Extrapolation of Monte Carlo–Derived Noble Gas Air-Submersion Effective Dose Rate Coefficients for Selected Occupational Settings to Other Geometries and Radionuclides

External exposures to airborne radioactivity at nuclear power sites are typically based on semi-infinite clouds with uniform concentrations and corresponding effective dose rate coefficients (EDRCs), along with the simplifying assumption that the radioactive clouds extend to infinity around the receptor of interest. The two regulatory models that are typically employed for finite-cloud adjustments to submersion doses, namely, Regulatory Guide 1.183 for hemispherical clouds and the International Commission on Radiological Protection (ICRP) document ICRP-30 for spherical clouds, were purposely oversimplified to facilitate their implementation. As a result, dose projections can be significantly underestimated under certain circumstances, particularly with radionuclides emitting low-energy photons and/or particles. In addition, these adjustments do not account for scatter radiation off surrounding walls, ceilings, and floors of typical occupational settings. In recognition of these limitations and for the mitigation thereof, Veinot et al. published an article [Rad. and Environ. Biophy., Vol. 56, p. 453 (2017)] that provided monoenergetic photon, electron, and positron EDRCs based on elaborate Monte Carlo computations for submersion in three typical occupational settings at nuclear facilities (namely, an office, a laboratory, and a warehouse). Included in the article were also EDRCs for exposure to 45 noble gases airborne within the said occupational settings. However, extrapolation of the results to other settings was limited due to geometry variations. Even so, as described in the present paper, the Veinot et al. article provided the basis for the definition of a straightforward model for extrapolation of the Monte Carlo–derived EDRCs to other occupational settings and radionuclides. The objective of the EDRC extrapolation model in this paper is to provide a simple but comprehensive approach to licensing-basis submersion dose calculations consistent with the recommendations in ICRP Publication 103. The model is reasonably accurate and applicable to submersion volumes that are smaller than an office and larger than a typical warehouse, as would be needed for control room habitability evaluations. It is emphasized that the model is only suitable for the external dose computation of airborne noble gases and particulates; dose contributions via the inhalation pathway and from direct shine from contaminated surfaces need to be evaluated separately. The EDRC extrapolation model was programmed into an Excel workbook (OccuSetEDRCs-R0.xlsx) that is available to interested parties; see Supplementary Data section for further details. The model can handle all 1252 radionuclides in ICRP-107 and submersion volumes ranging between about 40 and 4000 m³, with an upper estimated error of about 10% on average for large submersion volumes.

核电站场地的空气放射性外照射评估，通常以浓度均匀的半无限云团及对应的有效剂量率系数（effective dose rate coefficients, EDRCs）为基础，同时采用简化假设：放射性云团在受照受体周围无限延伸。当前用于有限云团浸没剂量修正的两类常用监管模型，分别为适用于半球形云团的《监管指南1.183》（Regulatory Guide 1.183），以及国际放射防护委员会（International Commission on Radiological Protection, ICRP）发布的、适用于球形云团的ICRP-30号出版物，二者均经过刻意简化以方便工程实施。由此导致在部分场景下剂量预测结果会被显著低估，尤其针对发射低能光子和/或粒子的放射性核素。此外，现有修正模型未考虑典型职业场所内周围墙壁、天花板及地板产生的散射辐射。鉴于上述局限性并为缓解其影响，Veinot等人于2017年在《辐射与环境生物物理学》（Rad. and Environ. Biophy.）第56卷第453页发表了一篇研究论文：针对核设施内三类典型职业场景（办公室、实验室及仓库）的浸没照射场景，通过精细的蒙特卡洛（Monte Carlo）计算，给出了单能光子、电子及正电子的有效剂量率系数。该研究还涵盖了上述职业场景中45种空气载带惰性气体的照射有效剂量率系数。不过由于几何构型存在差异，其研究结果外推至其他场景的适用性受到限制。即便如此，正如本文所述，Veinot等人的研究为构建一套简便的外推模型奠定了基础，该模型可将蒙特卡洛计算得到的有效剂量率系数外推至其他职业场景及放射性核素。本文提出的有效剂量率系数外推模型，旨在提供一种简洁且全面的方法，用于开展符合ICRP第103号出版物建议的许可基准浸没剂量计算。该模型具备合理的计算精度，适用于体积小于典型办公室、大于常规仓库的浸没场景，可满足控制室可居住性评估的相关需求。需特别强调的是，本模型仅适用于空气载带惰性气体及颗粒物的外照射剂量计算；经吸入途径摄入的剂量贡献，以及受污染表面直接照射产生的剂量，均需单独进行评估。该有效剂量率系数外推模型已被编入Excel工作簿（OccuSetEDRCs-R0.xlsx），可供相关方获取；详细信息请参见补充数据章节。该模型可处理ICRP-107收录的全部1252种放射性核素，适用浸没体积范围约为40至4000立方米，对于大体积浸没场景，平均估算误差上限约为10%。