Study On The Release Of Fission Products In Fuel Elements Of HTGR

The amounts of radionuclides in High Temperature Gas-cooled Reactors?HTGRs?under the normal operation condition are important for maintenance dose evaluating and the accumulated radionuclides in the primary loop under the normal operation condition are also essential for the evaluating the source terms of hypothetical accidents.Therefore,the conservative source terms calculation is crucial for the reactor safety analysis.The article analyzes the detail modeling and discretization of fission products diffusion in High Temperature Gas-cooled Reactors fuel elements with using STACY software,calculates the source terms under different conditions in of KüFA fuel elements heating test done by Jülich,compares the test result and organizes the independent verification.To study the conservativeness of source terms evaluation process and result is of great importance in evaluating the adaptation of STACY software in source terms design and verification.The results show that the main uncertainties are most likely introduced less by shortcomings of the model,but rather by input data strongly depending on material properties and exhibiting a broader variability.It is obvious from postcalculations that the strontium data are largely overpredicted in most cases.While the retention capability of UO₂ and the matrix material of the fuel spheres is visible,it is most probably the diffusive transport through the silicon carbide layer that appears very conservative and should undergo a thorough review.Metallic fission product release remained well below the expected levels from previous experience,which indicates release is largely overpredicted by the model calculations.The comparison of cesium and krypton release gives indication that during irradiation/heating particles with a failed SiC layer,but still intact oPyC layer may have developed.Silver release measurements are often unusual and inconsistent,and therefore extremely difficult for postcalculation.The results show that the STACY results are conservative,especially for the estimation of the important nuclides.The STACY program is feasible for engineering design and verification.