Abstract:
The hot cells located at the Jülich Research Center site are soon to be decommissioned. As consequence, the 157 pebble bed reactor fuel elements from scientific experiments being stored there since 1972 have to be disposed of. Roughly one third of the pebbles is spent fuel with burnups up to 21 %fima. As all pebbles generate a negligible amount of heat, they could be eligible for final disposal in the already licensed Konrad repository ? a deep geological repository in a former iron ore mine.
Initially, the state of knowledge concerning the pebbles was limited to data from EURATOM nuclear material and storage accounting. This proved to be insufficient to fulfil the requirements of the Konrad repository. Extensive research led to still existing private collections of documents as well as contemporary witnesses crucial for characterizing the pebbles. Based on the information obtained, visual inspection and measurement of the fuel pebbles, for each the type of nuclear fuel and its composition were identified. In case of the spent fuel, it was possible to determine the irradiation facility, irradiation length and overall burnup. Afterwards, a methodology to perform simplified burnup calculations with the code system SCALE 6.2 was devised. Where possible, SCALE calculations were verified and validated using historical inventory calculations and gamma ray spectrometry measurements which were also recovered. It was observed that the simplified SCALE models are sufficiently accurate for the purpose at hand. Taking the radiological characterization of the pebbles computed and comparing them with the Konrad acceptance criteria, it can be concluded that the 157 fuel pebbles can be transferred to Konrad for final disposal. To this end, all of the pebbles are put into internal containers backfilled with cement in groups of 16 or less. The latter have to be placed a licensed second class incident-tolerant waste container.
The project dealing with the disposing of 157 HTR pebbles illustrated that irradiated nuclear fuel from scientific experiments can be suitable for final disposal in the Konrad repository. Furthermore, simplified burn-up calculations as well as early recovering and securing of historical documentation / data were found to contribute significantly to a successful radiological declaration.