Safe storage for extended periods

30.07.2018

GRS investigates safety aspects of extended storage of spent fuel and other high-level radioactive waste

Behälter im Zwischenlager Nord (Quelle: EWN GmbH)

For high-level radioactive waste, such as spent fuel and vitrified waste from reprocessing, a repository is being sought in Germany as part of the site selection procedure. Until such a repository will be available, the waste is stored in dedicated facilities in so-called dual purpose casks (casks for transport and storage). Currently, it is assumed that the waste must be stored longer than previously planned, since at the time of expiry of the current operating licences of the storage facilities, no repository will be operational. Which safety aspects may be relevant for such an extended storage is an issue dealt with by GRS scientists in several research projects.  

Storage in Germany

In Germany, there are 16 storage facilities, of which 12 so-called decentralised storage facilities are located directly at the power plant sites. Here, spent fuel assemblies from the power reactors are stored. Three other central storage facilities are in Ahaus, Gorleben and Lubmin. For example, the Ahaus storage facility also receives nuclear fuel from research reactors. In Gorleben, mainly vitrified waste from reprocessing of German fuel assemblies and spent fuel storage casks are stored. In the ZLN (Zwischenlager Nord) storage facility in Lubmin, fuel from the Soviet-type reactors are stored as well as vitrified waste from the reprocessing plant in Karlsruhe. The storage facility in Jülich is a special case, where fuel pebbles are stored from the former Jülich AVR test reactor.

The operators of the on-site storage facilities are the respective power utilities. Since 2017, the central storage facilities have been operated by the federally owned Storage Company Gesellschaft für Zwischenlagerung (BGZ). An exception is the ZLN storage facility, which is operated by the also federally owned Entsorgungswerk für Nuklearanlagen (EWN). In 2019, decentralised storage facilities will also become the responsibility of the BGZ. The Federal Office for the Safety of Nuclear Waste Management (BfE) is responsible for the licensing of the storage facilities. Supervision of orderly operation is carried out by the authorities of the respective Land.

The storage facilities in Germany are designed as dry storage facilities. After use in the reactor, spent fuel assemblies are initially stored in the wet storage pool of the nuclear power plant for several years. If the activity and the heat have decayed sufficiently, storage in transport and storage casks (usually CASTOR® or TN® casks) is possible. The casks are loaded under water, drained and the cask interior is dried. Subsequently, the casks are sealed airtight with a double lid. The casks are filled with the inert gas helium. The inert gas limits corrosion effects to a minimum.

The storage facilities are constructed according to the so-called STEAG or WTI concept. Both concepts provide for a hall construction of reinforced concrete. An exception to this is the storage facility at the Neckarwestheim site, where a tunnel concept was implemented in a former quarry. In the STEAG and WTI concepts, the heat from the stored spent nuclear fuel is removed via natural air convection, which means that the ventilation of the storage room and the casks takes place via openings in the hall walls.

While the storage building is primarily designed to protect against the effects of weather and is an additional shielding, the casks must maintain their protective function not only under storage conditions but also under extreme loads under accident conditions.

Transportbehälterlager am Zwischenlager Ahaus (Quelle: Dietmar Rabich / Wikimedia Commons / “Ahaus, Transportbehälterlager -- 2014 -- 2321” / CC BY-SA 4.0)

Work of GRS on the topic of storage

The storage facilities are licensed to operate for 40 years. For example, the licence for the Gorleben storage facility expires in 2034, that for the Grohnde storage facility in 2042. An extension of the storage period requires a new licensing procedure where it is to be demonstrated that the safety requirements are still met. This means primarily that the so-called main safety functions must be fulfilled. These include the confinement of the radioactive inventory, residual heat removal, maintenance of subcriticality and the prevention of unnecessary radiation exposure. Regarding an extension of the operating licence, ageing processes are of particular importance, which change the stored radioactive inventory as well as the material of the casks over the years. In addition to technical issues, operational aspects, such as personnel planning or documentation and the preservation of knowledge over longer periods of time, are also taken into account.

GRS deals with the safety aspects of extended storage in several projects. In the context of work for the Federal Environment Ministry, the focus is on the question of whether the safety standard of the storage facilities can be maintained beyond the planned period of 40 years. For this purpose, GRS examines and assesses safety-relevant aspects – also based on its own calculations – and makes recommendations as necessary. Moreover, options for additional monitoring measures and investigation methods are being examined. GRS cooperates with partners such as the Öko-Institut, TÜV Nord EnSys and the Federal Institute for Materials Research and Testing (BAM).

So far, it is assumed that the fuel assemblies will be repackaged again in other containers for disposal. For this purpose, however, it is important that the fuel assemblies are intact and manageable. On behalf of the Ministry for Economic Affairs and Energy, GRS develops methods and software programs that allow a simulation of extended storage with all thermo-mechanical processes. The simulations can be used to determine, among other things, the impact of temperature and mechanical stress on the fuel assemblies. By extending the GRS-developed TESPA-Rod  simulation code it is possible to derive predictions on the integrity of the fuel assemblies over longer periods of time.

GRS also participates actively in the international exchange of experience on the issue of storage and, among other things, organised a workshop in June 2018, which was attended by more than 60 experts from Germany and abroad. The focus here was on the technical aspects that may be important for extended storage.

Find out more

Information provided by the Federal Office for the Safety of Nuclear Waste Management (BfE) on storage
Betreiber der dezentralen Zwischenlager: Gesellschaft für Zwischenlagerung (BGZ)
Recommendation of the Nuclear Waste Management Commission (ESK) “Guidelines for dry cask storage of spent fuel and heat-generating waste“
Bericht Deutschlands zur sechsten Überprüfungskonferenz im Mai 2018  on the BMU’s website