Dealing with damaged fuel21 March 2017
A new process to encapsulate defective fuel rods is being trialled at Doel in Belgium. By Kay Muenchow
Operators of nuclear power plants have to find solutions to safely store defective fuel rods – at the latest before the decommissioning of the plant. Areva has recently developed an encapsulation technology that safely encloses each defective fuel rod in an individual gas- tight capsule. These capsules are assembled in skeletons which can be transported
and placed in the dry-store together with non-defective fuel rods. The encapsulation procedure takes place in the spent fuel pool. A treatment with hot gas ensures dryness of the defective rod. The technology can be applied for each kind of fuel (uranium, or mixed oxide) and cask. The first implementation is currently under way at the Belgian Doel plant.
While nuclear power plants are in operation a certain number of fuel assemblies can incur damage. The question arises of how operators should handle the defective fuel assemblies and it has to be answered by the time the plant reaches decommissioning.
A number of solutions are currently available for handling sound fuel rods, such as storage in casks. However, a technical option has to be developed for handling defective fuel rods (DFR).
Fuel rods are considered to be sound if they fulfil their designated function, namely providing a safe enclosure for the nuclear material and gaseous products without a requirement for any special treatment. They either exhibit no damage at all, or show only minor damage.
In other cases they are considered to be defective. The safety requirements for spent fuel rods include confinement of radioactivity, ensuring they can maintain a subcritical condition, and structural integrity. If these criteria are no longer met, the fuel rods have to be considered as defective.
Areva NP – whose business operations contribute to nuclear plant construction, provide plant services and fuel supply – offers advanced solutions for spent fuel assembly management. These solutions provide dedicated defective fuel rod management solutions fulfilling various and stringent safety requirements.
In France, spent fuel is reprocessed. Assemblies are transported to the reprocessing plant in La Hague, where the fuel rods are cut into pieces and the unused fissionable material is separated by means of a chemical process. Various licensed means of transportation are available. Pursuant to European regulations damaged and defective fuel rods have to be transported in dedicated capsules. These capsules are placed in canisters inside the transport casks, after an extension to the licence of the cask has been obtained.
The advantage of Areva NP’s encapsulation technology is that nuclear power plant operators no longer have to continue storing defective fuel assemblies, so they can avoid any complications which might occur during a long intermediate storage time in the fuel pool. Transportation storage casks, e.g. those of the Areva TN family (such as TN24) are licensed for a period of around 40 years. This means that after this time high-level radioactive wastes have to be transferred to new casks and canisters in cases where the licence cannot be extended any longer.
Areva has accumulated many years of experience in fuel reprocessing and related transportation management. For example, fuel assemblies from various European countries and Japan have been treated at the La Hague reprocessing plant in France. For legal reasons this option is not open to German plant operators because there has been a ban in force since 2005 on transporting spent fuel for reprocessing. In Germany, as a result, spent fuel has to be placed in interim storage in specially designed facilities close to the nuclear power plant, while they await availability of a final repository.
According to the regulations in force, several options for storage are available.
Dry interim storage
Interim storage has been practiced in the USA for decades. Defective fuel assemblies are subjected to a series of special treatments in preparation for transportation and interim storage. Depending on the level of damage defective assemblies are managed using special casks with multilayer shielding. Using a dry storage cask, the defective fuel assemblies are placed in an interim storage area together with sound fuel assemblies. Areva has been developing and implementing such solutions for many years, and they are in compliance with the guidelines laid down by the US Nuclear Regulatory Commission (NRC). The benefit of this solution is its short- term cost effectiveness.
Encapsulation for long-term storage
In case reprocessing or intermediate storage is not the preferred option, Areva has developed an advanced method for preparing fuel rods for transportation and long-term storage that complies with the legal safety framework in place in Germany. The principle is also applicable in other countries. All defective fuel rods in a nuclear power plant, whether containing uranium or MOX pellets, can be treated using this technology.
This concept is based on placing of individual fuel rods separately in welded capsules. The capsule consists of a tube and two end plugs. After being welded together safely it avoids any fission product release over time. The capsules provide the first protective shell and ensure safe confinement of radioactive substances and at the same time adequate heat removal, as well as radiation shielding. The capsules are specially designed for dry long-term storage using specific durable corrosion-resistant material. This concept has been deployed for more than ten years for underwater storage and transportation.
The industrial process for encapsulation
The industrial encapsulation process begins with pretreatment, i.e. brushing and opening of the cladding of the defective fuel rods. The individual fuel rods are then transferred underwater into the capsules and dried inside the capsules. These operations are performed in the spent fuel pool because the water significantly reduces the radiation dose. Areva NP also provides the safety analysis and definition of the configuration to maintain confinement of the defective fuel.
Areva has developed a hot gas system for drying fuel rods. The system is pressurised with argon and heated. Heating causes the water to be vaporised and the flow of argon forces moisture to the outlet of the drying fuel zone. The moisture condenses in the cooling zone and is thus separated from the system. Argon circulation continues until a defined degree of dryness is reached. After reaching a certain level of pressure and temperature within this system, fuel can be assumed as physically dry. The fuel rod capsules are then closed and welded tightly, and the welds subsequently inspected. This newly developed process ensures both leak-tightness and a sufficiently high degree of dryness.
Several capsules containing defective fuel rods are then assembled in a skeleton. This skeleton holds a specific number of encapsulated fuel rods shaped in a similar way to a fuel assembly. The skeletons are placed together in an Areva transport and storage cask, for example a TN24. The casks are used for the transport and interim storage of irradiated and spent fuel assemblies. They are designed to ensure that safety goals are achieved in terms of shielding, leak-tightness and integrity, ensuring subcriticality and safe decay-heat removal.
The encapsulated fuel rods are prepared for safe transportation, for dry interim storage and, if necessary, also for direct long-term storage.
The technology for encapsulating defective fuel rods is independent of the type of transport and storage cask. All licensed casks with encapsulated defective fuel rods can be used for subsequent transportation and interim dry storage.
The first use of Areva’s defective fuel management technology is taking place in Belgium, where Synatom is in charge of all nuclear fuel cycle activities for commercial nuclear power plants and Engie Electrabel is the operator of the country’s Doel and Tihange reactors. Both companies reviewed possible options for implementing dry storage solutions of defective fuel rods and selected the Areva technology. At the moment, final engineering and procurement is ongoing. The onsite activities are scheduled for end of 2017.
Today, nuclear power plant operators in Europe have at their disposal a safe process for defective fuel management. The responsible supervisory authority in France has already granted a licence for Areva’s transport and storage casks with new content. The Belgium authority has validated this transport licence as well as the storage licence. In addition, a transport licence for the Areva transport cask has already been received in Germany and France.
About the author: Kay Muenchow studied Nuclear Technology at the Technical University Zittau. Afterwards he held several positions in the fuel service and fuel design activities of Areva Group in Germany and France. Since 2013 Kay Muenchow has been project manager for fuel encapsulation.