Neutron economy

Neutron economy is defined as the ratio of excess

weighted average

based primarily on the energies of the neutrons.

Nuclear fission is a process in which the

neutron spectrum. Those neutrons may cause other nuclei to undergo fission, leading to the possibility of a chain reaction. However, the neutrons can only cause another fission under certain conditions based on their energy; high-energy, or "relativistic", neutrons will often fly right through another nucleus without causing fission. The chance that a neutron will be captured increases greatly when its energy is about that of the target nucleus, which is known as a "thermal neutron". In order to maintain a chain reaction in a nuclear reactor, a neutron moderator is used to slow the neutrons down. This moderator is often used as the coolant that is used for energy extraction as well, and the most common moderator is water. The neutrons also slow due to elastic and inelastic collisions

with fuel and other materials in the reactor.

A fission reactor is based on the idea of maintaining criticality, where every fission event leads to another fission event, no more and no less. As fission of uranium releases two or three neutrons, this means some of the neutrons must be removed as part of the overall process. Some will be lost purely due to geometry, those released travelling outward from the outer edge of the fuel mass will not have a chance to cause fission, for instance. Others will be absorbed through various processes in the mass, and still others will be deliberately absorbed by control rods or similar devices to maintain the correct overall balance.^[3] The process of moderating the neutrons almost always leads to some of them being absorbed as well.

Neutron economy is a measure of the number of neutrons being released that can cause fission compared to the number needed to maintain the chain reaction. This is not simply an accounting of the total number of neutrons, as it also includes a weighting based on the energy. Thus, remaining high-energy neutrons are not a major part of the "overall economy" as they do not maintain the chain reaction. The quantity that indicates how much the neutron economy is out of balance is given the term reactivity. If a reactor is exactly

subcritical

.

The term "neutron economy" is used not just for the instantaneous reactivity of a reactor, but also to describe the overall efficiency of a

nuclear waste

to "burn off" some of the more radioactive components.

References

^ "DOE Fundamentals handbook Vol1" (PDF). h1019v1. US Department of Energy. Archived from the original (PDF) on 19 March 2014. Retrieved 17 October 2014.
^ "DOE Fundamentals handbook Vol2" (PDF). h1019v2. US Department of Energy. Archived from the original (PDF) on 3 December 2013. Retrieved 17 October 2014.
^ "Engineering Physics 4D3/6D3 - Nuclear Reactor Analysis". Retrieved 25 January 2013.
^ Meneley, A.R. Dastur. Synertistic Fuel Cycles of the Future (PDF) (Technical report).
^ "CANDU Reactors". Archived from the original on 2012-02-25. Retrieved 25 January 2013.

[1] "DOE Fundamentals handbook Vol1" (PDF). h1019v1. US Department of Energy. Archived from the original (PDF) on 19 March 2014. Retrieved 17 October 2014.

[2] "DOE Fundamentals handbook Vol2" (PDF). h1019v2. US Department of Energy. Archived from the original (PDF) on 3 December 2013. Retrieved 17 October 2014.

[3] "Engineering Physics 4D3/6D3 - Nuclear Reactor Analysis". Retrieved 25 January 2013.

[4] Meneley, A.R. Dastur. Synertistic Fuel Cycles of the Future (PDF) (Technical report).

[5] "CANDU Reactors". Archived from the original on 2012-02-25. Retrieved 25 January 2013.

[3]

See also

References