469:, 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
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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
298:– even very slightly – the number of neutrons would increase exponentially at a high rate, and very quickly the reactor would become uncontrollable by means of external mechanisms. 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
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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
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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
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and thus either happens at fission, or nearly simultaneously with the beta decay, immediately after it. The various half lives of these decays that finally result in neutron emission, are thus the beta decay half lives of the precursor radionuclides.
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133:) that yields a new nucleus (the emitter nucleus) in an excited state that emits an additional neutron, called a "delayed" neutron, to get to ground state. These neutron-emitting fission fragments are called delayed neutron precursor atoms.
125:, 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, on average, two or three free neutrons (in average 2.47), called
50:(or actually, a fission product daughter after beta decay), any time from a few milliseconds to a few minutes after the fission event. Neutrons born within 10 seconds of the fission are termed "prompt neutrons".
458:, are almost the same thing, but not quite; they differ in the case a rapid (faster than the decay time of the precursor atoms) change in the number of neutrons in the reactor.
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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.
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of the precursor nuclides – which are the precursors of the delayed neutrons – happens orders of magnitude later compared to the emission of the
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is termed a delayed neutron. The "delay" in the neutron emission is due to the delay in beta decay (which is slower since controlled by the
69:. A small fraction of them are excited enough to be able to beta-decay by emitting a delayed neutron in addition to the beta. The moment of
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The lower percentage of delayed neutrons makes the use of large percentages of plutonium in nuclear reactors more challenging.
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375:{\displaystyle \beta ={\frac {\mbox{precursor atoms}}{{\mbox{prompt neutrons}}+{\mbox{precursor atoms}}}}.}
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444:{\displaystyle DNF={\frac {\mbox{delayed neutrons}}{{\mbox{prompt neutrons}}+{\mbox{delayed neutrons}}}}.}
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the residual nucleus in such a highly excited state that neutron emission instead of
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However, thanks to the delayed neutrons, it is possible to leave the reactor in a
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65:(usually beta decay) and the resulting nuclides are unstable with respect to
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large nuclides fission into two neutron-rich fission products (i.e. unstable
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Talamo, A.; Gohar, Y.; Division, Nuclear
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388:The delayed neutron fraction (DNF) is defined as:
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27:Delayed emission of neutrons after nuclear fission
137:Delayed Neutron Data for Thermal Fission in U-235
325:The precursor yield fraction β is defined as:
314:, even in reactors requiring active control.
77:. Hence the neutron that originates from the
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520:Addison-Wesley, 2nd Edition, 1983, page 76.
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105:Delayed neutrons are associated with the
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463:effective fraction of delayed neutrons
385:and it is equal to 0.0064 for U-235.
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518:Introduction to Nuclear Engineering,
121:as an example, this nucleus absorbs
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286:Importance in nuclear reactors
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544:"Nuclear Data for Safeguards"
156:Yield, Neutrons per Fission
531:Physics of Nuclear Kinetics
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454:These two factors, β and
461:Another concept, is the
533:, Addison-Wesley, 1965.
312:inherent reactor safety
95:nuclear reactor control
590:Cite journal requires
493:Nuclear chain reaction
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300:resonance absorptions
97:and safety analysis.
46:event, by one of the
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321:Fraction definitions
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150:Decay Constant (s)
32:nuclear engineering
640:Nuclear technology
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107:beta decay
83:weak force
71:beta decay
67:beta decay
336:β
279:0.000273
259:0.000748
239:0.002568
219:0.001274
199:0.001424
179:0.000215
159:Fraction
101:Principle
621:Archived
477:See also
276:0.00066
256:0.00182
236:0.00624
216:0.00310
196:0.00346
176:0.00052
114:occurs.
59:nuclides
645:Neutron
190:0.0305
170:0.0124
40:neutron
575:991100
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471:50 pcm
267:0.230
247:0.610
230:0.301
210:0.111
187:22.72
167:55.72
144:Group
117:Using
290:If a
270:3.01
250:1.14
227:2.30
207:6.22
119:U-235
53:In a
38:is a
596:help
571:OSTI
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408:=
405:F
402:N
399:D
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