Quantity of ¹³⁵I released from the AGR-1, AGR-2, and AGR-3/4 experiments and discovery of ¹³¹I at the FPMS traps during the AGR-3/4 experiment

Abstract

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. tristructural 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 HPGe detector provides individual isotopic information, while the NaI(Tl) scintillator is used as a gross count rate meter. During irradiation, the 135m Xe concentration reaching the FPM detectors is from both direct fission and by decay of the accumulated 135 I. About 2.5 hours after irradiation (ten 15.3 minute 135m Xe half-lives) the directly produced 135m Xe has decayed and only the longer lived 135 I remains as a source. Decay systematics dictate that 135m Xe will be in secular equilibrium with its 135 I parent, such that its 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 135 I released from the fuel particles by measurement of the 135m Xe at the FPM following reactor shutdown. In this paper, the 135 I released will be reported and compared to similar releases for noble gases as well as the unexpected finding of 131 I deposition from intentional impure gas injection into capsule 11 of experiment AGR-3/4. viii ix ACKNOWLEDGMENT