Effects of Cavities and Fuel Lumps on Migration Area

6 Effects of Cavities and Fuel Lumps on Migration Area 

An elementary approach frequently used for estimating the increase in the average path length consists of applying a uniform correction factor to all macroscopic cross sections to account for the effective reduction in density due to the presence of the holes. On this basis, it is easy to show that the corrected diffusion length Lh (with holes) is related to the diffu- 

It has been demonstrated by D. J. Behrens that the estimate given by (10.221) is the first-order isotropic correction term for small holes in a more general result which takes into account the distribution and shape of the holes. The results obtained by Behrens for the diffusion length are summarized in the work which follows, and extension is made to the calculation of the age, using a procedure suggested by the work of Kaplan and Chernick. Finally, the estimates of the age and diffusion length obtained in this way are used to compute the migration area and thence the multiplication constant of the reactor. 

When both the size and spacing of the holes are small compared to the mean free path in the moderator, the expression (10.223) is well approximated by (10.221) moreover, it may be shown that in any event 

Some representative values of L%IL are given in Table 10.8 for various hole sizes and spacings so as to display the relative accuracy of Eq. (10.221). 

---