Study on the migration law of airborne radioactive nuclides and the spatiotemporal distribution of dose under the accident condition of cover-gas leakage in the lead-based reactor of the ADANES system

[Background]: Cover-gas leakage is one of the typical postulated accident scenarios for the lead-cooled reactors in the Accelerator-Driven Advanced Nuclear Energy System (ADANES). Understanding the migration of airborne radionuclides in such confined spaces is important for source-term analysis and dose assessment in accident conditions. [Purpose]: This study aims to establish a CFD-based approach for simulating the migration of airborne radionuclides released from the cover-gas system of an ADANES lead-cooled reactor under a small-leak scenario, and to obtain the corresponding time-dependent activity and dose fields in the reactor building. [Methods]: A three-dimensional CFD model of the cover-gas plenum and reactor building (including a ventilated confinement cell and a negative-pressure containment) was developed. The transient argon leakage through an equivalent small opening was solved using a pressure-based solver and the realizable k–ε turbulence model. Based on the tracer-gas concept, a conditional mean age formulation was introduced: an age-of-air–type scalar was solved together with the tracer concentration, and their ratio was used as the cumulative residence time of the cover gas in each cell. This residence time was then used as the decay and dose-conversion time for gaseous rare-gas radionuclides, which allowed the calculation of time-resolved activity concentration and immersion dose-rate fields in the building. [Results]: Under the assumed boundary conditions, the cover-gas leakage evolves from a subsonic compressible jet to a low-velocity plume as the system pressure decreases. For a given geometry, the discharge through the ventilation duct is strongly influenced by the containment underpressure level and the ventilation velocity. As time progresses, under-ventilated regions inside the confinement cell remain at relatively high immersion dose rates, while in the larger containment volume the high-dose cloud is mainly controlled by the leak location and spreads only locally. [Conclusions]: The proposed CFD–conditional-mean-age framework provides a practical tool for analyzing airborne radionuclide migration and dose distribution in confined spaces under cover-gas leak scenarios. The results can offer engineering reference for the design and assessment of the ADANES lead-cooled reactor building, including confinement layout, ventilation arrangements and accident consequence evaluation.