Conditioning of Plutonium Waste for Long-Term Interim Storage
Authors: Ledebrink F.-W. , Faber P. 
Published: April 2020

In the period since Germany’s experimental final repository ASSE was closed in 1978, around 5000 drums of conditioned plutonium-bearing radioactive waste from mixed-oxide (MOX) fuel fabrication have accumulated in the interim storage facilities of Siemens AG’s MOX fuel fabrication plant in Hanau, Germany — formerly ALKEM GmbH, now Siemens Decommissioning Projects (Siemens DP). Another 5000 drums will arise in the course of decommissioning and dismantling the MOX plant which has now been underway for some months. Hopes that a final waste repository would soon be able to go into operation in Germany have remained unfulfilled over the last 20 years. Also, the agreements reached between Germany’s electric utilities and the Federal Government regarding the future of nuclear energy have not led to any further progress in connection with the issue of radwaste disposal. A concrete date for a final repository to start operation has still not been set. The German Federal Government estimates that a geologic repository will not be needed for at least another 30 years. Since the opening of a final storage facility is not foreseeable in the near term, Siemens is taking the necessary steps to enable radwaste to be safely stored in aboveground interim storage facilities for a prolonged period of time.

Conditioning of radwaste from MOX fuel fabrication by cementing it in drums was started in 1984 in the belief — which was justified at that time — that final storage at the Konrad mine would be possible as of 1995. The quality requirements specified for the waste drums were therefore based on the Konrad acceptance criteria. The operating license for the storage facilities at Hanau at which these drums are presently in interim storage is limited to 20 years and will be expiring in 2004. The drums have not suffered any corrosion to date and, according to past experience, are not expected to do so in the future. However, permission to keep the drums in interim storage for a longer period of time in their current form would be extremely difficult to obtain as their corrosion resistance would have to be demonstrated for a further 30 years.

The present goal is therefore to create a waste form suitable for interim storage which needs no maintenance over a long-term period, incorporates state-of-the-art technology and will probably not require any further treatment of the waste packages prior to emplacement in a final storage facility. At the same time, the highest possible degree of safety must be assured for the time during which the waste remains in interim storage.

This goal can be attained by conditioning the drums such that they satisfy the requirements currently specified for final storage at the Konrad repository (1). In practice, this means immobilizing the cemented waste drums in concrete inside steel “Konrad Containers” (KCs). The KCs themselves and the concrete backfill represent two further barriers which not only serve as radiation shielding but also protect the drums against corrosion as well as any possible release of radioactive materials in the event of accidents occurring during interim storage. As the KCs are cuboid in shape, they can be stacked in space-saving configurations and are thus particularly suitable for interim storage. Also, due to their extremely heavy weight, theft of the waste packages can be practically ruled out.

Despite the fact that the agreements with the German Federal Government have failed to bring opening of the Konrad repository within reach, it is nevertheless a good idea today to condition radwaste in a manner that renders it suitable for ultimate storage there. The agreements between the Government and the utilities are expected at least to result in a land use permit being issued for the Konrad mine before the end of 2001. At present there are no facts known that could cause the safety of this facility to be questioned. Only recently, Germany’s International Nuclear Technology Commission (ILK) confirmed Konrad’s suitability and demanded that it be placed in operation without further delay (2). Even if its operation should, in fact, be blocked by political lobbies, potential legal action or economic considerations, the alternative repository at Gorleben could possibly become operable in approximately 30 years’ time. Gorleben was planned right from the start to be able to accommodate waste packages based on the Konrad acceptance criteria. This means that any waste packages designed for storage at Konrad could likewise be handled and stored at Gorleben. The processes used by Siemens for conditioning of radwaste conform to the recommendations of the “Guidelines for the Control of radioactive Waste with negligible Heat Generation” issued by the German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety (BMU) in 1989 (3).