Half-lives and delayed neutron

Table I. Half-lives and delayed neutron emission probabilities...

Fissile Elements. Reactor activation of fissile elements and counting of the delayed neutrons emitted in the decay of short-lived fission products provides a very specific method of analysis. Unfortunately, in order to provide discrimination between the different fissile isotopes (or elements) it is necessary to irradiate the samples in two different flux spectra thus utilizing the differing rates of reaction of fast and thermal fission. Decay curve analysis is impractical because of the similarity of the half-lives of delayed neutron groups from all the fissile isotopes. 

Delayed Neutrons - Those neutrons with an eaisslon time after the fission process that is delayed by the half-life of certain fission products (precursors). An arbitrary time lini3.t exists (10 sec.) to distinguish between prompt and delayed neutrons. The delayed neutron fractions and half-lives are different for the various fissionable isotopes. This means that the delayed neutron characteristics of a pile are exposure-dependent. 

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-... 

Data on delayed neutron half-lives and yields from fast fission in ... 

In specifying the source term here, separate account has to be taken of the contributions of prompt and delayed neutrons. For accurate analysis of the time variance of the flux, the fractional yields and half-lives of the six delayed neutron groups have to be fed into the source term. The method of solution is given in many reactor physics texts, and will not be repeated here. To illustrate the time behavior predicted by the solution, we may consider a reactor where, starting from an initially critical condition, the reactivity is suddenly increased from zero to a small positive value, such as p = 0.001. Assuming six delayed neutron groups, the time dependence of the flux after the reactivity increase is given by an expression of the form... 

Through use of a "rabbit" facility at the Argonaut reactor, the delayed-neutron parameters of U are measured, A small sample of is irradiated to equilibrium in a neutron flux and subsequently rapidly removed to a shielded counting pig. By means of a neutron counter and a time analyzer, the neutron emission versus time is determined. From these data, at least four of the five so-called groups can be resolved and characterized in terms of their half-lives and relative abundances. 

Because of the delayed neutron activities of 55 sec and 24 sec which are derived from j9 -decay half-lives of Br87 and I137, respectively (the neutron emission is instantaneous and follows the -emission), the two mass chains 86 and 136 gain in yield at the expense of the 87 and 137 chains, respectively. However, in general, these effects are small, amounting to a few percent of the yield of a chain at most. 

In the equations above, 6 is the mean lifetime for the pronq>t neutrons. As discussed in 14.7.1, some fission products decay by leading to an excited state which emits a neutron, -delayed neutron emission. For example Kr decays partly by -delayed neutron emission it is a daughter of Br with a half-life as long as 56 s. A large number of such neutron emitting fission products have been discovered, all with shorter half-lives, see Table 19.5. In reactors where the moderator contains D or Be atoms, y,n reactions with energetic y-rays from shortlived fission products and from activation products is also a source of delayed neutrons. The delayed neutrons have lower kinetic raergies ( 0.5 MeV) than the prompt ones and amount to < 1 % of the total number of fission neutrons emitted the fraction of delayed neutrons, /3, is 0.27 % for 0.65 % for 0.21 % for Pu, and 0.52% for Wh the delayed neutrons are taken into account, the ective... 

In a full analysis i.e., the prompt as well as delayed y-emission analysis, the short-lived nuclides are determined first. Actually the most intense y-radiations are measured first and then observation about their decrease of intensity is made. (This will happen for y-radiations of those isotopes which have short half-lives). Using a fast rabbit system, each sample is irradiated separately (but together with a comparator in order to calculate the neutron flux) for 5-30 s. Sample and comparator are measured separately after a waiting time from seconds (the element selenium has a nuclide with a half-life time of 17.5 s and thus needs to be measured as quickly after irradiation as possible) to as much as 20 min. After the analysis has been completed for all samples, a waiting time of 5-7 days is required before irradiating them again in order to determine the other elements with longer half-life times. 

From Table 3,3, the precursor with the longest half-life is bromine-87 the half life is 55.7 seconds. Bromine-87 is significant because when the reactor is shutdown and prompt and shorter lived delayed neutrons have decayed away, the core neutron population is maintained by delayed neutrons produced by the decay of bromine-87. The decay scheme for bromine-87 is shown in Figure 3,2. Approximately 30 percent of bromine-87 nuclei decay by beta emission to krypton-87 at ground state energy while 70 percent decay by beta emission to an excited state of krypton-87. Of the later 70 percent, about two percent of the excited krypton-87s... 

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. 

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