Using the fuel burnup as objective function, it is desired to determine the initial distribution of the fuel in a reactor in order to obtain the maximum energy possible, for which, without changing a fixed initial fuel mass, the results for different initial fuel and control poison configurations are analyzed and the corresponding running times compared. One-dimensional, two energy-group theory is applied to a reflected cylindrical reactor using U-235 as fuel and light water as moderator and reflector. Fissions in both fast and thermal groups are considered. The reactor is divided into several annular regions, and the constant flux approximation in each depletion step is then used to solve the fuel and fission-product poisons differential equations in each region. The computer code OPTIME was developed to determine the time variation of core properties during the fuel cycle. At each depletion step, OPTIME calls ODMUG, [12] a criticality search program, from which the spatially-averaged neutron fluxes and control poison cross sections are obtained. A uniform initial fuel distribution was chosen as a benchmark and the results for several different fuel configurations were analyzed. Two different initial control poison distributions were investigated for each fuel configuration: a uniform and a fuel proportional distribution. Using an iterative approach fuel was moved from the low burnup regions toward the high burnup regions; reactor running times were in this way increased from 9000 to 11,500 hours in the fuel proportional control poison distribution case and from 9000 to 11,000 hours in the uniform control poison distribution case. Beyond this point not only did the running time not increase, but no criticality was reached. / Ph. D.