delayed neutron

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delayed neutron

[di′lād ′nü‚trän]
(nuclear physics)
A neutron emitted spontaneously from a nucleus as a consequence of excitation left from a preceding radioactive decay event; in particular, a delayed fission neutron.

Delayed neutron

A neutron emitted spontaneously from a nucleus as a consequence of excitation remaining from a preceding radioactive decay event. Analogously, delayed emission of protons and alpha particles is also observed, but the known delayed neutron emitters are more numerous, and some of them have practical implications. In particular, they are of importance in the control of nuclear chain reactors.

In a 235U nuclear reactor, about 0.7% of the neutrons are delayed, the others being prompt. In a conventional, moderated reactor, the prompt neutrons are born, slowed down, and reabsorbed to produce the next fissions in a cycling time of about 1 millisecond. (In a fast-neutron reactor, the time is much shorter.) Consequently, if the reactor were to become overcritical (more neutrons generated per millisecond than are absorbed or leak out), the chain reaction would exponentiate or “run away,” and the reactor might overheat itself and possibly cause a dangerous accident unless the control rods could respond within a few milliseconds to correct the situation. The fortunate presence of the delayed neutrons eases the situation, because so long as the reactor operates within the margin of 0.7% (“delayed critical”), the control rods can take as long as several seconds to respond, and thus the chain reaction comes within the range of easy and leisurely control. See Reactor physics

References in periodicals archive ?
Although the delayed neutrons correspond to less than 1% of the total neutrons emitted by the system, they play a fundamental role in the control of a thermal reactor.
The methodology for calculating delayed neutrons is also based on the TechnionHAMMER, TWODB and CITATION programs.
In the second part of the work the fraction of delayed neutrons and the coefficient of temperature reactivity were calculated.
The fraction of delayed neutrons will be associated with the maximum amount of reactivity that can be inserted into the nucleus, without being jeopardized, entering the prompt criticality state.
A sensitivity benchmark exercise was organized within the scope of the Uncertainty Analysis in Modeling (UAM) project [40,41] of the OECD/Nuclear Energy Agency (NEA) to compare the available and stimulate the development of new, deterministic, and stochastic methods and codes for the sensitivity and uncertainty computations of the effective multiplication factor ([k.sub.eff]) and the effective delayed neutron fraction ([[beta].sub.eff]).
The measurements were performed on critical eigenvalue ([k.sub.eff]), and material buckling, reaction rate ratios, material worth, fission rate, and capture rate distributions as well as effective delayed neutron fraction [[beta].sub.eff] were measured.
In the scope of the CHANDA project of the European Commission a sensitivity and uncertainty analysis of the neutron multiplication factor [k.sub.eff] and the effective delayed neutron fraction [[beta].sub.eff] was performed to identify the most important nuclear data for neutron-induced reactions for criticality calculations of the latest MYRRHA designs.

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