The Swiss Federal Interim Storage Facility

Since radioactivity was discovered, radioactive materials have been accumulating in medicine, industry, and research as well as in power generation. Those materials that have not cooled down in a short time through radioactive decay and hence cannot emit any more harmful radiation have to be disposed of safely. Therefore in Switzerland the radioactive waste materials are supposed to be stored in a deep geological repository. The search for a suitable location in Switzerland for such a deep repository is currently under way. Until this has been established, the accumulated radioactive wastes are gathered into decentralised interim storage facilities. Some of these are located at the individual nuclear power plants. In addition, there are two central interim storage facilities in the canton of Aargau. These two central interim storage facilities are located close to each other, although organisationally they are completely separate.

On the one hand there is the Zwilag, operated by Zwilag Zwischenlager Würenlingen AG, a stock company owned by the Swiss nuclear power plant operators. Here a central storage possibility exists for low-, intermediate-, and high-level radioactive wastes from the nuclear power plants. The Zwilag campus directly abuts the campus of the Paul Scherrer Institute PSI.

On the other hand, on the PSI campus itself is the central facility for Switzerland, the Swiss Federal Interim Storage Facility (BZL). The BZL belongs to the federal government and is operated on its behalf by PSI. The BZL serves for the storage of all radioactive wastes that fall within the federal government's area of responsibility. These are wastes from medicine, industry, and research – MIR wastes for short. These include only low- and intermediate-level radioactive wastes.

Low- and intermediate-level radioactive wastes from medicine, industry, and research

The MIR wastes that are conditioned and temporarily stored at PSI stem, in the case of medicine for instance, from radiation therapy (accelerator wastes) or from the use of radioactive tracers, which for example make tumours in the body visible. Industry utilises radioactive materials for material testing or to improve the shelf life of food products. Lastly, in research too, work is done with radioactive substances, or materials become activated by exposure to ionising radiation. Our mission is to minimise the traces that the human uses of radioactivity inevitably leave behind", says Joachim Müth, head of the section for Dismantling and Waste Management at PSI. "That is a long-term task and a generational question. We want to meet this responsibility.

At present in the Swiss Federal Interim Storage Facility there are MIR wastes with an overall activity of around 10 quadrillion becquerels (10¹⁶ Bq). The unit becquerel indicates the quantity of radioactive decay per second. To make it easier to conceive of this quantity, if only approximately, explanatory texts often refer to the potassium content of the human body. Potassium is essential for human metabolism. It naturally contains radioactive potassium-40, and it has been calculated that, depending on age, gender, and other factors, the potassium-40 activity of the human body amounts to about 40 to 60 becquerels per kilogramme of body weight. Put another way: The body of a person who weighs 75 kilograms contains between 3,000 and 4,500 becquerels of radioactivity from potassium-40.

Of the overall activity level of wastes stored in the BZL, the radioactive substance tritium accounts for the largest share. These tritium wastes originate, for example, in the Swiss watch industry (luminous dials) or come from “Exit” signs used in the past, escape route lighting, eyepieces such as gun sights, or labelling solutions.

How do the wastes get into the BZL?

The Federal Office of Public Health (BAG), in consultation with PSI, annually conducts a collection drive for radioactive wastes from the areas of medicine, industry, and research. Within the framework of this collection action, businesses as well as individual people can register their radioactive wastes with BAG (information can be found here: http://i.psi.ch/szfV). They then receive advice on how to pack their wastes properly for transportation to PSI. Costs are incurred for the waste disposal. PSI gets in contact with the businesses to inspect the wastes, their documentation, and their packaging on site. After that, PSI seals the containers and checks to make sure they are transportable. Finally, a specialised transport company picks up the waste on behalf of the business and conveys it to PSI.

Conditioning of wastes at PSI

The waste materials delivered by the transport company are first stored at PSI, then sorted for further treatment and, when it is feasible, cleaned. The goal is to separate contaminated from uncontaminated components so that the smallest possible volume of radioactive waste is left over. To further minimise the volume, the wastes are compressed if possible. For this purpose, each of the 200-litre barrels to be filled is positioned under a compactor. The waste is first filled into the barrel by trained staff members wearing protective gear – so-called operators – and subsequently compressed with the compactor.

While the operators are conditioning the MIR wastes in this manner in an operation box, two to three more staff members watch from outside the box to ensure that the work is accomplished precisely and safely. In addition, they log and document the entire material flow. We know our materials and activity levels very precisely and document them almost down to the gramme, Müth says. Through the conditioning, the MIR wastes are brought into a storable and safe form; then they are transferred to the BZL. The continuous and multiply redundant documentation ensures that the inventory of the BZL is known with precision.

With waste products that can be burned, the barrels are incinerated in the plasma oven of Zwilag to further reduce the volume. Afterwards both burnt and unburnt as well as incompressible wastes, such as for example steel parts or pieces of concrete, are hardened. This is accomplished by pouring a lime-cement-based mortar over and around the waste. Thus the radioactive materials are ultimately encased in concrete, and the radiation is shielded. In this condition, the wastes are stored at the BZL.