Critical comparison of INAA and ICP-MS applied in the characterization of purity of TRISO fuel and substrates to its production

The application of inductively coupled plasma mass spectrometry (ICP-MS), both in solution and laser ablation (LA) mode, and instrumental neutron activation analysis (INAA) in the nuclear material analysis are presented in this paper. The possibility of each technique for the chemical characterization of substances used during TRISO fuel production and its advantages and limitations are discussed based on the obtained results of the analysis of real materials used in TRISO fuel production in the Institute of Nuclear Chemistry and Technology. The paper also reports the application of INAA and LA-ICP-MS to the verification of the purity of the protective layers of pyrolytic carbon (PyC) and silicon carbide.

Raman Spectroscopy Studies of TRISO-Particle Fuel

Development of High Temperature Gas-cooled Reactors opens new horizons for nuclear power in Poland. Good understanding of the failure-free performance of Tri-structural ISOtropic (TRISO)-particle fuel is a key for the safe and efficient operation of those reactors. It is also essential to avoid potential errors in TRISO fuel production for the HTR program in Poland. In a longer perspective to ensure the highest possible quality of the TRISO fuel fabrication and storage before loading into the reactor core, it is necessary to control the quality of the TRISO-particle fuel in order to understand the aging of fresh fuel. Nevertheless, such a solution requires to determine whether the passage of time affects the occurrence of changes in TRISO fuel layer’s structure and at the same time whether it contributes to increasing the probability of damage to the examined fuel material. For this purpose, it is planned to perform an experiment on different types of TRISO fuel, produced in different periods of time. The comparative analysis will be based mainly on the experimental method of Raman spectroscopy.

Microstructural and Micro-Chemical Evolutions in Irradiated UCO Fuel Kernels of AGR-1 and AGR-2 TRISO Fuel Particles

AGR-1 and AGR-2 tristructural-isotropic (TRISO) fuel particles were fabricated using slightly different fuel kernel chemical compositions, modified fabrication processes, different fuel kernel diameters, and changed 235U enrichments. Extensive microstructural and analytical characterizations were conducted to correlate those differences with the fuel kernels’ responses to neutron irradiations in terms of irradiated fuel microstructure, fission products’ chemical and physical states, and fission gas bubble evolutions. The studies used state-of-the-art transmission electron microscopy (TEM) equipped with energy-dispersive x-ray spectroscopy (EDS) via four silicon solid-state detectors with super sensitivity and rapid speed. The TEM specimens were prepared from selected AGR-1 and AGR-2 irradiated fuel kernels exposed to safety testing after irradiation. The particles were chosen in order to create representative irradiation conditions with fuel burnup in the range of 10.8 to 18.6% fissions per initial metal atom (FIMA) and time-average volume-average temperatures varying from 1070 to 1287°C. The 235U enrichment was 19.74 wt.% and 14.03 wt.% for the AGR-1 and AGR-2 fuel kernels, respectively. The TEM results showed significant microstructural reconstructions in the irradiated fuel kernels from both the AGR-1 and AGR-2 fuels. There are four major phases: fuel matrix of UO2 and UC, U2RuC2, and UMoC2—in the irradiated AGR-2 fuel kernel. Zr and Nd form a solid solution in the UC phase. The UMoC2 phase often features a detectable concentration of Tc. Pd was mainly found to be located in the buffer layer or associated with fission gas bubbles within the UMoC2 phase. EDS maps qualitatively show that rare-earth fission products (Nd et al.) preferentially reside in the UO2 phase. In contrast, in the irradiated AGR-1 fuel kernel, no U2RuC2 or UMoC2 precipitates were positively identified. Instead, there was a high number of rod-shaped precipitates enriched with Ru, Tc, Rh, and Pd observed in the fuel kernel center and edge zone. The differences in irradiated fuel kernel microstructural and micro-chemical evolution when comparing AGR-1 and AGR-2 TRISO fuel particles may result from a combination of irradiation temperature, fuel geometry, and chemical composition. However, irradiation temperature probably plays a more deterministic role. Limited electron energy-loss spectroscopy (EELS) characterizations of the AGR-2 fuel kernel show almost no carbon in the UO2 phase, but a small fraction of oxygen was detected in the UC/UMoC2 phase.

TRISO Fuel Performance Modelling with BISON

Modeling of tristructural isotropic (TRISO)-coated particle fuel is being refined in the fuel performance code BISON. New developments include the implementation of an updated set of material properties, TRISO failure mechanisms, fission product diffusion parameters, and the design of a Monte Carlo scheme that allows BISON to calculate the probability of fuel failure within a population of TRISO particles and the subsequent fractional release of key fission products.