|
|
| (One intermediate revision by one other user not shown) |
| Line 1: |
Line 1: |
| In [[nuclear engineering]], a '''prompt neutron''' is a [[neutron]] immediately emitted by a [[nuclear fission]] event, as opposed to a [[delayed neutron|delayed neutron decay]] which can occur within the same context, emitted after beta decay of one of the [[fission product]]s anytime from a few milliseconds to a few minutes later.
| | Myrtle Benny is how I'm known as and I feel comfy when people use the complete title. It's not a common thing but what she likes doing is base jumping and now she is attempting to make cash with it. California is exactly where her house is but she requirements to transfer because of her family. Hiring is my occupation.<br><br>Look at my blog post over the counter std test ([http://bonetoob.com/user/PRKV Read the Full Guide]) |
| | |
| Prompt neutrons are emitted with life times on the order of 10<sup>−22</sup> seconds after exitation of the nuclide that emits the neutron. This emission is controlled by the [[nuclear force]] and is extremely fast. By contrast, so-called delayed neutrons are delayed by the time delay associated with beta decay (mediated by the weak force) to the precursor excited nuclide, after which neutron emission happens on a prompt time scale (i.e., almost immediately).
| |
| | |
| ==Principle==
| |
| | |
| Using [[Uranium-235|U-235]] as an example, this nucleus absorbs [[thermal neutron]]s, and the immediate mass products of a fission event are two large fission fragments, which are remnants of the formed U-236 nucleus. These fragments emit two or three free neutrons (2.43 on average), called ''prompt'' neutrons. A subsequent fission fragment occasionally undergoes a stage of radioactive decay that yields an additional neutron, called a ''delayed'' neutron. These neutron-emitting fission fragments are called ''delayed neutron precursor atoms''.
| |
| | |
| Delayed neutrons are associated with the [[beta decay]] of the fission products. After prompt fission neutron emission the residual fragments are still neutron rich and undergo a beta decay chain. The more neutron rich the fragment, the more energetic and faster the beta decay. In some cases the available energy in the beta decay is high enough to leave the residual nucleus in such a highly excited state that neutron emission instead of [[gamma ray|gamma emission]] occurs.
| |
| | |
| '''Delayed Neutron Data for Thermal Fission in U-235'''<ref>Lamarsh, Introduction to Nuclear Engineering</ref>
| |
| | |
| {| class="wikitable"
| |
| |-
| |
| ! Group
| |
| ! Half-Life (s)
| |
| ! Decay Constant (s<sup>−1</sup>)
| |
| ! Energy (keV)
| |
| ! Yield, Neutrons per Fission
| |
| ! Fraction
| |
| |-
| |
| | 1
| |
| | 55.72
| |
| | 0.0124
| |
| | 250
| |
| | 0.00052
| |
| | 0.000215
| |
| |-
| |
| | 2
| |
| | 22.72
| |
| | 0.0305
| |
| | 560
| |
| | 0.00546
| |
| | 0.001424
| |
| |-
| |
| | 3
| |
| | 6.22
| |
| | 0.111
| |
| | 405
| |
| | 0.00310
| |
| | 0.001274
| |
| |-
| |
| | 4
| |
| | 2.30
| |
| | 0.301
| |
| | 450
| |
| | 0.00624
| |
| | 0.002568
| |
| |-
| |
| | 5
| |
| | 0.614
| |
| | 1.14
| |
| | -
| |
| | 0.00182
| |
| | 0.000748
| |
| |-
| |
| | 6
| |
| | 0.230
| |
| | 3.01
| |
| | -
| |
| | 0.00066
| |
| | 0.000273
| |
| |}
| |
| | |
| ==Importance in nuclear fission basic research==
| |
| | |
| The standard deviation of the final kinetic energy distribution as a function of mass of final fragments from low energy fission of uranium 234 and uranium 236, presents a peak around light fragment masses region and another on heavy fragment masses region. Simulation by Monte Carlo method of these experiments suggests that that those peaks are produced by prompt neutron emission.<ref>R. Brissot, J.P. Boucquet, J. Crançon,C.R. Guet, H.A. Nifenecker. and Montoya, M., "Kinetic-Energy Distribution for Symmetric Fission of 235U", Proc. of a Symp. On Phys. And Chem. Of Fission, IAEA. Vienna, 1980 (1979)</ref><ref>[http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=APCPCS000947000001000326000001&idtype=cvips&gifs=yes | M. Montoya, E. Saettone, J. Rojas, "Effects of Neutron Emission on Fragment Mass and Kinetic Energy Distribution from Thermal Neutron-Induced Fission of 235U", "AIP Conference Proceedings", American Institute of Physics, Volume 947/October, 2007, {{doi|10.1063/1.2813826}}, pp. 326-329]</ref><ref>[http://www.ejournal.unam.mx/rmf/no535/RMF005300506.pdf M. Montoya, E. Saettone, J. Rojas, "Monte Carlo Simulation for fragment mass and kinetic energy distribution from neutron-induced fission of U 235" , Revista Mexicana de Física 53 (5) 366-370, oct 2007]</ref><ref>[http://rmf.fciencias.unam.mx/pdf/rmf/54/6/54_6_440.pdf M. Montoya, J. Rojas, I. Lobato, "Neutron emission effects on final fragments mass and kinetic energy distribution from low energy fission of U 234", Revista Mexicana de Física, 54(6) dic 2008]</ref> This effect of prompt neutron emission does not permit to obtain primary primary mass and kinetic distribution which is important to study fission dynamics from saddle to scission point.
| |
| | |
| ==Importance in nuclear reactors==
| |
| | |
| If a [[nuclear reactor]] happened to be [[prompt critical]] - even very slightly - the number of neutrons and power output would increase exponentially at a high rate. The response time of mechanical systems like control rods is far too slow to moderate this kind of power surge. The control of the power rise would then be left to its intrinsic physical stability factors, like the thermal dilatation of the core, or the increased [[resonance absorption]]s of neutrons, that usually tend to decrease the reactor's reactivity when temperature rises; but the reactor would run the risk of being damaged or destroyed by heat.
| |
| | |
| However, thanks to the delayed neutrons, it is possible to leave the reactor in a [[subcritical]] state as far as only prompt neutrons are concerned: the delayed neutrons come a moment later, just in time to sustain the chain reaction when it is going to die out. In that regime, neutron production overall still grows exponentially, but on a time scale that is governed by the delayed neutron production, which is slow enough to be controlled (just as an otherwise unstable bicycle can be balanced because human reflexes are quick enough on the time scale of its instability). Thus, by widening the margins of non-operation and supercriticality and allowing more time to regulate the reactor, the delayed neutrons are essential to [[Passive nuclear safety|inherent reactor safety]] and even in reactors requiring active control.
| |
| | |
| ==Fraction definitions==
| |
| | |
| The factor β is defined as:
| |
| | |
| :<math>
| |
| \beta = \frac{\mbox{precursor atoms}}
| |
| {\mbox{prompt neutrons}+\mbox{precursor atoms}}.
| |
| </math> | |
| | |
| and it is equal to 0.0064 for U-235.
| |
| | |
| The delayed neutron fraction (DNF) is defined as:
| |
| | |
| :<math>
| |
| DNF = \frac{\mbox{delayed neutrons}}
| |
| {\mbox{prompt neutrons}+\mbox{delayed neutrons}}.
| |
| </math>
| |
| | |
| These two factors, β and ''DNF'', are not the same thing in case of a rapid change in the number of neutrons in the reactor.
| |
| | |
| Another concept, is the ''effective fraction of delayed neutrons'', which is the fraction of delayed neutrons weighted (over space, energy, and angle) on the adjoint neutron flux. This concept arises because delayed neutrons are emitted with an energy spectrum more thermalized relative to prompt neutrons. For low enriched uranium fuel working on a thermal neutron spectrum, the difference between the average and effective delayed neutron fractions can reach 50 pcm (1 pcm = 1e-5).<ref>[http://www.osti.gov/bridge/product.biblio.jsp?query_id=1&page=0&osti_id=991100 Deterministic and Monte Carlo Analyses of YALINA Thermal Subcritical Assembly]</ref>
| |
| | |
| ==See also==
| |
| | |
| *[[prompt critical]]
| |
| *[[Critical mass (nuclear)|critical mass]]
| |
| *[[nuclear chain reaction]]
| |
| | |
| ==References==
| |
| {{Reflist}}
| |
| | |
| ==External links==
| |
| *[http://lpsc.in2p3.fr/gpr/PPNPport/node47.html Hybrid nuclear reactors:delayed neutrons]
| |
| *[http://www.pipeline.com/~rstater/nuke1a.html Beta is not the delayed neutron (population) fraction]
| |
| | |
| [[Category:Nuclear technology]]
| |
| [[Category:Neutron|Prompt]]
| |
Myrtle Benny is how I'm known as and I feel comfy when people use the complete title. It's not a common thing but what she likes doing is base jumping and now she is attempting to make cash with it. California is exactly where her house is but she requirements to transfer because of her family. Hiring is my occupation.
Look at my blog post over the counter std test (Read the Full Guide)