A measurement system has been designed and built for the specific application of measuring the effective thermal conductivity of a composite, nuclear-fuel compact (small cylinder) over a temperature range of 100 • C to 800 • C. Because of the composite nature of the sample as well as the need to measure samples pre-and postirradiation, measurement must be performed on the whole compact non-destructively. No existing measurement system is capable of obtaining its thermal conductivity in a non-destructive manner. The designed apparatus is an adaptation of the guardedcomparative-longitudinal heat flow technique. The system uniquely demonstrates the use of a radiative heat sink to provide cooling which greatly simplifies the design and setup of such high-temperature systems. The design was aimed to measure thermalconductivity values covering the expected range of effective thermal conductivity of the composite nuclear fuel from 10 W · m −1 · K −1 to 70 W · m −1 · K −1 . Several materials having thermal conductivities covering this expected range have been measured for system validation, and results are presented. A comparison of the results has been made to data from existing literature. Additionally, an uncertainty analysis is presented finding an overall uncertainty in sample thermal conductivity to be 6 %, matching well with the results of the validation samples.

h i g h l i g h t sMeasured the ETC of surrogate TRISO fuel compacts experimentally and numerically. The ETC of the surrogate fuel compacts varies between 50 and 30 W m À1 K À1 . A new model/approach to analyze the effect of constituent materials on ETC is proposed. a b s t r a c t Accurate modeling capability of thermal conductivity of tristructural-isotropic (TRISO) fuel compacts is important to fuel performance modeling and safety of Generation IV reactors. To date, the effective thermal conductivity (ETC) of TRISO fuel compacts has not been measured directly. The composite fuel is a complicated structure comprised of layered particles in a graphite matrix. In this work, finite element modeling is used to validate an analytic ETC model for application to the composite fuel material for particle-volume fractions up to 40%. The effect of each individual layer of a TRISO particle is analyzed showing that the overall ETC of the compact is most sensitive to the outer layer constituent. In conjunction with the modeling results, the thermal conductivity of matrix-graphite compacts and the ETC of surrogate TRISO fuel compacts have been successfully measured using a previously developed measurement system. The ETC of the surrogate fuel compacts varies between 50 and 30 W m À1 K À1 over a temperature range of 50-600°C. As a result of the numerical modeling and experimental measurements of the fuel compacts, a new model and approach for analyzing the effect of compact constituent materials on ETC is proposed that can estimate the fuel compact ETC with approximately 15-20% more accuracy than the old method. Using the ETC model with measured thermal conductivity of the graphite matrix-only material indicate that, in the composite form, the matrix material has a much greater thermal conductivity, which is attributed to the high anisotropy of graphite thermal conductivity. Therefore, simpler measurements of individual TRISO compact constituents combined with an analytic ETC model, will not provide accurate predictions of overall ETC of the compacts emphasizing the need for measurements of composite, surrogate compacts.

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Concentrating solar power (CSP) with thermal energy storage has potential to provide grid-scale, on-10 demand, dispatachable renewable energy. As higher solar receiver output temperatures are necessary for 11 higher thermal cycle efficiency, current CSP research is focused on high outlet temperature and high 12 efficiency receivers. The objective of this study is to provide a simplified model to analyze the thermal 13 efficiency of multi-cavity concentrating solar power receivers. The model calculates an optimal aperture 14 flux that maximizes the local efficiency, constrained by a maximum receiver working temperature. Using 15 this flux, the thermal efficiency, receiver temperature, and heat transfer fluid (HTF) temperature are 16 calculated based upon an optimized flux distribution. The model also provides receiver design and HTF 17 heat transfer requirements to achieve the necessary overall thermal efficiency. From the results, possible 18 HTFs can be investigated to determine which ones are feasible. A case study was performed on a multi-19 cavity tube receiver design to demonstrate the use of the model. The case study receiver design had an 20 effective absorptivity of 99.8%, and was modeled with conservative values for thermal constraints. It was 21 found that a HTF with a minimum convection coefficient between 250-500 W•m-2 •K-1 , depending on the 22 convective heat transfer to the environment, is necessary to achieve a thermal efficiency greater than 90% 23 for the receiver. The general model can provide a design guideline for attainable thermal efficiencies of 24 multi-cavity concentrating solar power receivers given thermal constraints and heat transfer conditions. 25

The Transient Reactor Test (TREAT) Facility provided thousands of transient irradiations and plays a crucial role in nuclear-heated safety research. The facility's flexible design and multi-mission nature saw historic experiments for numerous reactor fuels and transient types but was never specifically adapted to address very-brief pulse transients akin to postulated Light Water Reactor (LWR) Reactivity Initiated Accidents (RIA). Since the behaviors of fuel under these conditions depends strongly on energy input duration and resulting cladding time/temperature response under pellet cladding mechanical interactions, this three-year project was conceived to investigate new pulse-narrowing capabilities. Kinetic models showed incremental improvements for minor facility enhancements including increased reactivity step insertions (to initiate the power pulse) and slightly-increased transient rod drive speed for pulse termination ("clipping"). Replacing peak fuel assemblies, repositioning non-transient control rods to hold down the "hot side" of the core, and balancing against required excess reactivity needs, the limiting fuel assembly power can be reduced by ~20 %. This is a remarkable discovery of latent capability in TREAT, not only in enabling the subject capabilities for reduced pulse widths, but also significantly "uprating" the core's transient energy capacity. Incremental improvements can likely enhance TREAT's capability into BWR-relevant missions; briefer PWR pulse shapes can only be achieved with an advanced clipping system. Helium-3 ( 3 He) was found to offer the greatest benefits for clipping design and overall performance. A unique concept showed great promise for enabling a 3 He-based system with a reasonable cost; this design concept will likely become the focal point of future work.