Fission importance function

The fission importance function, /+(x), is the ultimate increase in the total fission rate in the critical reactor as a result of one fission neutron born at x. It is related to the source-importance function as follows ... 

The computational effort required for implementing the different perturbation theory expressions is formulation dependent. For example, it is more difficult and expensive to calculate the perturbation in the fission-kernel, as compared with the perturbation in the first-flight kernel, 3. Moreover, there are several approaches for the computation of a given perturbation theory approximation. Consider the approximation One approach is to compute the reactivity using Eq. (88) in which (j> is substituted for and Q is expressed in terms of importance functions needed for these computations are the unperturbed ones. 

Of course, such results as expressed in general terms by Eq. (13.14) encompass much more complicated behaviors than the simple one-velocitj absorption. Even in the one-velocity case changes in the diffusion coefficient bring about somewhat different considerations than those presented above the energy dependence of the changes has not been discussed here at all. Still it is a broadly applicable conclusion that the importance function tends to behave space-wise as the neutron flux, and thus the statistical weight, in so far as changes in the absorption and fission cross sections are concerned, tends to vary spatially as the square of the flux. 

The products of nuclear fission reactions are radioactive and disintegrate according to their own time scales. Often disintegration leads to other radioactive products. A few of these secondary products emit neutrons that add to the pool of neutrons produced by nuclear fission. Very importantly, neutrons from nuclear fission occur before those from radioactive decay. The neutrons from nuclear fission are termed prompt. Those from radioacth e decay arc termed delayed. A nuclear bomb must function on only prompt neutrons and in so doing requires nearly 100 percent pure (or Pu) fuel. Although reactor... 

The major androgen or androgen precursor produced by the adrenal cortex is dehydroepiandrosterone (DHEA). Most 17-hydroxypregnenolone follows the glucocorticoid pathway, but a small fraction is subjected to oxidative fission and removal of the two-carbon side chain through the action of 17,20-lyase. The lyase activity is actually part of the same enzyme (P450cl7) that catalyzes 17tt-hydroxylation. This is therefore a dual function protein. The lyase activity is important in both the adrenals and... 

A knowledge of the size distribution function of the radioactive debris and the specific activity of individual fission product chains as a function of particle size suffice to define many important radiological properties of the land-surface nuclear explosion. If is the function of a radionuclide or fission mass chain distributed between particle sizes Di and D2, then... 

While fission product mobility is mostly a function of the chemical properties of the element, the initial physical form of the contamination can also be important. For radioactive contaminants... 

The radioactivity of important radionuclides in spent fuel as a function of time after the fuel is removed from a reactor is indicated in Table 13.7. Figures 13.18 and 13.19 describe the water dilution volumes for the radionuclides in spent fuel and reprocessed waste as a function of time. (The water dilution volume is the volume of water needed to dilute the amount of a given isotope in the waste to a concentration safe for ordinary use.) Ordinant values in Figs. 13.18 and 13.19 can also be viewed as a measure of the radioactivities of the isotopes as a function of time. For both spent fuel and reprocessed waste, the chief source of radioactivity for the first 10 to 100 y is the fission products "Sr and " Cs. Thereafter, up to about 10,000 y. Am and (briefly) Pu isotopes are the dominant... 

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