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.
Khorshidi et al. (Wed,) studied this question.