The development and application of two quantitative methods that could be used as part of the decisionmaking process in the licensing of nuclear power plants are described. These methods are (a) the use of quantitative screening criteria to assess the adequacy of the safety functions in existing plants and (b) the use of value/impact or cost /benefit analysis to determine limits to the cost-effective expenditure on“back-fitting” to improve safety. It also is shown that the results obtained by the two methods are not necessarily compatible with one another. As an example, the two methods are applied to the question of improving the decay heat removal function for light water reactors. Screening criteria are presented for this function for both pressurized water reactors and boiling water reactors. The value/impact assessment is carried out as a function of site population, existing plant features, and new plant features.