The AGR-1 experiment is a multiple fueled-capsule irradiation experiment being conducted in the Advanced Test Reactor (ATR) in support of the Advanced Gas Reactor (AGR) Fuel Development and Qualification Program. The experiment began irradiation in the ATR with a cycle that reached full power on December 26, 2006 and ended with shutdown of the reactor for a brief outage on February 10, 2007 at 0900. The AGR-1 experiment will continue cyclical irradiation for about 2.5 years. In order to allow estimation of the amount of radioiodine released during the first cycle, purge gas flow to all capsules continued for about 4 days after reactor shutdown. The FPMS data acquired during part of that shutdown flow period has been analyzed to elucidate the level of 135I released during the operating cycle.

The Advanced Gas Reactor-1 (AGR-1) experiment is the first experiment in a series of eight iow-enriched uranium oxycarbide tri-isotropic (TRISO) coated particle fuel (in compact form) experiments scheduled for irradiation in the Advanced Test Reactor (ATR) located at the Idaho National Laboratory (INL). The experiment began irradiation in the ATR with a cycle that reached full power on December 26, 2006 and will continue irradiation for about 2.5 years. Six separate test fuel capsules are being irradiated in an inert sweep gas atmosphere with on-line fission product monitoring of each capsule's effluent to track fission gas emissions from the fuel during irradiation. The online fission product monitoring system incorporates 7 HPGe spectrometers and 7 Nal(Tl) gross radiation detectors to monitor and quantify the fission gas releases that are important indicators of fuel performance. Details of the design and operation of this detection system and the preliminary results of the fuel performance measurements are presented in this paper.