Based on a simplified solution of the balance equations for concentration of uranium and plutonium isotopes and a set of two-group microscopic cross sections, isotopic ratios, 235U/U, Pu/U, and 239Pu/235U, are calculated as a function of fuel burnup for pressurized water reactor spent fuel. The two-group cross sections for 235U, 238U, 239Pu, and water are collapsed into equivalent thermal-group constants, with the fast-to-thermal flux ratio obtained through a two-group criticality consideration. For this purpose, parasitic neutron captures are represented through a simple semiempirical relationship. The calculational model, incorporated as the BURN code, yields isotopic ratios that compare favorably with three major data sets from the ISTLIB data bank.