Experiments and operational modeling of dry and wet deposition of chlorine 36 in gaseous and particulate form on grassland

Chlorine 36 (³⁶Cl, T_1/2 = 301,000 years) is a radionuclide of both natural and artificial origin that can be released in gaseous or particulate form during the decommissioning of nuclear facilities or the recycling of nuclear waste. Once released into the atmosphere, ³⁶Cl can be transferred to terrestrial ecosystems through dry and wet deposition. However, due to analytical constraints, knowledge about these deposits remains limited. Given its relatively high mobility in the geosphere and high bioavailability, understanding the behavior of ³⁶Cl in the environment is crucial for human impact studies and the assessment of the safety of radioactive waste storage. Therefore, the objectives of this thesis are to quantify dry and wet deposits on ³⁶Cl on grass, followed by modeling these deposits based on parameters related to ground cover and atmospheric turbulence. To achieve this, a sampling system (gas, particles, rainwater, and grass) and micro-meteorological measurement equipment were installed at the IRSN La Hague Technical Platform (PTILH), located 2 km from the Orano La Hague facility, which chronically emits small quantities of ³⁶Cl. To determine the concentrations in the various samples, an original radiochemistry method was developed and validated. Following the collection of samples, chlorine extraction via alkaline fusion and specific chemical preparation for ³⁶Cl measurement by Accelerator Mass Spectrometry (AMS-ASTER) at CEREGE were carried out. The alkaline fusion extraction of NIST leaf standards allowed for the determination of an average extraction yield of 83 ± 5% and method validation. ³⁶Cl levels in the atmosphere and rainwater at the study site were on average one to two orders of magnitude above natural background levels. Gaseous ³⁶Cl represented an average of 72.9% of atmospheric ³⁶Cl, while 27.1% was in particulate form. Enrichment of ³⁶Cl was observed in grass samples, indicating the transfer of ³⁶Cl from the atmosphere to the grass during experimental campaigns at the study site. Dry deposition velocities of ³⁶Cl on grass were determined for the first time in this study, with values ranging from 1.0 x10^-3 to 1.1 x10^-2 m.s^-1. Measurements of ³⁶Cl deposition flux showed that dry deposition contributed on average to 51.9% of the total ³⁶Cl deposition at our study site, compared to 48.1% for wet deposition. The study of wet deposition flux also highlighted the existence of a 'rainout' phenomenon in the environment. Parametrization of dry and wet ³⁶Cl deposition, both gaseous and particulate, was carried out based on meteorological data and adaptation of existing models. The values calculated by the modeling were in good agreement with experimental data, with less than a threefold difference for dry deposition velocities and less than a fourfold difference for wet deposition flux. Combining the dry deposition model with the wet deposition model revealed that ³⁶Cl deposits on PTILH are dominated by the gaseous fraction, contributing 91.9% to the total deposition (dry and wet deposition by 'washout'), with 8.1% attributed to the particulate fraction. Regarding deposition mechanisms, it was calculated that dry deposition accounts for an average of 41.6% of ³⁶Cl deposits, compared to 15.0% for wet deposition by 'washout' and 43.4% for wet deposition by 'rainout'. The results obtained and the knowledge acquired during this study will contribute to improving the characterization of the transfer of radionuclides in the environment, including their impact on humans. (author)