Proportional Prompt’ neutrons

Since the total number of neutrons in the next generation will be proportional to k and the number of next-generation prompt neutrons will be proportional to kp, it follows that the fraction of prompt neutrons in the next generation will be kp/k. Similarly the fraction of next-generation delayed neutrons will be kdi/k, for I = 1 to 6. The delayed neutron fraction for group i is given the symbol ft, so that... 

In Figure 4.1.c, the delayed neutron fraction effectively decreases in proportion to the positive reactivity added to the reactor while the prompt neutron fraction increases. This action can be accounted for in equation (4.10) by adding reactivity to the prompt neutron fraction and subtracting reactivity from the... 

As explained briefly in Chapter 2, the dynamic response of a reactor system is critically dependent on the fact that a small proportion of the secondary neutrons produced as a result of fission is emitted with a delay of up to several minutes after the fission has occurred. To illustrate the importance of the delayed neutrons, let us consider a very simplified argument leading to the rate of flux increase that would occur in a reactor if the neutrons produced by fission were all prompt neutrons, i.e., emitted within a time of the order of 10 s after fission. 

Zvara estimates that counting prompt neutrons from spontaneous fission could be the most sensitive method of detection of superheavy elements, having a limit of 10 g/g for a half-life of 2 x 10 yr. Ter-Akop yan et report the construction of a large He proportional counter which is intended to be used in this type of work. 

This experiment is designed to determine the reactivity worth of the control rods by a pulsed neutron technique. A burst of neutrons is injected into the reactor, and the decay rate of the resultant neutron flux is measured. The decay rate measured is that of the prompt fission neutrons and is proportional to the prompt critical reactivity of the reactor. Measurements will be made with the reactor in subcritical conditions and at delayed critical. The decay rate at delayed critical yields the constant of proportionality between the decay rate of the neutron flux and the reactivity in dollars. This constant is equal to the ratio of the effective delayed-neutron fraction to the prompt-neutron lifetime. 

Since a nuclear reactor is a statistical system, it will show fluctuations in neutron intensity. These fluctuations, or pile noise, are not commonly considered of interest in themselves, but only as interference to other experiments. However, since the nature of the pile noise depends strongly on important reactor parameters, its study can enable the determination of quantities less easily accessible by other means. In particular, Moore (f) points out that the noise spectrum of such a system, that is, the mean square noise amplitude per unit band width, is proportional to the square modulus of the transfer function or to the Fourier cosine transform of the autocorrelation function. Thus, observation of the noise spectrum of a reactor could yield information about the shape of its transfer function. To test this technique, pile noise analyses were done on various low-power experimental reactors at Argonne National J aboratory. Since these reactors operate at such a low level that power effects on reactivity do not appear, the shape of the low-frequency portions of their transfer functions would depend only on fairly well-known delayed neutron parameters, and thus would be of little interest. However, the high-frequency rolloff portion of the transfer function is strongly dependent on the quotient of the effective delayed neutron fraction over the prompt neutron... 

The situation in the core was one in which a small change in the proportion of steam could lead to an increase in core reactivity which generated enough neutrons for the chain reaction to expand without waiting for the delayed neutrons—a prompt critical situation. 

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