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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.
Wiki User
∙ 12y ago
Wiki User
∙ 13y agocontrol rods
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What must happen for a nuclear to make electricity?
a chain reaction
What must happen for nuclear reactor to make electricity?
a chain reaction
A nucleus captures two neutrons and decays to produce one neutron is this process likely to produce a chain reaction?
no because the decay requires 2 neutrons to take place and only gives off 1 neutron
For a fission reaction to fail to be maintained as a chain reaction each fission must produce less than additional fission reaction(s).?
In a continuous chain reaction at a steady rate, which gives a steady power output from a fission reactor, every fission gives rise to another fission. This happens because in every fission 2 or 3 neutrons are produced, some are absorbed or lost, but if just one is captured by another fissionable nucleus, the chain reaction will continue. Apex answer = 1
Why does a chain reaction occur when uranium 235 undergoes fission?
Neutrons do this. When 235U undergoes fission, the result is the production of two daughter atoms, each very roughly half the mass of the original atom, a number of neutrons, and heat. The neutrons then can collide with other fissile or fissionable atoms causing them to undergo fission much more quickly than they otherwise would.
How is a chain reactor controlled?
Since the continued chain reaction of a nuclear fission reactor depends upon at least one neutron from each fission being absorbed by another fissionable nucleus, the reaction can be controlled by using control rods of material which absorbs neutrons. Cadmium and boron are strong neutron absorbers and are the most common materials used in control rods. A typical neutron absorption reaction in boron is In the operation of a nuclear reactor, fuel assemblies are put into place and then the control rods are slowly lifted until a chain reaction can just be sustained. As the reaction proceeds, the number of uranium-235 nuclei decreases and fission by- products which absorb neutrons build up. To keep the chain reaction going, the control rods must be withdrawn further. At some point, the chain reaction cannot be maintained and the fuel must be replenished
What is meant by chain reaction in nuclear reactor?
Basically a chain reaction (nuclear or chemical) is a self sustaining auto-catalytic reaction.In a nuclear reactor it is a neutron chain reaction, where each neutron released in every fission event can trigger another fission event. In a nuclear reactor the excess neutrons must be disposed of, which is the purpose of the control rods so that the reaction can be kept at some desired constant rate.
Why does a nuclear fission reaction generally not result in an uncontrolled chain reaction?
A nuclear fission reaction generally does not result in an uncontrolled chain reaction because of the release of binding energy. This contributes heat and energy into the reaction, which tends to reduce the density of the fissile material, i.e. making it larger, and thus making it subcritical.The hard part in creating an uncontrolled fission reaction is in holding the fissile material in a supercritical geometry long enough to convert all of it. This requires enormous pressure and high technology.Also, the result of a fission reaction includes neutrons that are often too energetic to properly go on and create subsequent reactions. In a controlled reaction, a moderator is required to "slow down" the neutrons. In an uncontrolled reaction, the dynamics are such that only prompt neutrons are needed to support the chain reaction, a state we call super prompt criticality, but in order to sustain this, the enrichment of the fuel must be above a certain level, typically greater than 20 percent U-235. Modern weapons are in the high 80's and 90's percent.
Describe a characteristic of a fissionable substance that is essential for a chain reaction to sustain itself?
Firstly it must be able to capture neutrons which then have a high probability of causing its nuclei to undergo fission or splitting, and the fissions must produce enough further neutrons so that for every fission that occurs, another one will follow.
What must happen for a nuclear to make electricity?
a chain reaction
What must happen for nuclear reactor to make electricity?
a chain reaction
Describe a characteristic of a fissionable substance that is essential for a chain-reaction to sustain itself?
The fission process is sustained by neutrons. A neutron entering a nucleus and causing fission must be replaced in order to cause the next fission, and so on. So the fissionable substance must emit more neutrons when fission occurs, and enough of them so that despite some being absorbed by the moderator and some leaking from the reactor boundary, there is still enough to maintain the chain reaction. Uranium 235 emits on average about 2.5 neutrons per fission (you might say what is half a neutron, but this is explained by the fact that fissions have a range of possible results, with different numbers of neutrons emitted, and the average is 2.5).
What reaction must uranium undergo in order to release energy?
Nuclear fission with thermal neutrons
What must happen in order for a nuclear chain reaction to happen?
The number of neutrons existing in the critical core of the reactor must be steady or increasing, if it is decreasing the reaction will reduce and eventually stop. When a nucleus fissions (ie one of U-235) roughly 2.5 neutrons are produced, on average. Some of these are absorbed by the moderator and other incore materials, and some are lost at the core boundaries, but if for every nucleus that is fissioned, one neutron is then absorbed into another U-235 nucleus causing another fission, then the process will continue at a steady rate and we can call that a chain reaction. If there is a surplus of neutrons the population will increase and this could go on exponentially increasing, but this is contolled by the neutron absorbing control rods so that the neutron flux, or reactor power, is maintained at a steady level, for normal steady operation. In order for a nuclear chain reaction to take place, each nucleus must produce an average of one neutron that causes fission of another nucleus.
A nucleus captures two neutrons and decays to produce one neutron is this process likely to produce a chain reaction?
no because the decay requires 2 neutrons to take place and only gives off 1 neutron
How does the uranium and plutonium react in the nuclear reactor?
To set off a fission reaction (the reaction that occurs in a nuclear reactor), a person must first pump a neutron into a heavy nucleus. So if a neutron is pumped into a uranium or plutonium nucleus capable of undergoing fission, the nucleus splits in two, and releases more neutrons, which hit more nuclei, which in turn send out even more neutrons, thus setting off a chain reaction where every time a neutron hits a nucleus, the nucleus splits in two and sends out more neutrons.
Nuclear fission chain reactions do not naturally occur for a couple of reasons. One reason is that uranium-235 atoms are not found in large concentrations. Engineers counteract this with what process?
Enrichment. apex.
