QUANTITATIVE EVALUATION OF RADIATION FIELD BEHAVIOR IN SUBTERRANEAN TUNNEL ENVIRONMENTS USING MONTE CARLO MODELING

Abstract

Subterranean transportation and utility tunnel systems present unique radiological response challenges due to restricted geometry, limited ventilation, and complex photon scattering environments. In the event of an accidental or deliberate radiological release, accurate characterization of the radiation fields within these environments is critical for first responder protection and risk-informed decision-making. However, conventional open-air dose-rate assumptions may not adequately represent radiation transport behavior in confined tunnel geometries. This research quantifies gamma radiation transport and effective dose rates in representative subterranean tunnel configurations using Monte Carlo N-Particle (MCNP) simulations. Photon-emitting radionuclides of operational relevance, including Cs-137, Co-60, Ir-192, and Ra-226 (with progeny in equilibrium), were modeled under varying source strengths and spatial configurations. Fluence-to-dose conversion was performed using established gamma dose rate constants and the International Commission on Radiological Protection (ICRP)-based dose conversion methodologies. Dose fields were evaluated as a function of distance, geometry, and material composition to assess deviations from inverse-square behavior observed in free-field environments. Results demonstrate that tunnel geometry significantly alters photon fluence distribution through enhanced scattering and wall backscatter effects, resulting in spatial dose profiles that differ measurably from open-air assumptions. In certain configurations, dose rates at extended distances were sustained above free-field expectations, indicating potential underestimation of responder risk when applying standard outdoor models. The magnitude of deviation was radionuclide-dependent and influenced by photon energy spectra and tunnel material composition. These findings support the need for geometry-specific modeling in subterranean radiological response planning and provide quantitative insight for operational health physics assessments. The work contributes to improved dose prediction accuracy in confined environments and informs emergency response doctrine for underground radiological incidents.