- Transport Containers for Spent Nuclear Fuel
- 2. Transport Containers for Fresh Fuel Assemblies
Transport Containers for Spent Nuclear Fuel
What are CASTOR Containers used for?
The name CASTOR is an acronym for "Cask for Storage and Transport of radioactive Material". CASTORs were developed to transport spent fuel elements from nuclear power plants and highly radioactive vitrified waste material from reprocessing to their intermediate storage sites. They serve to shield off radiation and prevent the release of radioactivity.
These casks belong to the group of so-called type B packages. Spent fuel elements and HLW can be stored in these containers for up to 40 years. CASTORs are manufactured by GNS Gesellschaft für Nuklear-Service mbH in Essen, Germany.
The major component of the CASTOR is its 30 to 40 centimetre thick cask body made of ductile iron. Depending on the design type, several longitudinal boreholes filled with polyethylene rods have been incorporated into the wall to improve neutron shielding. The casks, which are loaded under water, are nickel-plated on the inside and coated with polyester resin on the outside.
Most of the casks have cooling fins to accommodate fuel rods or HLW that are still generating heat. The lid system consists of a so-called double-barrier sealing system upon which a third protective cover is placed during storage. During transportation, both the lid side as well as the bottom side are protected by large steel-plate shock absorbers. CASTORs are approximately six metres long, have a diameter of approx. two metres and can weigh up to 140 tons when loaded.
Tests with CASTOR Containers
BAM works together with the Federal Office for Radiation Protection (BfS) to perform regulatory type approval testing on CASTORs. BfS tests the radiation shielding and the criticality safety of the containers. BAM assesses the mechanical and thermal design of the containers and performs various tests and calculations with respect to their accident safety. During these assessments, the containers are subjected to mechanical and thermal tests.
The mechanical examination consists of several drop tests. During the first run, the container must be dropped from a height of nine metres onto an unyielding target— whereby the aim is to cause the maximum possible damage. In the second test series, the container is made to drop from a height of one metre onto a fixed steel bar having a diameter of 15 centimetres and a height of 20 centimetres. Here too the container is aligned in different positions so that the maximum possible damage is obtained.
An additional crush test is performed for "light" containers weighing up to 500 kilograms and having a specific weight of less than 1000 kilograms per cubic metre. In this test, a weight of 500 kilograms is made to drop on the lying container from a height of nine metres.
BAM performs drop tests for smaller containers in its test facilities in Berlin-Lichterfelde. Larger containers are tested at the institute's drop test stand in Lehre, and since 2004 in Horstwalde. The "unyielding" foundation of these droptest facilities consists of four/five metres thick reinforced concrete, into the surface of which a 20 centimetre thick steel plate has been let. This arrangement ensures an impact force during testing that is so high that, under real conditions, this force can only be achieved with considerably higher impact speeds.
The worldwide first test of a full-size spent fuel transport container of the type CASTOR Ia was performed in 1978. First test of a full-size spent fuel transport container of the type CASTOR
Both in this test as well as in all subsequent drop tests the CASTORs proved to be accident-proof.
In supplementary tests, a 1:2 scale model of a spent fuel transport container was dropped from a helicopter out of a height of approx. 200 metres several times. The safety relevant shieldings and the seals were not impaired.
For thermal testing, the CASTOR is subjected to a half-hour fire which fully encloses the container. The arerage flame temperature must be 800 degrees Celsius.
CASTOR containers are also subjected to a water submergence test. In this test they must withstand an overpressure of two megapascal. This corresponds to the pressure at a depth of 200 metres. The test is generally performed by calculation.
BAM performs drop tests for smaller containers at its test facilities in Berlin-Lichterfelde. Larger containers are tested at the institute's drop test stand in Lehre, and since 2004 in Horstwalde. The fire-test stand is located there.
Within a period of 28 years, BAM has performed more than 100 drop tests and fire tests with CASTORs and comparable containers as well as more than 100 tests with other type B containers.
One of these tests was performed in April 1999, when an accident was orchestrated in which a CASTOR container was subjected to the explosion of a railway tank car filled with propane. The fireball of the exploding tank car was more than 150 metres high, and parts of the propane tank flew up to 200 metres from the point of the explosion.
The CASTOR container was hurled seven metres from its test stand, it performed a summersault and penetrated the ground one metre with its cover side.
The cask and its seals withstood the effects of the explosion although the test was performed without the protective shock absorbers. The CASTOR would even be resistant to a plane crash. This was shown by firing a 1000 kilogram steel shaft projectile with a speed of 300 m/s at a test container.
Calculations Supplement Tests
Not all potential accidents can be performed experimentally. That is why most stressanalyses on containers are now simulated by computer by the Finite-Element-Analysis (FEM). The advantage: The results of calculations are better suited than the results of tests when determining when the material reaches its stress limits. Every single component can be subjected to a mechanical and thermal calculation under the widest range of different conditions. Numerical tests performed in this way reveal stresses and deformations at points that are not accessible at all during real tests.
Comparative data from earlier tests are incorporated in the calculations. The results of the computer simulation are then taken into account in later designs, for example. Thus, for example, experience with POLLUX transport containers has helped to make CASTORs safer.
Working with models and simulations corresponds to the worldwide agreed regulations for the transport of radioactive substances, the "IAEA Regulations for the Safe Transport of Radioactive Material" (1,1 MB)
Whether the safety margins determined mathematically for the container are actually adequate, can only be investigated experimentally. Thus, for example, a twelve centimetre deep notch was applied to a CASTOR VHLW container. This defective test probe was then dropped from a height of 14 metres and should prove that it remains its integity even when defective. The result: The CASTOR remained intact.
1.2 CONSTOR and Other Transport Containers
The CONSTOR V/TC (acronym for CONcrete STORage Cask, manufacturer GNS mbH) is a steel-sandwich cask consisting of an inner and an outer liner made of steel and the intermediate space filled with heavy concrete named CONSTORIT (iron aggregate frame and hardened canent paste). The cask is designed to be used for 32 fuel elements from a pressurised water reactor or 69 fuel elements from a boiling water reactor, respectively.
With two impact limiters on its ends, the package is almost 7500 mm long and has a diameter of 3500 mm. Because of the mass of the full-scale cask of 181.000 kg, it was hitherto impossible to perform real drop tests with it worldwide. This only became possible with the
new BAM drop-test facility
Videofilm: The drop test of CONSTOR V/TC (WMV, 4 MB)
The horizontal drop test went successful for the cask design which demonstrated all relevant safety functions like the Helium leakage rate of the closure lid. The measurements of strains and deceleration during impact have been used by BAM and GNS to compare real occurring stresses with the pre-calculation from computer simulations. The collected data also helps to make the calculations obtained by the finite element method (FEM) more perfect.
Drop Test with the MSF-69BG full-scale cask
Beginning in September 2004 a comprehensive test sequence was performed by BAM using a 127.000 kg full-scale MSF-69BG cask test model manufactured by Mitsubishi Heavy Industries Ltd.(MHI), Japan. The overall dimensions of the cask with impact limiters are 6900 mm long and 3100 mm in diameter. The MSF casks are destined to store spent fuel elements from nuclear power plants. In order to examine the leak-tightness of the closure lid system of the cask, the test object has been dropped from several drop heights from 0,3 up to 9,3 metres with different drop test positions like inclination of 10 degrees (slap-down) as well as lid side (vertical) position.
Tests outside of BAM
The safety of CASTOR and comparable cask designs has been demonstrated internationally by lots of mechanical tests. The overall results confirm the knowledge gained by BAM.
Here are a few examples:
The U.S. Sandia National Laboratories performed high-speed crash tests to examine transport casks for fuel elements. In these tests, the test specimens were crashed into a massive concrete wall at speeds of 97 km/h und 135 km/h. The casks showed no serious damage. The casks also withstood a vertical crash with a diesel locomotive moving at a speed of 131 km/h, as well as a trip on a railway carriage moving with the same into a concrete wall.
In 1992, a chilled MOSAIK cask also withstood drops from heights of 9 and 18 metres respectively without damage although crack-like defects had been imparted to the cask before the test. One year later, the GNS company dropped a MOSAIK cask onto a runway made of concrete from a height of 800 metres.
At the Japanese CRIEPI test centre, a cask quite similar to the German CASTOR V cask has been tested with a considerable crack. The pre-damaged test specimen was cooled down to -40 degrees Celsius to increase the risk of brittle fracture. As a results of the drop onto an unyielding target no crack growth was observed, the cask remained sealed.
In England the Magnox transport cask system was subjected to a crash-test. A diesel locomotive with three waggons, accelerated to a speed of 160 km/h, collided with a standing Magnox cask. For comparison: As measurements have shown, the cask is loaded with a lower momentum than in a 9 metre drop test onto an unyielding target.
1.3 POLLUX Containers for Permanent Storage of Radioactive Materials
What are POLLUX casks used for?
The POLLUX cask is developed by the Gesellschaft für Nuklear-Behälter mbH (GNB) for transport and the direct permanent storage of spent fuel elements in deep geological formations, e.g. closed salt mines, etc. However, it is not yet certain whether POLLUX will one day be used in this manner.
The POLLUX consists of an inner and outer cask. The inner cask is made of forged steel and tightly closed by a welded lid to take up compressed nuclear waste and ensure that radio nuclides are locked in. The outer cask made of ductile cast iron serves to shield off the radioactive radiation. It is closed by a massive screw cap.
Depending on the respective type of nuclear fuel rods, POLLUX could take up 18 to 30 fuel elements from pressurized water reactors or boiling water reactors. The cask has a diameter of almost 2 metres, its length is approx. 5.50 metres, and the weight is 65 tons.
The POLLUX must comply with the traffic regulatory requirements for type B(U) packagings and the german atomic law. It could be used for transport, intermediate storage and permanent storage.
Tests with a prototype of the POLLUX cask
BAM performed in total six regulatory drop tests using an original-sized prototype of the POLLUX cask: The tests included a drop from a height of nine metres onto an unyielding target vertically down onto the lid side, as well as a drop in horizontal position. The shock absorbers in the transport configuration were deformed; the container itself showed no cracks or deformations.
The container also remained undamaged when another drop test was carried out with the bottom facing downwards, as well as a test in which the container was dropped from a height of five metres onto concrete slab without using shock absorbers. On the other hand, the concrete foundation was struck through by the trunnions, and several deep cracks ran into the slab.
The resistivity of the cask’s lid edge was tested in an oblique drop from a height of nine metres. In this test, both shock absorbers were severely damaged. However, the threaded area of the outer cask’s lid showed not even the most minimal deformation: The lid could be unscrewed without any problem.
Strain and deceleration m easurements performed on the cask showed that the severest stresses only occur after the main impact, during the phase of free rebound. This phenomenon is a result of the interaction between loose single masses: The 34 tons outer cask contains the 21 tons inner cask, which contains again a massive steel cylinder with a weight of 10 tons simulating a real content. Due to elastic spring reactions between these single masses, so-called relative movements with internal impacts are caused. These impacts between the single masses take place offset by a few milliseconds and cause considerable material stresses.
After completion of the drop test series, the POLLUX was transported back to GNB and opened in the presence of BAM experts. None of the container components showed any damage that could have affected the function of the POLLUX, the shielding or the safe enclosure of radioactive material.
1.4 Cast Containers for Thermally Non-Conducting Radioactive Waste
Tests with Type VI Cast Containers
Type VI cast containers were developed for radioactive waste that generates no heat. These containers are made of GGG 40 nodular cast iron, and are manufactured by GNS. GGG 40 refers to cast iron with nodular graphite, which has similar ductility properties to that of steel.
With its dimensions of 2 x 1,7 x 1,6 metres, the cast container appears like a cube that has been slightly drawn upwards. Its walls are 15 centimetres thick and empty it weighs 18,4 tons. The circular cover is screwed to the container body and sealed with an elastomeric sealing ring.
The BAM has performed numerous tests with type VI cast containers: Since 1991, lifting tests, stacking tests and drop tests have been performed. In 1995, these tests were supplemented by tests in which the specimen is cooled down to -20 degrees Celsius beforehand, in order to increase the brittleness of the material. Furthermore, five crack-like faults were purposely imparted to the container before it was dropped from a height of five meters on to a foundation having characteristics corresponding to the floor of the planned permanent storage facility. The container survived the crash without becoming leaky.
These drop tests were followed by a fire test at the fire test stand in Lehre. To simulate a full container, the cast container was filled with ion-exchange resin. The container was then subjected to a fire for one hour, to measure the resulting changes in pressure and to detect any possible leakages. During the course of the test, the inner wall of the container reached a temperature of almost 400 degrees Celsius. The leakage rate remained considerably lower the permissible maximum value.
2. Transport Containers for Fresh Fuel Assemblies
The acronym ESBB stands for Einzel-SNR-Brennelement- Behälter (that is "Single SNR fuel element container"). The ESBB is made for the transport of one unirradiated SNR fuel element each from the so-called Kalkar rapid breeder that never went into operation. It consists of a stainless steel fuel rod cladding in which the radioactive inventory of sintered pellets is enclosed.
The container itself consists essentially of a four metre long seamless steel tube with a diameter of 160 millimetres. The first prototype had a welded-in bottom and a screwed in plug. This plug could be expanded with the help of a spindle. The plug pressing against the tube from the inside sealed up the container. However, the total of six drop and fire tests performed by BAM during the year 1997 showed that the design was not suitable. Even the first drop test resulted in deformations of the plug so that the container became not tight. The radially pressed metal gasket proved to be unsuitable.
The manufacturer thus undertook a design change on the ESBB. The plug was welded to the container. The metal gasket was replaced by a tight welded seam.
In 1998, again six drop tests were performed using two modified prototypes. Four times the containers dropped down onto the unyielding target from a height of nine metres, twice they dropped down onto a rigid bar from a height of one metre. In the oblique and horizontal drop they partly showed clear deformations: In the drop from nine metres, with the longitudinal axis slanting 20 degrees, the container was bended by 30 millimetres. However, the welded seam remained undamaged, as helium leakage tests showed. The ESBB container can thus be considered accident-proof.
2.2 Other Containers
In 1999, BAM performed drop tests with transport containers of type RA-3D for the fuel element manufacturer ENUSA Industrias Avanzadas S.A., Spain.
The RA-3D container was not tested at BAM test facility in Germany, but near Salamanca, Spain, at the factory premises of the manufacturer ENUSA. The reason for this: The fuel rods loaded in the test specimen were filled with uranium oxide pellets made of natural uranium. The necessary approval for performing such tests was given more quickly in Spain than this would have been possible in Germany. Three experts from BAM traveled to Spain with measuring equipment of BAM to perform the usual drop tests from heights of 1,2 metre, and 9 metres, as well as a drop from one metre on to a rigid bar made of mild steel.
In 2000, protective containers for unirradiated MOX fuel elements made by Nuclear Cargo + Service GmbH (NCS), Hanau, were tested.
The MOX protective containers were tested as 1:2 scale models. The originals were developed for the transport of unirradiated MOX (uranium mixed oxide) fuel elements for boiling-water reactors and pressurized-water reactors.
The outer shell of the protective container consists of 8 millimetre steel plate (4 millimetres in the model) covering a 30 millimetre heavy layer of fir wood (model: 15 millimetres). The steel plate of the inner container is located 20 centimetres and 35 centimetres deeper (model: 10 cm).
The intermediate space serves as a crash zone to absorb impacts in crash situations. For example, in the drop test onto a rigid bar, the outer steel shell was torn open, the wood splintered but it was not struck through. A 54 millimetre high buckle resulted, which however did not come into contact with the inner container. The fuel elements remained undamaged.