Benchmarking PHITS for radiation dose assessment from fly ash-blended concrete using MCNP6 and ResRad-Build

In this study, the PHITS Monte Carlo code is applied for the first time to evaluate indoor radiation exposure associated with fly ash-blended concrete, and its performance is systematically benchmarked against the widely used ResRad-Build program and MCNP6 code. A reference room conforming to RP-112 dimensions (5 m × 4 m × 2.8 m) was modeled, and eight concrete mixtures containing 0-50% fly ash were investigated. The activity concentrations of ²²⁶Ra, ²³²Th, and ⁴⁰K, along with material composition ratios and densities, were used as input parameters for simulations with PHITS, ResRad-Build, and MCNP6. The annual effective doses calculated using PHITS ranged from 0.378 to 0.489 mSv.y^-1. The maximum relative deviations between PHITS and MCNP6 and between PHITS and ResRad-Build were 5.9% and 7.9%, respectively. PHITS exhibited the closest agreement with the reference dose estimate derived from measured activity concentrations, with a maximum relative deviation of 3.6%. These findings clearly demonstrate that PHITS is a highly reliable and effective tool for assessing indoor radiation exposure from building materials.