The sensitivity of light water reactor (LWR) fuel cycle parameters and costs to uncertainties in thermal nuclear data and methods is examined using a code package developed at Rensselaer Polytechnic Institute. Cross sections averaged over the thermal energy (<1- or 2-eV) group are shown to have an important economic role for LWRs. When it has been determined that fuel cycle parameters and costs are sensitive to a specific thermal group cross section, it becomes desirable to determine how specific energy-dependent cross sections influence fuel cycle parameters and costs. The FASTT code was written to compute detailed sensitivity coefficients using either a direct or a perturbation technique. Multigroup cross-section sensitivity coefficients vary with fuel exposure. After computing the changed exposure-dependent thermal group cross section, new fuel cycle parameters and costs are computed by a sequence of fuel depletion, core analysis, and cost codes. One can therefore obtain the change in fuel cycle cost for different fuel cycle options induced by a change in the shape of a detailed thermal cross section. A striking feature of our thermal analyses is the (usually) overwhelming importance of the hardened Maxwellian energy region (0.01 to 0.1 eV). The FASTT code is also used to determine the importance of the frequency distribution used to compute neutron scattering kernels based on the incoherent approximation. The sensitivities to Nelkin's scattering data are not large. A method, having potentially large implications for LWR design, is developed for obtaining correspondence among different scattering kernels.