An examination of tag gas designs for the Fast Flux Test Facility and the Clinch River Breeder Reactor Plant indicates that an unambiguous identification of an assembly with a breached pin should be possible under normal operating conditions when 1% or less of the tag gas escapes; correction for the background tag gas in the argon cover gas improves the probability of correct identification. The probabilistic method of Schmittroth has been further developed for use in situations in which the escaped tag gas does not permit a unique identification of a failed assembly.