Transporting damaged fuel2 January 2019
As Germany’s nuclear plants close, damaged spent nuclear fuel must be managed. But it may present an opportunity for dealing with the damaged fuel assemblies more cost-effectively. Franz Hilbert and Maik Hennebach propose a novel solution.
IN GERMANY, OBTAINING A LICENCE for decommissioning and dismantling of a reactor pressure vessel (RPV) and its internals requires there to be no fuel in the nuclear plant.
Complete fuel assemblies containing undamaged fuel rods are placed into heavy dual-purpose casks or, in other countries, placed in dry storage canisters which are put in storage vaults.
However, in each German reactor (and most likely in all reactors in the world) a small number of damaged fuel rods have accumulated over the years of operation. This material requires the same safeguarding measures as the bulk of fuel assemblies – bearing huge costs in comparison with its small amount. The removal of these damaged fuel rods might become the bottleneck in dismantling the plant.
Daher Nuclear Technologies has developed tools to prepare damaged fuel for transport (using underwater brazing technology) and to transport it in its NSC 45 package.
About NCS 45
The NCS 45 package (Figure 1) has a main body with a stainless-steel double shell (blue) which encloses thick lead shielding (grey). The package is equipped with a rotary lock at the lid side, and a push plug at the plug side. All openings are sealed off during transport with bolted stainless-steel lids (red) and double-O-rings for leakage testing. The front of the package (with the lid) is protected by large shock absorbers (orange) against the normal and accidental conditions of transport. The mass of the package as presented for transport is 22.5t.
The NCS 45 packaging can be loaded and unloaded while submerged in water (in a fuel pool), or under dry conditions attached to a hot cell.
For dry handling the rotary lock lid can be removed and reinstalled outside the hot cell to avoid surface contamination of the lid; the rotary lock provides sufficient shielding in closed position even without the rotary lock lid. The push plug can be used to push the radioactive material into the hot cell or to pull the material into the packaging.
The content can consist of undamaged and damaged UOX or MOX fuel rods, from all types of reactors currently in use (PWR, BWR and FBR). The burnup might be up to 120GWd/MgHM and the cooling time as low as 120 days after discharge from the reactor. There are no restrictions on cladding material, pellet diameter or active length. The total length of the fuel rods is restricted by the total length of the cavity (4620mm). The number of fuel rods which can be transported depends on burnup, cooling time and condition of the fuel rods (damaged, undamaged or encapsulated).
It accommodates UOX fuel enriched to 5% or 7% (by weight), or MOX fuel of up to 100% Putot and max. 100% uranium with maximum 94% fissile Pu in Putot and maximum 94% U-235 in U. The maximum fissile mass for MOX fuel may be up to 8.25kg in the chemical form of oxide or carbide, depending on burnup and cooling time.
The NCS 45 package received its first type B(U)F-96 certificate of package approval in 2008, and the first NCS 45 was put in operation in 2009. The country of origin of the certificate is Germany. Currently Rev. 4 of the certificate is valid until 2020. The certificate has been validated in several European countries; earlier revisions were also validated in the USA, but that was allowed to lapse for to economic reasons.
Underwater brazing equipment
Fuel rods with a burnup greater than 55GWd/tHM must be encapsulated for transport in welded or brazed capsules. The NCS 45 can transport more encapsulated damaged fuel rods than non-encapsulated damaged fuel rods.
Encapsulation of damaged fuel rods or fuel rod parts in a hot cell in welded capsules is a state-of-the-art technology. However, encapsulation of damaged fuel rods submerged in a fuel pool is a technological challenge. To meet that DNT has developed patented underwater brazing equipment (UBE, see Figure 2). The UBE consists of an upper and lower brazing chamber, which are connected by a tube. The upper brazing chamber has an opening at the top for loading and unloading the UBE, which is closed after loading with a lid. The lower brazing chamber is equipped with a lever to lift the lower and upper plug into the brazing position, after the draining and drying process is completed. Both brazing chambers contain the inductive heating system, comprising an inductive coil surrounding the tube and plug of the capsule, and the power source. An optical sensor connected by fibre optics to the power generator is used to control the temperature of the capsule during brazing.
Figure 3 shows the production process of brazed capsules. The capsule is made of a stainless-steel tube (yellow) with pre-mounted lower plug (blue). The tube is lowered under water into the UBE. Next, the fuel rod (red) is loaded into the capsule and the upper plug positioned on top of the capsule. The UBE is closed by placing a lid onto the upper brazing station, the water expelled. The cavity of the UBE and the capsule are dried using a vacuum drying method. Before brazing, the cavity of UBE and capsule are filled with helium. Then capsule, lower plug and upper plug are pushed by using the lever of the lower brazing station into their brazing positions. One after the other, the plug areas are heated by inductive heating to 1100°C to melt the solder. After cooling down, the brazed capsule is leak-tested using helium.
The German example
German utilities want to remove all reactor fuel as soon as possible after shutdown to decrease the need for safeguards measures and to facilitate dismantling. Transport and storage solutions for fuel assemblies are well established, so the bulk of nuclear material can be removed from the reactor pool after a cooling time. However, there are currently limited solutions for dealing with the small number of damaged fuel rods, plus the costs for dealing with the comparatively tiny amount of fuel are prohibitively high.
About half of the German reactor fleet is already shutdown and the other half is still in operation. The solution is simply to transport the damaged fuel rods from a permanently closed reactor to another reactor still in operation. This has the following advantages:
- The damaged fuel rods can be removed from the fuel pools of shutdown reactors in line with the removal of the fuel assemblies. It is no longer on the critical path for dismantling;
- Intermediate storage of the damaged fuel rods in dual purpose casks can be developed and licensed until the reactor where the fuel rods are accumulated is shutdown;
- The cost of storage for the damaged fuel rods is spread across more fuel rods accumulated from several reactors, instead of an individual reactor.
Removal of damaged fuel rods
Planning, licensing and performing of the removal of damaged fuel rods from a nuclear plant comprises the following major activities:
- Check fuel rod data and comparison with the allowable limits specified in the certificate of package approval of the NCS 45. If required, extension of the certificate to fit to the fuel data.
- Definition of a transport plan.
- Authorisation of operations.
- Cold trial using the NCS 45.
- Application for transport permit.
- Transport is performed.
The package NCS 45 can transport undamaged MOX fuel rods up to a burnup of 55GWd/tHM. Above that burnup and for damaged MOX fuel rods encapsulation in leak-tight welded or brazed capsules is required. To achieve this for fuel rods in a fuel pool, the DNT underwater-brazing technology is used. The general steps to produce these capsules containing fuel rods are as follows:
- Plan and perform cold trials at the plant;
- Get go-ahead for the operation by the supervising authority;
- Encapsulation and production of dry, leak-tight and helium-filled capsules.
Figure 5 shows the UBE in operation under water. The rectangular structure is the support and earthquake safety structure of the UBE. This support structure is variable and can be adapted to the situation in the fuel pool. The top of the UBE (the lid and the housing for the temperature sensors) can be seen in the lower left corner. The large black hose contains the power cables. The yellow hose contains the fibre optics for the temperature sensors. The flanged tube in the centre of the figure is the tool for the lid of the UBE. The stainless-steel hoses on the right side are for draining and vacuum drying of the UBE.
In conclusion, DNT has successfully planned and carried out several transport operations recently and has more in its order book. Together with underwater-brazing technology, the company can provide solutions not only for irradiated UOX fuel rods but also for MOX fuel rods and fuel rods with a high burnup, as well as damaged assemblies.
Author information: Franz Hilbert, Chief Operating Officer at Daher Nuclear Technologies GmbH; Dr. Maik Hennebach, Head of Design and Development at Daher Nuclear Technologies GmbH