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The close ofthe 1980s saw many trials and applications ofjiber in the loop or FITL. Most delivered traditional telephony (i.e., narrowband) services to the home, but a few provided both narrowband and entertainment video (i.e., broadband) services. During thattime, the local-exchange carriers (LECs) also formulated theirobjectives and strategies for deploying FITL. As they gained a betterunderstanding ofthe marketplace and the political realities ofdelivering video services, the LECs adopted an FITL-deployment strategy based on strict cost-effectiveness for the delivery oftraditional telephone services. However, any fiber-optic access architecture theyadopt has to supportbroadband services in the future. To address the costand service challenges ofFITL, the LECs and vendors must continually evaluate many alternative access architectures to identify potential advantages that can help fiber access achieve costparity with copper access. Total system costs must be considered including electrical and optical components, powering system, fiber and cable components, and life-cycle costs (Le., administration, maintenance, assignment, and provisioning operations).

The article contains sections titled: Introduction History of HTGRs and Coated Particle Fuel Development HTGRs Built and Operated HTGRs Designed but Not Built Modular Reactors (1980 s Design) Current Modular Reactor Designs Coated Particle Fuel Development Development in the UK Development in G ermany Development in the US Recent Directions Fuel Manufacturing Processes Fuel K ernel Manufacture Powder Metallurgical Process Sol–Gel Process Gel‐Precipitation Process for UO 2 Kernels Gel‐Precipitation Process for UCO Kernels Fabrication of PuO 2 Kernels Ceramic Coatings by CVD Pyrolytic Carbon ( PyC ) Coatings Silicon Carbide ( SiC ) Coatings Fuel Element Manufacture Spherical Fuel Elements for Pebble‐Bed Cores Hexagonal‐Block Fuel Elements for Prismatic Cores HTGR Fuel Materials Properties Properties of UO 2 Fuel Kernels Thermal Properties of UO 2 Mechanical Properties of UO 2 Swelling Rate of UO 2 Properties of Pyrolytic Carbon ( PyC ) Coatings Thermal Properties of PyC Theoretical Density of PyC Mechanical Properties of PyC Irradiation‐Induced Dimensional Change of PyC Properties of Silicon Carbide ( SiC ) Coatings Thermal Properties of SiC Mechanical Properties of SiC Swelling Rate of SiC Fuel Quality Control and Performance Evaluation Quality Control of UO 2 Kernels Quality Control of Coated Particles Quality Control of Spherical Fuel Elements Failure Statistics and Performance Requirements The Mathematics of Failure Statistics Statistical Analysis of Fuel Element Manufacture Statistical Analysis of Irradiation Performance Statistical Analysis of Accident Condition Performance Comparison to Performance Requirements Fuel Performance under Normal Operating Conditions Irradiation Testing of Modern UO 2 TRISO‐Coated Particles Accelerated Irradiations in Material Test Reactors HFR ‐ P 4 Test SL ‐ P 1 Test HFR ‐ K 3 Test FRJ 2‐ K 13 Test FRJ 2‐ K 15 Test FRJ 2‐ P 27 Test Conduct of the Accelerated Tests HTR MODUL Proof Tests HFR ‐ K 5 and HFR‐K 6 AVR Real‐Time Irradiation Testing Performance Assessment for Normal Operating Conditions Fuel Performance under Accident Conditions Simulation of Core Heat‐Up after Depressurization under Dry Conditions Analysis of Accident Simulation Testing Behavior after Water and Air Ingress Simulation of Water Ingress Simulation of Air Ingress HTGR Fission Product Generation and Transport Fission Product Generation Fission Product Transport Equivalent Sphere Model for Gas Release Recoil Release from Fuel Kernel Release from Post‐Irradiation Heating Tests Release of Short‐Lived Xe and Kr Isotopes Retention by a Single Coating Layer Release for a Single Shell Diffusion Data for Coating Layers Applicability and Uncertainties of Transport Data Transport and Release from Fuel Elements in Reactor Tests and HTGRs Fission Product Chemistry and CO Generation in UO 2 Coated Particle Failure Mechanisms Pressure‐Induced Failure Internal Gas Pressure Buildup Predicting Pressure‐Induced Particle Failure STRESS Modeling The R edlich– K wong Equation for Internal Gas Pressure Cracking of PyC Coatings due to Fast Fluence iPyC Failure iPyC – SiC Interface Debonding oPyC Failure Fast Neutron‐Induced PyC Cracking Kernel Migration (Amoeba Effect) Kernel Migration in UO 2 ‐Coated Particle Fuel Kernel Migration in Uranium Carbide Coated‐Particle Fuel Kernel Migration in Thorium Oxide Coated‐Particle Fuel Corrosion of the SiC Layer CO Attack of SiC Palladium– SiC Interactions Silver Migration through SiC Layer Rare Earth Fission Product Attack SiC Decomposition As‐Manufactured Defective Particles Contamination during Manufacture Irradiation‐Induced Failures Summary of Coated Particle Failure Mechanisms

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