SPECIAL REPORT | HOME AND DRY
Dry storage structure, systems or components must ensure that safety functions are fulfilled as they relate to criticality, shielding, confinement, heat transfer, structural integrity and retrievability. Source: Vermont Yankee
there are a wide range of potential accessibility issues, such as different annulus gaps and widths, entry pathways, welding processes, materials or weld locations. However, 95% of the global inventory is either bolted cask systems that contain bare fuel, or concrete storage overpacks that contain welded canisters. Bolted cask systems are more common in the USA and welded canisters outside the USA. Bolted cask systems have developed over the last 30–35 years. The report says, “They are established industrial technology and none has been involved in serious incidents or accidents, even loaded casks at Fukushima Daiichi during the accident”. Regulatory inspections of bolted cask systems are less
Credible ageing effects and plans for managing them for various components in dry cask storage systems are the first step in an aging management programme. Source: US DOE
technically challenging because all external components of bolted casks are accessible to visual inspections. There may be opportunistic inspections of the bottom if the cask is lifted for operational reasons. Inner components are protected by a nitrogen or helium atmosphere with pressure monitoring to detect leakage. Routine inspections are performed in situ, usually without the need for lifting and handling systems although the cask may be moved to a dedicated area for a more detailed visual inspection every 10 years. The casks are accessible at any time during normal operation so corrosion on the external surfaces is detectable without special inspections. Experience to date finds only signs of wear and tear and locally limited corrosion due to paint damage incurred during handling operations. Several operational occurrences have been reported of corrosion of trunnion bolts, which may affect safe handling
of the cask. It is controlled by inspecting for corrosion and if necessary bolts are replaced and resealed. New casks have a modified design in which all trunnion bolts are protected by a stainless steel ring with sealing, preventing the intake of borated water into the bolt shafts during loading. The Nuclear Energy Institute tasked the Institute of
Nuclear Power Operations (INPO) with establishing a clearinghouse of ageing related information relevant to dry cask storage SSCs. It contains positive information about inspection results that confirm storage integrity along with lessons learned that might require industry-wide corrective action. No significant repair actions have been taken to date after 33 inspection reports. There are examples of other actions: In September of 2016, evidence of corrosion was
discovered on the carbon steel outer shell of a Holtec HI–STORM 100 overpack at Plant Hatch. The corrosion had occurred behind a radiation placard that had fallen off and it was remediated by sandblasting away rust and repainting the cask. In October 2020, direct and camera inspections of the standardised NUHOMS HSMs and stainless steel canisters at Susquehanna observed chloride deposits on the canister but sampling showed no challenge to safety function In a typical example of bolted cask ageing management at North Anna the Orano TN–32 dry storage casks were painted white to better facilitate visual inspections and to provide protection from general corrosion.
Extending store lifetimes A number of dry stores systems have already reached their intended lifetime but a survey of the storage system condition has supported an extended licensed life. AMPs in many countries provide this type of information,
along with ageing management experience from nuclear power plants and the use of staff with power plant experience as storage facility operators. The IAEA report notes that during the period while it was being compiled, there was significant technological development in the field of inspections, evolving from lab scale to routine operation and they are now part of the renewal process for storage licences in the USA. One example is a monitoring method utilising temperature data from different locations of a canister. As the development of sensors gains momentum, they may offer completely new monitoring possibilities The report concludes, “As the storage durations are
continuing to be extended, AMPs will remain an important safety pillow for dry storage systems.” ■
32 | June 2026 |
www.neimagazine.com
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