Post-irradiation measurements on fuel rod plenum springs for transport and reuse of partly burnt fuel

After the closure of the Ringhals 1 BWR on the west coast of Sweden, it is planned to ship partly burnt fuel assemblies to the Forsmark plant on the east coast of Sweden for further operation to their target end-of-life discharge burnup. The fuel will be transported in a horizontal position in dry spent fuel casks normally used for shipment of spent fuel from the Swedish nuclear power plants to the CLAB central interim storage facility. In the framework of the transport qualification and the qualification of the fuel for re-use after the transport the plenum spring of a 1-cycle irradiated fuel rod was investigated. The compressed length of the spring in the plenum, its free length after removal, and its spring constant have been determined by hot cell post-irradiation measurements. Together these data can be used to determine the restraining force of the spring on the fuel column. The results demonstrate adequate transport margins, and enable an evaluation of unplanned transport events. In addition, cold pellet-clad gap measurements indicate that already after one annual cycle only a small part of the pellets in the fuel column still behave as intact solid cylinders without cracking and radial relocation. Cracked and relocated pellets require less restraint from the plenum spring to prevent pellet stack movement under horizontal acceleration during transportation.