Search results
Results from the WOW.Com Content Network
The higher the energy of the state that undergoes nuclear fission, the more likely a symmetric fission is, hence as the neutron energy increases and/or the energy of the fissioning atom increases, the valley between the two peaks becomes more shallow; for instance, the curve of yield against mass for 239 Pu has a more shallow valley than that ...
A fast neutron is a free neutron with a kinetic energy level close to 1 M eV (100 T J/kg), hence a speed of 14,000 km/s or higher. They are named fast neutrons to distinguish them from lower-energy thermal neutrons, and high-energy neutrons produced in cosmic showers or accelerators. Fast neutrons are produced by nuclear processes:
The remaining energy to initiate fission can be supplied by two other mechanisms: one of these is more kinetic energy of the incoming neutron, which is increasingly able to fission a fissionable heavy nucleus as it exceeds a kinetic energy of 1 MeV or more (so-called fast neutrons). Such high energy neutrons are able to fission 238
The neutrons that occur directly from fission are called prompt neutrons, and the ones that are a result of radioactive decay of fission fragments are called delayed neutrons. The term lifetime is used because the emission of a neutron is often considered its birth , and its subsequent absorption or escape from the core is considered its death .
The mere fact that an assembly is supercritical does not guarantee that it contains any free neutrons at all. At least one neutron is required to "strike" a chain reaction, and if the spontaneous fission rate is sufficiently low it may take a long time (in 235 U reactors, as long as many minutes) before a chance neutron encounter starts a chain reaction even if the reactor is supercritical.
If the fission requires an input of energy, that comes from the kinetic energy of the neutron. An example of this kind of fission in a light element can occur when the stable isotope of lithium, lithium-7, is bombarded with fast neutrons and undergoes the following nuclear reaction: 7 3 Li + 1 0 n → 4 2 He + 3 1 H + 1 0 n + gamma rays ...
This fission occurs when atomic nuclei grab free neutrons and form heavy, but unstable, elements. When it comes to nuclear energy , human engineering and the rest of the universe are a bit at odds.
238 U can produce energy via "fast" fission. In this process, a neutron that has a kinetic energy in excess of 1 MeV can cause the nucleus of 238 U to split. Depending on design, this process can contribute some one to ten percent of all fission reactions in a reactor, but too few of the average 2.5 neutrons [6] produced in each fission have ...