An investigation is made of considerations pertaining to the immediate detection of a fuel element failure of the order of 100 cm/sup 2/ of exposed fuel alloy, under certain conditions and requirements. The problem is viewed in the light of S3G conditions. Of the methods considered, fission gas detection is concluded to offer the best possibilities for immediate failure detection. A good choice of instrumentation for the detection of the fission gases is not given. Various types of detectors are considered, and it is recommended that a choice wait upon experimental evaluation. (auth)