[en] The Fast Flux Test Facility (FFTF) is a liquid-metal-cooled, fast neutron test reactor located on the Hanford Site. A dual computer system has been developed to monitor the reactor cover gas to detect and characterize any fuel or test pin fission gas releases. The system acquires gamma spectra data, identifies isotopes, calculates specific isotope and overall cover gas activity, presents control room alarms and displays, and records and prints data and analysis reports. The Fission Gas Monitor System (FGMS) integrates commercially available hardware and software, providing a reliable and easily maintained system. The design provides extensive automation of previous manual operations, reducing the need for operator training and minimizing the potential for operator error. The dual nature of the system allows either system A or B to be taken out of service for periodic tests or maintenance without interrupting the overall system performance. A control room color display tracks the specific activity of selected isotopes, provides alarms when a fission gas release is detected, and characterizes the nature and source of the release. Sources in the FFTF that can be uniquely identified include fuel pins, control rod absorber pins, and N- and M-type test pins which are irradiated in the reactor Materials Open Test Assembly (MOTA). The principal method used to characterize the source of the assembly leak involves the analysis of four isotopes, namely Xe-125, Xe-133, Xe-135, and Kr-85m, and the ratios of Xe-133/Xe-135 and Xe-133/Kr-85m. The computer analysis also compensates for Xe-133 buildup which reaches a saturation value after about 20 days of full-power operation. In addition, the computer calculates the specific activity of 20 isotopes which are used to help perform a detailed analysis of leaks from fuel assemblies