2023:11 Study of degradation of spent fuel’s structural material and core components under long-term wet storage

SSM perspective


Sweden is managing the spent nuclear fuel and core components from its commercial nuclear reactors with a national strategic plan. The fuel and core components are initially stored at the reactor site. After an on-site storage period to comply with transportation limits on decay heat and radiation, these materials are shipped to the Clab (Central Interim Storage Facility for Spent Nuclear Fuel). The Clab is located in Oskarshamn, Sweden. The Clab facility operates several linked underground storage pools which are expected to have a storage capacity of 11,000 tons. Figure 1 is a photo of one of the underground spent fuel storage pools at Clab. Clab is expected to be operational until 2070 to 2075.

Spent fuel and core components may reside at Clab for up to fifty years prior to final disposition or disposal in Sweden’s repository. Storing this volume of spent fuel and core components requires significant handling: moving fuel and core components into compact storage canisters and moving the canisters in the various pools. During this storage period, aging and structural degradation may compromise the integrity of the fuel and core components to be packaged in repository containers. Packaging or encapsulation in the copper disposal canisters requires handling and vacuum drying. Handling and drying of spent nuclear fuel at the end of an extended pool storage period is a structural integrity and fission/activation product containment concern. The release of fission and activation products into the Clab pool storage facilities due to this degradation would increase the risk and operational difficulties at Clab and later with encapsulation of spent fuel at Clink.

The design and operation of the Clab facility leverages a considerable body of data demonstrating safety and efficacy of pool storage of spent nuclear fuel and core components. Pool storage allows direct and indirect monitoring of the storage conditions and inspection of the items in storage up to the point of final encapsulation in disposal containers.


The A.N.T. International team used public domain data regarding the nuclear and mechanical design of fuel and core components from Sweden’s commercial nuclear power program. Clab conditions of storage were assessed regarding design basis fuel and core components as described in [EPRI, 2010] and using pool conditions described in [Gustafsson and Hagberth, 1978] and [Strasser et al., 2008]. Compliance with industry water chemistry guidelines was assumed [Svensk Kärnbränslehantering A, 2011]. This assessment identified potential threats to fuel and core component structural integrity. The team reviewed public domain industry literature for comparison and benchmarking purposes regarding material stored, conditions of storage, and surveillance methods. The authors have developed surveillance plans to identify and track the rate of degradation to confirm performance expectations.

The key mechanism that would lead to degradation of spent fuel and core components stored at Clab was determined to be corrosion and corrosion[1]related phenomena. No additional engineering measures were identified during the course of the study that may further mitigate or eliminate identified structural degradation threats due to corrosion. The authors concluded that spent fuel and core components may be safely stored at Clab for up to fifty years. The key assumptions leading to that conclusion include:

  1. Water quality at the Clab storage pools is consistent with the [Svensk Kärnbränslehantering AB, 2011] guidelines in a surveillance program provided in this report.
  2. Sufficient water circulation exists in the Clab storage pools to ensure adequate mixing and prevention of areas of stagnation.
  3. The spent fuel and core components are as described in [EPRI, 2010].
  4. Clab operators implement a water chemistry and material surveillance program similar to that provided in this report.
  5. Spent fuel and core components are properly handled.


The design and operation of Clab is an integral component of Sweden’s management of high-level nuclear waste generated in the course of electricity production. The choice of Clab as an interim pool storage facility allows the ability to monitor and inspect the integrity of spent fuel and core components throughout the interim storage period. This ensures the ability to address public and regulatory issues and concerns up to the point of final disposition of the items in the repository. It is hoped that this report become an integral part of SSM’s approach to the safe storage of spent nuclear fuel and core components from Sweden’s commercial nuclear power industry.

Need for further research

This report may be considered complete. As Clab operates, data from water chemistry monitoring program, material surveillance program and periodic inspection of representative fuel and core components may identify areas of further research and analysis.

It is recommended that operating experience from LWR storage pools be reviewed periodically for relevance to Clab.