The study reports on two passive techniques developed to determine the peak temperature reached by spent‑fuel cladding during routine transfer operations at the Gösgen Nuclear Power Plant (KKG). The first method uses irreversible temperature‑indicating lacquers that change colour when their melting point is exceeded. Thirteen different lacquers were applied to notches machined into aluminium bars, each bar being encapsulated in a welded steel tube to keep the lacquer dry and shield it from the pool water. The bars were then inserted into the guide tubes of the fuel assemblies, which were loaded into a 12‑assembly transport cask of the TN12 series. After the cask was vacuum‑dried, placed in a helium atmosphere, and moved from the internal to the external wet storage pool, the assemblies were unloaded and the sensors removed in the hot workshop. The lacquers covered a temperature range from 149 °C to 454 °C, providing 13 discrete thresholds that can be read visually after the transfer. The colour change is irreversible, allowing a one‑time measurement without the need for active monitoring equipment.

The second technique, developed by ORANO NPS, consists of a passive temperature sensor based on bimetallic washers arranged along a central rod inside a 300 mm stainless‑steel tube. The tube is sealed at both ends and contains a movable trolley that is driven upward by the thermal expansion of the washers. Because the trolley cannot return to its original position when the temperature falls, the final position after a transfer operation indicates the maximum cladding temperature reached. The sensor’s response is linear with temperature, and calibration performed by ORANO yields an estimated uncertainty of ±10 °C. Like the lacquer sensors, the bimetallic assemblies are encapsulated in steel tubes and inserted into the guide tubes by the Framatome fuel service team. The insertion, transfer, and retrieval procedures mirror those used for the lacquer sensors.

Experimental campaigns were carried out in 2011, 2022, and 2023. In the 2022 campaign, several lacquers were tested, each covering a distinct temperature band, and the results were compared with the bimetallic sensor readings. Although the data remain qualitative, the two independent methods produced consistent indications of peak temperatures, confirming the feasibility of passive monitoring. Further quantitative analysis is planned to compare measured values with predictions from thermal models.

The project is a collaboration between KKG in Switzerland and ORANO NPS in France, with technical support from Framatome for sensor insertion. The work was presented at a workshop held at Gösgen, where participants from the nuclear industry and research institutions discussed the findings. The study demonstrates that passive, irreversible temperature indicators can be integrated into routine spent‑fuel handling without additional instrumentation, providing a simple and reliable way to verify that cladding temperatures remain within safe limits during transfer operations.