Electrochemical Deconsolidation and Fission Product Distribution of Post-irradiated HTR-10 Spherical Fuel Element

Post-irradiation examination is an essential method for fuel performance evaluation. In order to study the distribution of radionuclides in fuel elements as well as the irradiation damage mechanism of TRISO coated fuel particles in spherical fuel element of 10 MW high temperature gascooled reactor (HTR10), a spherical element with moderate burnup of about 35 GW·d/tU was disassembled by twostep electrochemical deconsolidation method. The principle of this method is electrochemical oxidation of graphite. Coated fuel particles, deconsolidated graphite matrix powder and electrolytes from different regions of the element were obtained. The radionuclides in powder and electrolyte were quantitatively analyzed by the γ spectrum method. The efficiency of the γ spectrum measuring system was calibrated by a semiempirical method in order to withstand the interference by background contaminations. The results show that there are two types of radionuclides in the irradiated HTR10 spherical fuel element deconsolidated graphite matrix powder and electrolyte. Fission product nuclides such as 144Ce/144Pr, 137Cs/134Cs, 152Eu/154Eu/155Eu and 95Zr/95Nb, are generated by fission reaction of free uranium contamination in the graphite matrix. Activation product nuclides such as 75Se, 60Co, 54Mn and 46Sc, are directly generated by the activation of impurities in the graphite matrix, and also adsorbed or contaminated from the primary activation fragments by the graphite matrix. The distribution of radionuclides in the graphite matrix of the irradiated spherical fuel element was also determined. Electrolyte samples with higher reactivity of 137Cs and 144Ce are found, indicating that the coated fuel particles in the corresponding area may be damaged. The damage can cause the fuel kernel to contact the electrolyte during the electrochemical disintegration process, and part of the radionuclides is dissolved into the electrolyte. The consistency of volatile and nonvolatile fission products indicates that the damaged particle didn’t undergo long high temperature history. The results show that the content of various fission products in the graphite matrix of the irradiated HTR10 fuel element is negligible when the structure of the coated fuel particle is intact, which reflects the good inclusion of the TRISO coated fuel particles on the radioactive fission products. On the other hand, the activation products are uniformly distributed in the interior of the whole spherical fuel element, while the content of activation products increases significantly on the outer surface of the element and nonuniformly distribute. In this work, a platform and a method for electrochemical deconsolidation of spherical fuel elements were established, as well as an analysis and a test on deconsolidation samples. Results are important for research on damage mechanism analysis and radiation behavior of TRISO coated fuel particles.