Activation analysis of reactor building concrete for nuclear power plant decommissioning using the Chebyshev method

This study calculates neutron activation rate and elemental activity in reactor building concrete using SuperMC-3.4 software with the Chebyshev Rational Approximation Method (CRAM). Major elements such as Co, Fe, Ca, Ni, and C were analyzed over 60 years of reactor operation plus 5 years of cooling period. Co-60 and Ni-62 remain significant contributors to gamma dose even after prolonged cooling. The total gamma dose rate at the biological shield decreased from 6.8 mSv/h at shutdown to 0.42 mSv/h after 5 years (a reduction of 93.8%). Consequently, the activity of Co-60 decreased from 1.2 × 105 to 7.6 × 103 Bq/cm3. The results demonstrated that decommissioning after 60 years of operation plus 5 years of cooling meets the requirements of IAEA-GSR Part 6, maintains occupational exposure below the ICRP limits (20 mSv/year with protective measures), and supports ALARA-compliant radiation protection planning. To estimate the required shielding thickness, a standard lead apron (0.5 mm Pb) resulted in a dose reduction of approximately 20%. Also, mobile lead barriers (50 mm) led to a dose reduction of approximately 50%. Using combined shielding could achieve a potential reduction of 70–80%, thus enabling extended operation near concrete surfaces. Compared to international standards, the gamma dose rate at shutdown and after 5 years showed good agreement with reported reference values (deviation ± 9%) for various plant types, confirming the effectiveness of CRAM implementation. Cooling for 5 years after reactor shutdown reduces radiation dose by over 90%, enabling safer decommissioning. Cobalt-60 and Nickel-62 are the main radioactive elements remaining in concrete after years of cooling. The method confirms occupational radiation exposure can stay below international safety limits with protection.