b'Optical Fiber-basedA new method measures fission gas released into the plenum region of nuclear Fabry-Perot Interferometryfuel rods to improve thermal and mechanical modeling of reactors.for Fission Gas Detection U nderstanding and predicting fission gas release is extremely important for both thermal and mechanical modeling of a reactor. Fission gases generally precipitate into bubbles that cause fuel swelling, resulting in pellet-cladding gap closure and mechanical interaction. Fission gas that releases to the fuel rod free volume (ultimately the plenum) causes pressure build-up and thermal conductivity degradation of the rod filling gas.TOTAL APPROVED AMOUNT:The objective of this research was to develop a sensor using optical fibers for in-pile $182,000 over 3 years detection of fission gas in the plenum region of fuel rods. The data from the new PROJECT NUMBER:sensor can be used to validate the physics-based models of fission gas release and 19P43-007 build-up in the plenum.PRINCIPAL INVESTIGATOR:Currently, for pre-irradiated and re-fabricated test rodlets, the released fission Austin Fleming gas (predominately xenon) is mixed with the backfilled helium. It is a technical challenge to measure the fission gas concentrations due to limited space and gas volume. Typical xenon measurements take advantage of the strong emission and absorption peaks in the near infrared wavelengths, which is impossible to implement inside the fuel cladding.This research used xenons high index of refraction as compared to helium to detect the concentration of fission gas in the plenum region, which is a novel application of this interferometric technique. The interferometry is based on a measurement of phase rather than amplitude, which minimizes the possible impact of radiation darkening on the measurement.TALENT PIPELINE:Austin Fleming, INLRussell L. Heath Distinguished PostdocSohel Rana, student at Boise State University(a) Fiber optic sensor made by splicing silica capillary tubes to single mode optical fiber. (b) Interference spectrum from sensor for varying argon and helium concentrations. The index of refraction can be determined from tracking an individual peak or the separation between peaks. 25'