What role does neutrons play in fission?

Answer 1

In nuclear fission an atom is broken up into two or more new particles. Sometimes this happens all by itself, but when we want it to happen more rapidly we use a critical assembly of fissile fuel and a moderator, which multiplies the neutron flux. The moderator slows down the neutrons produced in each fission which can then cause further fissions, resulting in a chain reaction. Each fission also releases energy as heat. The chain reaction is controlled at a steady level by control rods which absorb neutrons.

Answer 2

In nuclear fission an atom is broken up into two or more new particles. Sometimes this happens all by itself, but when we want it to happen more rapidly we use a critical assembly of fissile fuel and a moderator, which multiplies the neutron flux. The moderator slows down the neutrons produced in each fission which can then cause further fissions, resulting in a chain reaction. Each fission also releases energy as heat. The chain reaction is controlled at a steady level by control rods which absorb neutrons.

Answer 3

The role of the neutrons is absolutely crucial. When U235 fissions, 2 or 3 neutrons are emitted. These have high energy, but the U235 nucleus is split most efficiently by slow neutrons, so they are slowed down by a moderator, water, heavy water, or graphite. Some neutrons are absorbed in the moderator, some are lost at the core boundary and some in other structures or control rods. The latter can be varied so that just enough neutrons survive to encounter further U235 nuclei and so cause further fissions, in fact a steady state chain reaction is set up. Since each fission releases a certain amount of heat, the neutron flux is a direct indication of the reactor power level, and instruments to measure the flux tell the operator what is happening, ie steady power or increasing/decreasing power.

Answer 4

The neutron may be absorbed by a nucleus, for example the nucleus of a U-235 atom, transform it into another kind of nucleus, and thus provoke instability. The new nucleus in this case is of a kind that quickly disintegrates.

Answer 5

initially a neutron is fired into the first nucleus, this then splits and produces 3 more neutron with it. two of those enter the just split nucleus and so on and so on.

neutrons need to be controlled in order for the nucleus to absorb them, so in the reactors they insert control rods which slow the velocity if the neutrons.

Answer 6

on a basic level, neutrons are crucial in sustaining the fission reaction, when a nucleus splits it emits two similarly sized nuclei and some neutrons ( 2 or 3), these neutrons are then absorbed by the next fissionable nuclei, the addition of a neutron to the nuclei causes and instability which triggers the nuclei to rip apart emitting energy and more neutrons to continue the chain reaction.

without neutrons being emitted by the split nuclei the reaction would stop and wouldn't be known as a chain reaction.

Answer 7

The nuclear fission reaction is an equilibrium state where the average number of fissions (splits) of Uranium-235 or Plutonium-239 is balanced exactly by the number of neutrons produced by those fissions and that participate in the next series of fissions. I say "that participate" because not every neutron produced by a fission causes a new fission. Each fission can generate between two and three or so neutrons, and we want, in a controlled reaction such as a power plant, there to be exactly one neutron produced per fission that contributes to the next fission.

This is what we call KEffective = 1, or stable criticality. In order to accomplish this, many things come into play. The geometry of the core, the temperature, the pressure, and the average thermal energy of the neutrons has to be all controlled very tightly. The most important part is the thermal energy of the neutrons. In a fission event, the neutrons produced have too much energy, and they won't go on to create more fission events. We need to slow them down, in a process called moderation, usually done with water, but sometimes with other things, into a very narrow range of energy in order to sustain criticality. Control rods play a major part in this, because slowing them down too much, or absorbing them, will shut the reaction down.

In the normal state of affairs, we use the control rods to start and stop criticality, and to make minor adjustments in power while at low and medium power, and we use the moderator's automatic stabilization process via negative temperature coefficient, to maintain the reaction in equilibrium, at high power.

By the neutron chain reaction described by Leo Szilard in his british patent 630726, filed in 1934. However it was not discovered until 1939 by Otto Hahn et al, that Uranium-235 could sustain this reaction.

Answer 8

Neutrons and protons are both classified as nucleons, which is to say, particles that are found in the atomic nucleus, and all nucleons attract other nucleons by means of what is poetically known as the strong nuclear force. However, protons also repel other protons by means of the electromagnetic force, since they all have positive charges and like charges repel. So with protons you get both an attraction and a repulsion toward other protons, but with neutrons you get only the attraction. As a nucleus gets larger, it becomes harder for the strong nuclear force to hold the nucleus together, because it is an extremely short range force. Even a large nucleus, which by any usual standard is an extremely small object, becomes too large for the strong nuclear force, at which point the electrostatic repulsion that protons exert on other protons will overcome the attraction of the strong nuclear force, unless you also have neutrons, holding things together. However, as the nucleus gets ever larger, at some point even the neutrons can't hold it together. That is why the very heavy elements tend to have an unstable nucleus, and beyond a certain point, they are so unstable that they cannot exist for more than a tiny fraction of a second.