdrawing control rods very slowly, stopping periodically and repeating step 3. Each time the neutron multiplication factor should increase.If the multiplication factor keeps increasing in step 4 while the control rods are stopped, you have reached critical. Remove the neutron source and the neutron flux in the reactor should decline slightly then become constant.You have now safely started your nuclear reactor. Congratulations!If anything unexpected happens reinsert all control rods, remove the neutron source, find and fix the problem, and restart the procedure.
Why did the chernobyl nuclear disaster occur?
it was april 26, 1986, it occured because of an attempt was made to insert the control rods into the reactor core quickly, after having taken them out, and an effect was the the core structure of graphite blocks had changed and would not allow the control rods to push into the holes of the blocks, leading to rapid heat production,and the explosion.
What does nuclear moderation accomplish in a nuclear reactor?
Moderation slows or reduces the energy of neutrons in a nuclear reactor. By doing this, moderation allows continuation of the chain reaction. Neutrons will only cause more fission events when they have a specific range of energy, but they have too much energy when they are first emitted from their precipitating event, hence the need for moderation. Moderation also regulates the reaction. In the light water moderated reactor, for instance, a common design, water is the moderator. Water is also the heat sink, carrying away the energy of the reaction to make steam which spins turbines and makes electricity. If reactivity were to increase, temperature would also increase, causing an increase in the number of voids in the water. This reduces the effectiveness of the moderator and tends to decrease reactivity. Similarly, if reactivity were to decrease, temperature would decrease, causing voids to decrease, ultimately causing reactivity to increase. Conversely, if the load changes, that will reflect back into the water temperature, causing reactivity to adjust accordingly. It is a self-stabilizing situation. It is also a safety designed system. If there were a sudden loss of heat sink, such as a turbine load rejection, temperature would go up, causing a decrease in reactivity. If there were a steam line break, causing a depressurization incident, the water would flash to steam and the reactor would go instantly subcritical. In both of these scenarios, there would be time to insert the control rods, forcing the reactor further subcritical, and giving the emergency core cooling systems time to startup.
What is the purpose of control rods in nuclear reactors?
A nuclear reactor requires the neutrons released from one reaction to trigger the fission of other nuclei. Control rods are required to absorb some of these neutrons so as to prevent a runaway chain reaction.
How does a nuclaer reactor work?
A nuclear reactor is composed of the following:fissionable fuelcontrol rodssafety rodsmoderator (omitted in a fast fission reactor)cooling systemMost reactors are what are called thermal reactors, what this means is slow (aka thermal) neutrons are used in the chain reaction to cause fissions in the fuel. A moderator (e.g. water, graphite, heavy water) is required to slow the fast neutrons emitted by fissions to this slow speed before too many of them are captured by uranium-238, which removes them from the chain reaction.Fast reactors do not need a moderator as they use highly enriched fuel with most of the uranium-238 removed.The control rods can be inserted and removed as needed and are made of a material (e.g. cadmium) that easily captures neutrons, which removes them from the chain reaction.The safety rods are only inserted in an emergency, in some reactor designs they cannot be removed once inserted. Like the control rods they are made of a material (e.g. boron, cadmium) that easily captures neutrons, which removes them from the chain reaction.The cooling system removes heat from the reactor (and in power reactors carries it to the turbine room where it is used to generate electricity). Frequently there are redundant cooling systems, including an emergency one that floods the reactor core with water.Starting a nuclear reactor is requires a special procedure to do it safely:Insert all control rods as far as they go.Attach a neutron source to the reactor.Measure the neutron multiplication factor (neutron flux ÷ neutrons provided by the neutron source) of the reactor, it will be low and constant.Slightly remove one control rod.Measure the neutron multiplication factor of the reactor, it will be a bit higher than before and constant.Repeat steps 4 and 5 until the neutron multiplication factor is no longer constant, but slowly rises on its own without further removal of control rods. The reactor is now very very slightly supercritical.Remove the neutron source from the reactor.The neutron flux in the reactor should settle to a constant value again and stay there. The reactor is now exactly critical.The reactor is now fully started. To increase power level, slightly remove control rods. To decrease power level, slightly insert control rods.
