Comprehensive Analysis of Burnup-Driven Fission Product Evolution and Radiological Releases in VVER-1200 in Bangladesh’s First Nuclear Power Plant

This study evaluates the evolution of radionuclide inventories in nuclear fuel as a function of burnup up to 19.808 MWD/kg, focusing on noble gases, halogens, alkali metals, tellurium-group isotopes, noble metals, lanthanides, and cerium-associated actinides. The results demonstrate distinct burnup-dependent behaviors governed by radionuclide production, radioactive decay, neutron capture, and transmutation processes. Noble gas isotopes exhibited continuous accumulation, stabilization, or early depletion depending on half-life, while halogen and alkali metal isotopes showed strong sensitivity to decay characteristics and fuel depletion. Tellurium-group isotopes displayed mixed behavior, with lower-mass tellurium isotopes accumulating continuously, whereas higher-activity isotopes reached early maxima and declined. Noble metal isotopes were predominantly accumulation-driven, reflecting sustained production throughout irradiation. Lanthanides generally exhibited delayed or early peak activities followed by near-equilibrium stabilization, although selected isotopes and minor actinides accumulated continuously. Cerium-related isotopes and transuranic nuclides, particularly ²³⁹ Np, ²³⁸ Pu, ²³⁹ Pu, and ²⁴¹ Pu, demonstrated persistent growth with increasing burnup, highlighting their importance for long-term radiotoxicity and spent-fuel management. Comparison with NUREG-1940 reference values showed overall agreement, although most calculated inventories were underestimated by approximately 15–50%. The study demonstrates that fuel burnup significantly influences fission product inventories and radiological source terms in a VVER-1200 reactor. Higher burnup results in larger inventories of key radionuclides and increased potential radiological releases. The findings provide useful information for reactor safety analysis, accident consequence assessment, and emergency preparedness relevant to the Rooppur Nuclear Power Plant.