[en] The NRC has issued a revised ECCS rule which allows the use of best estimate computer codes for safety analysis, providing the uncertainty of the calculations are quantified and compared with acceptance limits contained in 10 CFR Part 50. To support the revised rule, the NRC and its contractors and consultants have developed and demonstrated a methodology to quantify uncertainty called CSAU (Code Scaling, Applicability and Uncertainty). The methodology consists of three primary elements containing 14 Steps. The first element, ''Requirements and Capabilities,'' which contains the first six steps, is described and demonstrated in this paper. The objective of this element is to characterize the important contributors to uncertainty. The objective is accomplished by determining the applicability of a code to analysis of a transient in a Nuclear Power Plant through comparison of the scenario- and plant-dictated requirements with the simulation capabilities of the code. 13 refs., 7 figs., 4 tabs