| Popis: |
Thorium as a nuclear fuel has received renewed interest, because of its widespread availability and the good irradiation performance of Th and mixed (Th,U) oxide compounds as fuels in nuclear power systems. Early HTGR development employed thorium together withhigh-enriched uranium (HEU). After 1980, HTGR fuel systems switched to low-enriched uranium (LEU). After completing fuel development for the AVR and the THTR with BISO coated particles, the German program expanded its efforts utilizing thorium and HEU TRISOcoated particles in advanced HTGR concepts for process heat applications (PNP) and directcycle electricity production (HHT). The combination of a low-temperature isotropic (LTI) inner and outer pyrocarbon layers surrounding a strong, stable SiC layer greatly improvedmanufacturing conditions and the subsequent contamination and defective particle fractions in production fuel elements. In addition, this combination provided improved mechanical strength and a higher degree of solid fission product retention, not known previously with high-temperature isotropic (HTI) BISO coatings. The improved performance of the HEU (Th,U)O$_{2}$ TRISO fuel system was successfully demonstrated in three primary areas of development: manufacturing, irradiation testing under normal operating conditions, and accident simulation testing. In terms of demonstrating performance for advanced HTGR applications, the experimental failure statistic from manufacture and irradiation testing are significantly below the coated particle requirements specified for PNP and HHT designs at the time. Covering a range to 1300°C in normal operations and 1600°C in accidents, with burnups to 13% FIMA and fast fluences to 8×10$^{25}$ n/m$^{2}$ (E>16 fJ), the performance results exceed the design limits on manufacturing and operational requirements for the German HTR-Modul concept, which are 6.5×10$^{-5}$ for manufacturing, 2×10$^{-4}$ for normal operating conditions, and 5×10$^{-4}$ for accident conditions. These performance statistics for the HEU (Th,U)O$_{2}$ TRISO fuel system are in good agreement with similar results for the LEU UO$_{2}$ TRISO fuel system. |
