b'results have indicated that fuel stressestion. This facility will support initial change rapidly at the initiation of theand ongoing in-pile TWIST experi-LOCA due to the redistribution of thements by providing detailed charac-stored energy in the fuel where theterization of the thermal-hydraulic center portion of the pellet decreasesperformance and the ability to develop in temperature and the outer peripheryand qualify instrumentation, both of increases. It is believed that this maywhich would not be easily achievable have an impact on FFRD behavior asin a nuclear environment.both the fuel stress state and tempera- References:ture are important when describing an[1.] Nuclear Energy Institute, The overpressurization of fission gas-filledEconomic Benefits and Challenges pores in the fuela leading hypothesiswith Utilizing Increased Enrich-of the driving force of fuel fragmenta- ment and Fuel Burnup for Light-tion [2]. In general, temperature rampWater Reactors, 2019.rate effects in the cladding and the fuel have been shown to have strong[2.] L. O., Jernkvist, Modelling of impacts on both cladding and fuelFine Fragmentation and Fission behaviors including fission gas releaseGas Release of UO 2Fuel in Ac-[3], never evaluated during an integralcident Conditions, EPJ Nuclear LOCA experiment and requiring fissionSciences and Technologies, Vol. 5, heating of the specimen. Iss. 11, 2019.The design of TWIST has also focused[3.] A. Yamauchi, "Study on the Re-strongly on its instrumentation packagelationship Between Fuel Frag-which is needed to provide data to thementation during a LOCA and multiscale advanced modeling effortsPellet Microstructure," Journal of within DOE R&D programs. SeveralNuclear Science and Technology, first-of-a-kind in-situ instrumenta- pp. 1-13, 2021.tion approaches have been developed and are being integrated into TWIST including: optical fiber based noncon-tact temperature measurements in the cladding balloon region, an electro-impedance sensor to measure cladding balloon extent, and the TREAT Fuel Motion Monitoring System to provide in-situ monitoring of fuel relocation. Along with the design of the in-pile TWIST experiment vehicle, an out-of-pile equivalent experiment facility has been designed and is under construc-2021|AFC ACCOMPLISHMENTS 147'