Gas tagging consists of the addition to nuclear reactor fuel pins of small amounts of gas having a unique isotopic composition for each assembly; when an assembly fails during subsequent irradiation, the tag gas which is released along with the fission gas, makes it possible to locate the defective assembly by a mass spectrometric analysis of the reactor cover gas. Location of the ratios of the tag gas isotopic concentrations on curved surfaces in a three-dimensional tag-ratio space enables the three ratios corresponding to failure of a single fuel assembly to be distinguished from those formed from any combination of two or more failed assemblies. Three prototypic designs have been analyzed for the fast flux test facility (FFTF) reactor, and some alternative design possibilities have been suggested. Based upon these results, current FFTF gas tag designs incorporate to a certain extent the principle of curved surfaces.