Determination of the Quantity of I-135 Released from the AGR Experiment Series

A series of three Advanced Gas Reactor (AGR) experiments have been conducted in the Advanced Test Reactor (ATR) at Idaho National Laboratory (INL). From 2006 through 2014, these experiments supported the development and qualification of the new U.S. tri-structural isotropic (TRISO) particle fuel for Very High Temperature Reactors (VHTR). Each AGR experiment consisted of multiple fueled capsules, each plumbed for independent temperature control using a mix of helium and neon gases. The gas leaving a capsule was routed to individual Fission Product Monitor (FPM) detectors. For intact fuel particles, the TRISO particle coatings provide a substantial barrier to fission product release. However, particles with failed coatings, whether because of a minute percentage of initially defective particles, those which fail during irradiation, or those designed-to-fail (DTF) particles, can release fission products to the flowing gas stream. Because reactive fission product elements like iodine and cesium quickly deposit on cooler capsule components and piping structures as the effluent gas leaves the reactor core, only the noble fission gas isotopes of Kr and Xe tend to reach FPM detectors. The FPM system utilizes High Purity Germanium (HPGe) detectors coupled with a thallium activated sodium iodide NaI(Tl) scintillator. The germanium detector provides individual isotopic information, while the NaI(Tl) scintillator is used as a gross count rate meter. During irradiation, the ^135mXe concentration reaching the FPM detectors is from both direct fission and by decay of the accumulated ¹³⁵I. About ~2.5 hours after irradiation (ten 15.3 minute ¹³⁵mXe half-lives) the directly produced ^135mXe has decayed and only the longer lived ¹³⁵I remains as a source. Decay systematics dictate that ^135mXe will be in secular equilibrium with its ¹³⁵I parent, such that it's production rate very nearly equals the decay rate of the parent, and its concentration in the flowing gas stream will appear to decay with the parent half-life. This equilibrium condition enables the determination of the amount of ¹³⁵I released from the fuel particles by measurement of the ^135mXe at the FPM following reactor shutdown. In this paper, the ¹³⁵I released will be reported and compared to similar releases for noble gases as well as the unexpected finding of ¹³¹I deposition from intentional impure gas injection into capsule 11 of experiment AGR-3/4. (author)