A novel assessment of the conservatism in the UNF-ST&DARDS decay heat calculations has been performed. UNF-ST&DARDS is used to quantify the uncredited margin in safety analysis calculations for spent nuclear fuel (SNF) storage, transportation, and disposal systems. The goal of the assembly-specific as-loaded safety analysis approach in UNF-ST&DARDS is to determine the time-dependent realistic state of the SNF systems; however, it is desirable to conservatively estimate safety analysis parameters, such as decay heat, for a given set of fuel characteristics. The primary source of conservatism in the generic UNF-ST&DARDS assembly-specific as-loaded analysis (also referred to as bounding within UNF-ST&DARDS) is the conservative assumptions of various reactor operational parameters that attempt to envelop wide spectrum of reactor operating scenarios. The assessment in this paper is necessary to demonstrate that sufficient decay heat conservatism is retained in the UNF-ST&DARDS bounding as-loaded spent fuel analysis methodology. This paper also demonstrates the time dependent impact of various parameters such as last cycle power on decay heat values. A comparison between the UNF-ST&DARDS bounding decay heat calculations and calculations performed using a detailed description of the fuel assembly operating histories, referred to as detailed calculations, was performed using recently acquired data. The data used to perform this evaluation are from one set of 3019 assemblies from a US boiling water reactor (BWR) site and one set of 2117 assemblies (952 8 × 8, and 1165 10 × 10) from a Swedish BWR reactor. Analyses of the US data involved two sets of assumptions for the bounding calculations and produced two data sets. The first analysis, in which the cycle-wise burnups were derived for the bounding calculations from the detailed data, generated the derived data set; the second, in which the assumptions associated with incorporating US nuclear fuel data survey (Form GC-859) data were included in the calculations for a subset of the same assemblies, generated the GC-859 data set. When bounding assumptions were used, the average level of conservatism (overestimation of decay heat) ranges between 9.0% and 17.7% for the derived data set, between 11.4% and 32.3% for the GC-859 data set, between 10.1% and 62.6% for the Swedish 8 × 8 fuel and between 8.3% and 44.7% for the Swedish 10 × 10 fuel. The level of conservatism and the scatter in the ratio between bounding and detailed data increase significantly for the 100- and 200-year cases for derived US and Swedish data sets. The GC-859 data set had large conservatisms in the early cooling times that initially shrank with time and then increased for the 100-year and 200-year cooling times. These results show that, while UNF-ST&DARDS can be used to calculate assembly decay heat based on assembly characteristics and operating history to identify potential significant margins to the licensing basis decay heat calculations, the decay heats calculated by UNF-ST&DARDS are still conservative compared to more detailed calculations for the range of assemblies and operating conditions assumptions evaluated in this study.