Monitoring and Condition Assessment of the TUK-123/UKKh-123 Casks Containing BN-350 Spent Nuclear Fuel

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.