2. For an initial mixture of U?' and UP the eta factor, defined by (3.5.6), near the beginning of an irradiation at a low flux level is given by "(1) =0)-:'(+()' where 's and ," are the parameters for the pure isotopes and the El are one-group macro- scopic cross sections; also it has been assumed that Pu* is produced instantaneously by the absorption of a neutron in Us and the effects on , of the depletion of Us as well as the formation of fission products and higher plutonium isotopes have been neglected. Find that enrichment of U?' such that, at r = 0, dy'd = 0. (Use pis = 2.08; p* = 2.12; 0 = 1015 b; of' = 680 b; of" = 7b.)