During the decommissioning of the BN-350 reactor, the spent nuclear fuel (SNF) was transferred to long-term dry storage in TUK-123 transport and storage cask systems designed for transportation and long-term storage with a design service life of approximately 50 years. The TUK-123 system consists of a UKKh-123 storage package, which is a sealed metal-concrete cask (MCC), and a protective-damping cover (PDC) used only during transportation. Radiation characteristics are a key quantitative criterion for assessing the safety of long-term storage in the absence of direct access to fuel and cask components. This paper presents the results of a computational study of the radiation characteristics of BN-350 SNF under dry storage conditions as of 1 January 2025. Spatial distributions of the ambient dose equivalent rate were determined for normal storage conditions and for accident scenarios involving partial failure of fuel assembly (FA) canisters and fuel redistribution. It was established that in the near-field region, the dose fields are formed predominantly by long-lived fission products and activation nuclides, whereas the neutron contribution is determined mainly by the spontaneous fission of actinides and (α, n) reactions. The results obtained provide a quantitative basis for assessing the radiation safety of long-term BN-350 SNF dry storage.

Long-term dry storage is a key stage in managing spent nuclear fuel (SNF) from sodium-cooled fast reactors, for which monitoring supports aging management and future fuel-cycle decisions. This study presents a monitoring-based condition assessment of BN-350 SNF stored in 60 UKKh-123 metal–concrete casks at the dedicated long-term container storage site of the «Baikal-1» research reactor complex. The casks were transported to and placed at this site in 2010, and their condition has been monitored annually. The assessment integrates dosimetric records, full-container campaigns from 2018 and 2023, and detailed inspection data for selected packages. The monitored indicators included external gamma dose equivalent rate, surface contamination, external surface temperature, visual condition, and leak-tightness of detachable sealing connections. For all 60 packages, the maximum gamma dose equivalent rates were 5.8–25.7 μSv/h in 2018 and 2.0–20.3 μSv/h in 2023, far below the nominal 500 μSv/h value specified for normal operation by the technical operating conditions. Surface contamination remained within applicable limits, and external surface temperatures were below the nominal 84 °C operating limit. A detailed inspection confirmed low gamma dose rates and satisfactory leak-tightness. The results support continued controlled storage with periodic monitoring and safety reassessment.