Gabriele Hampel, Wolfram H. Knapp
Medical University of Hannover
Department of Nuclear Medicine
Carl-Neuberg-Str. 1, D-30625 Hannover, Germany

Kurt Ebbinghaus, Dirk Haferkamp
NOELL-KRC
D-97064 Würzburg, Germany

A research reactor of type TRIGA Mark I was installed in the Department of Nuclear Medicine at the Medical University of Hannover (MHH) for the production of isotopes with short decay times for medical use in the early seventies. Since new production methods have been developed, the reactor has become obsolete and the MHH decided to decommission it. As the fuel elements were delivered by General Atomics of San Diego, the MHH intends to take part in the „Research Reactor Spent Nuclear Fuel Acceptance Program" of the United States. Probably in the second quarter of 1999 all 76 spent TRIGA fuel elements will be shipped to Idaho National Engineering and Environmental Laboratory (INEEL) in one cask of type GNS 16. Due to technical reasons within the MHH a special Mobile Transfer System, which is being developed by the company Noell-KRC, will be used for reloading the fuel elements and transferring them from the reactor to the cask GNS 16. A description of the main components of this system as well as the process for transferring the fuel elements follows.

A research reactor of type TRIGA Mark I was installed in the early seventies at the MHH. The reactor is located in the basement of the building which houses the Department of Nuclear Medicine.

In 1972 the approval for reactor operation was received and 71 TRIGA fuel elements with aluminum claddings were delivered by General Atomics of San Diego. Operation started in 1973 at a power level of 250 kw with 60 fuel elements in the core. In 1984 five additional fuel elements with stainless steel claddings were delivered by General Atomics. These fuel elements were installed in the core in 1991. Nevertheless a reduction of the power level began at this time. In 1993 the power level was reduced to 100 kw and since the beginning of 1997 the reactor has been in an inactive operation phase.

The TRIGA reactor has been used for the production of medical isotopes with short decay times for use in research and diagnosis in the nuclear medicine. For example the isotope Fluorine-18 with a decay time of 1.78 hours was produced for bone scintigraphy or the isotope Xenon-135 with a decay time of 9.10 hours was produced for the diagnosis of lung function.

Another application was the activation analysis in medical, biological and geological research for the detection of very small amounts of chemical elements. For example the amount of cadmium in food like fish and vegetables or the amount of diesel soot from public buses in Vienna were detected.

There were three different groups of users of the TRIGA reactor during its time of operation between 1973 and 1996: The Department of Nuclear Medicine with a share of 45 %, other MHH departments with a share of 17 % and other institutes outside the MHH with a share of 38 %.

In the first years of operation the reactor was mainly used by the MHH, in the last years it was mainly used by institutes outside the MHH. In addition, the number of irradiations decreased rapidly since 1993. It was then that the MHH decided to decommission the reactor.

The reasons for the decommissioning are that new methods for producing medical isotopes with a cyclotron or nuclide generator have been developed and a power level of 100 kw is not sufficient for most activation analyses. Continued reactor operation would require new fuel elements, new technical instrumentations and new instrumentations for radiation protection at high cost. This all makes no sense, because there are no applications for continued operation in the MHH. Therefore the reactor must be decommissioned within the next years and the fuel elements have to be removed as soon as possible.

During operation the following personnel structure and responsibilities are organized in accordance with current German regulations: The license holder is the State of Lower Saxony, represented by the Ministry of Science and Culture, in turn represented by the Chancellor of the MHH, who is responsible for the TRIGA reactor. The Chancellor then appoints a deputy as Operation and Administrative Chief. The MHH personnel includes a Reactor Manager, a Radiation Safety Officer, a Safety Officer for On-site Safety and Protection, a Chief Operator, two Operators and two Technicians.

For the whole decommissioning the MHH has engaged the company Noell-KRC as general contractor, which is responsible for planning and completing the project, licensing and approval, site staff, documentation, quality control, radiation protection, safety regulations and fire protection.

To perform this mission the general contractor will join MHH in the licensing procedure and will also be appointed as a license holder.

A total of 76 spent TRIGA fuel elements must be removed. Each fuel element consisted of a homogeneous solid mixture of uranium and zirconium hydride with 8 wt.% uranium enriched by 20 % U-235 upon receipt.

As all fuel elements were delivered by General Atomics, the MHH intends to take part in the „Research Reactor Spent Nuclear Fuel Acceptance Program" of the United States, represented by the Department of Energy (DOE).

All TRIGA fuel elements (aluminum and stainless steel clad) will be delivered to INEEL probably in the second quarter of 1999.

The shipment will be carried out by truck from the MHH to the German port of Bremerhaven, by vessel to Charleston Naval Weapons Station and by train or truck to INEEL. From MHH to Charleston Naval Weapons Station transport will be organized by the consortium NCS / GNS and from Charleston Naval Weapons Station to INEEL by DOE / INEEL.

The title to all fuel elements shall vest in the United States upon arrival on United States soil at the Charleston Naval Weapons Station.

Only one transport cask of type GNS 16 is required to ship the 76 fuel elements. This cask must have a certification of approval for use in Germany and a validation for transport in other countries, especially in the United States. Both procedures have been initiated.

In addition, authorization to ship by the German Federal Board of Radiation Protection, permission to export, permission to import into the United States and the participation of the Euratom Agency are required.

Due to technical reasons (e.g. the transport cask weighs 15 t, but the maximum floor weight capacity in the radiological building is only 2 t / m² ) and the location of the reactor within the radiological building, it is not possible to place the cask inside the reactor facility. Therefore a temporary building with an area of about 15 m x 4.5 m and a height of about 9.5 m will be erected next to the radiological building to house the transport cask GNS 16.

The building will also be used to prepare the waste from dismantling the reactor for transport.

For the transfer of the fuel elements from the reactor to the transport cask a special Mobile Transfer System, which is being developed by the company Noell-KRC, will be used.

The main components of this Mobile Transfer System are

• Loading Units for 6 (5) fuel elements
• Special Transfer Cask
• Transfer Vehicle
• Mobile Reloading Facility for the Loading Units
• Air Cushion Gliding Track

This Mobile Transfer System is being developed especially for use at the TRIGA reactor of the MHH. With only small changes to the equipment it can also be used for the removal of spent fuel elements from similar reactor facilities.

Fig. 1 shows the reactor facility on the ground floor of the radiological building. The fuel elements will be loaded from the reactor tank into one of the storage pits. From this pit they will be drawn into the Special Transfer Cask, which will be set on a Transfer Vehicle using a hoisting device. The Transfer Vehicle together with the Special Transfer Cask will be moved through the reactor area to the elevator.

In detail the transfer process of the fuel elements at the MHH includes the following steps:

• Preparation of one storage pit
  A Loading Unit, which has been especially developed to accommodate TRIGA fuel elements, is placed in the storage pit. Two different kinds of Loading Units are being developed: One type with loading channels for 6 and the other for 5 fuel elements. The Loading Unit for 5 fuel elements will be used for fuel elements which are not completely straight.
• Reloading of fuel elements from the reactor tank into the Loading Unit
  One fuel element is loaded from the reactor tank into the Loading Unit in the storage pit using the MHH flask, which is part of the normal equipment for operation. This process is repeated until the Loading Unit is filled.
• Loading the Special Transfer Cask with Loading Units
  The Special Transfer Cask is set on the storage pit and the full Loading Unit is drawn into this cask (see fig. 2).

• Preparation of the Special Transfer Cask for Transfer within the MHH
  Since the fuel elements are wet the Loading Unit inside the Special Transfer Cask is air-dried. After that the Special Transfer Cask is hoisted onto the Transfer Vehicle and tied down by safety belts. A protective hood is placed over the cask, producing a below atmospheric pressure. Then a propelling apparatus is coupled to the transfer vehicle.
• Transfer from the reactor facility to the temporary building
  The Transfer Vehicle is moved by the propelling apparatus through the reactor area to the elevator, which lifts the vehicle one meter up to the level of the basement floor. From here the Transfer Vehicle is moved about 80 m along the corridors to the temporary building.

• Movement of the Loading Unit from the Special Transfer Cask into the cask GNS 16 (see fig. 4)

• Transfer Route on MHH grounds
  The cask GNS 16 will be moved outside the temporary building by means of the Air Cushion Gliding Track. From here it will be placed by the mobile crane on the 20-ft-container. The cask will be transported from the MHH to the port at Bremerhaven by truck.

The transfer process described above offers maximum safety at all times. Potential load drop has been taken into acount by specific provisions and design features in keeping with the German safety regulations.

During removal, transfer and shipment of the fuel elements all measures will be taken in concurrence with the minimization principle as described in Art. 28 (1) of German Radiation Protection Ordinance.

The group equivalent dose during removal and transfer of the fuel elements is estimated to be about 15 man-mSv. For 3 or 4 persons involved the equivalent dose will be approximately 4 - 5 mSv for each person during a time of about 3 months for carrying out the procedure.

There is no danger of an additional radiation dose for MHH personnel, patients, students, visitors or others who are not involved in the work.

The whole decommissioning will take place according to the following schedule:

The reactor was finally shut down in January 1997. The approval for the transfer of the fuel elements from the reactor core into the transport cask is expected to be received by the end of 1998. The transfer of the fuel elements will be carried out in the first quarter of 1999. The shipment of the fuel elements to the United States is intended for the second quarter of 1999. Directly after this the dismantling of the reactor will begin. The reactor facility will be released from the provisions of the German Atomic Law in the second half of 2000. After this the MHH will start with the preparations for converting the reactor facilities for new applications.

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