Precursors, delayed neutron

Six groups of fission products are particularly important to the control engineer. These decay to form daughter nuclei that are in a sufficiently excited state to throw off a neutron on formation. Such neutrons are called delayed neutrons, and the six groups of fission products are known as delayed neutron precursors. Delayed neutrons make up less than 1% of the total number of neutrons liberated by fission, but the fact that they are released on a much slower timescale than the prompt neutrons liberated at the time of fission renders the control of nuclear reactors relatively easy. 

Figure 21.3 Neutron absorption and the production of prompt neutrons and delayed-neutron precursors.

Let the production of the nuclei of delayed neutron precursor i due to absorption in a fission event be... 

SCfi. We may see from Figure 21.3 that kjnc delayed neutron precursor nuclei per m are produced at every absorption event, which events occur every St seconds. It follows that... 

Following the procedure of Section 21.6, we may substitute rtc = (n/l)St and let 5f 0 to give the rate of production of delayed neutron precursor nuclei per... 

The rate of increase of concentration of delayed neutron precursors is the rate of production minus the rate of dscay ... 

The derivation has been based on the variables, n, Ct and S, being understood as density terms, expressed per unit volume of the reactor core. However, it is possible to multiply both sides of each equation by the reactor core volume and so produce an equivalent set of equations in the total number of neutrons and delayed neutron precursors contained within the core.]... 

The longest-lived example of such a delayed neutron emitter (or delayed neutron precursor) is the nuclide Br. It undergoes P decay with a half-life of 55 s. The daughter nucleus is the... 

SSm - Sodium sanqjling to allow transfer and monitoring of delayed neutron precursors... 

A a decay constant of delayed neutron precursor group 1, and... 

In the system under consideration (a noncirculating-fuel bare thermal reactor), it is physically reasonable to suppose that the spatial variation of the concentration of delayed-neutron precursors is proportional to that of the neutron flux and that this mode persists even though the magni-... 

This model would have some application in the early stages of the flux rise after t = 0, and before the concentration of the delayed-neutron precursors has been built up significantly over its steady-state value. For example, Eq. (9.122) is a good approximation to (9.121) when the transient effects, represented by the second term, are significantly larger than those due to the stable reactor period, i.e., when... 

During the minutes following a reactor scram, reactor power decreases on a negative 80 second period, corresponding to the half-life of the longest-lived delayed neutron precursors, which is approximately ... 

Delayed neutron precursors decay by beta decay. Which ONE reaction below is an example of beta decay ... 

The half-life of the longest-lived group of delayed neutron precursors is 55 seconds. 

Delayed neutron generation time, r, is the weighted average of the mean generation times of the six delayed neutron precursors. Table 3.7 is a compilation of delayed neutron data for uranium 235 from Table 3.4. 

Delayed generation time, t, is the weighted average of the mean generation times of the six delayed neutron precursors. 

During a positive reactivity addition (supercritical), the shortest lived delayed neutron precursors are formed more rapidly than the longer lived precursors, As a result, y shortens to 10 seconds. 

There is a limit to the negative period that can be developed in a reactor by negative reactivity additions. Soon after the insertion of a large amount of negative reactivity such as a scram, the prompt neutron population decreases to a low level. Neutron population is predominantly the result of delayed neutrons which are produced by fission product decay. Within a short time, 2-3 minutes, all of the short lived delayed neutron precursors have decayed away. At this point, and from this point on, the core neutron population is sustained by decay of the longest lived fission product precursor, bromine-87, with a half life of = 55.72 seconds. Since the rate at which core neutron population decreases is determined by radioactive decay of bromine-87, an effective reactor period can be calculated by setting equations (2,9) and (4.7) equal. Neutron population, N will be used to replace activity, A, and power, P, respectively in the two equations. 

The next longest lived delayed neutron precursor is 1137 with a 24,4 second half life. Using the seven half life rule, 1137 will decay away to less than ix of its steady state operating value in 7x24.4 - 171... 

As explained in Chapter 2, the delayed neutrons are not emitted from the direct products of the fission, but from nuclei which are formed by subsequent jS decay of these products. While many of the delayed neutron precursors have been identified, it is more convenient in practice to analyze the time behavior of the delayed neutrons by an empirical division into a number of groups, each characterized by a single decay constant, or half-life. It is found that the characteristics of the delayed neutrons from all the fissionable isotopes of interest can be adequately described by the use of six groups. The half-lives and yields of the delayed neutron groups for the fissile isotopes and Pu, and for the fertile isotope are sum-... 

The effective delayed neutron fraction ( Seff) represents an important reactor kinetics parameter. In circulating-fuel systems, because of the delayed neutron precursor drift, the Seff calculation requires special techniques. The coupled neutronics/CFD simulations represent a necessary step for the accurate calculation of the effective delayed neutron fraction in the MSFR (Aufiero et al., 2014). Fig. 7.3 shows the distributions of the prompt (right) and delayed (left) neutron sources obtained in OpenFOAM and adopted to calculate fieti in the nominal MSFR conditions. 

Neutronic characteristics of MSRs have been explored in the literature. Flow effects were considered when calculating the effective multiplication factor and fast neutron, thermal neutron, and delayed neutron precursor distribution of the liquid-fuel MSR based on the multigroup neutron diffusion equation and delayed neutron precursor conversation equation (Zhang et ah, 2009b Cheng and Dai, 2014 Zhou et ah, 2014). Spatial kinetic models were developed for better neutronic analysis of the MSRs (Zhang et ah, 2015 Zhuang et ah, 2014). 

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