APPENDIX II: COMMERCIAL CASKS FOR SPENT FUEL TRANSPORT
A. Persson
OKG AB, Oskarshamn, Sweden
Abstract. This report provides information on the status of spent fuel storage and transport in Sweden. There is one central pool storage system (CLAB) for all 11 reactors in operation in Sweden. CLAB is interim spent fuel storage until the final geological disposal facility becomes available. Shipment of spent fuel is carried out by 10 dedicated transport casks, the maintenance of which follows the safety report. Since 1985 when the transport started, significant experience was gained on cask maintenance and occasional repairs
Spent fuel situation
Sweden has today 11 reactors in operation, located at four sites along the coast. All the spent fuel is transported to one Central Interim Storage for Spent Fuel, CLAB. This is a pool storage facility.
The CLAB facility is owned by SKB, Swedish Nuclear Fuel & Waste Management Co, they are also responsible for the transport system. At CLAB we have today a total of 19583 fuel elements in store representing 3800 tons. Total capacity in CLAB will be 8000 tons. CLAB is an interim storage and spent fuel stored there will in the future be taken up and put in copper canisters for final disposal in a deep geological repository 500 meters down in the crystalline bedrock.
Transport system
The transports to CLAB started in 1985 and since then we have shipped approximately 1300 casks.
Transports are done by sea, with the ship M/S Sigyn and a fleet of 10”transport only” casks Transnucleaire model TN17/2. Transports are regulated by national laws as well as the IMDG-code and ADR.
A time schedule for planned transports is made up for one year in advance. Before transports, fuel to be shipped is reported to SKB for approval and a check is done to make sure it is covered by the license for the transport cask and CLAB.
Advance notice for each transport is sent to competent authorities.
Handling of casks is carried out in a rather traditional manner, loading under water in reactor fuel pool. A protective skirt is fitted around the cooling fin area of the cask before lowering into the pool.
After loading, the cask is taken out of the pool drained from all water and vacuum dried inside. Then the cask is decontaminated and sent to CLAB.
At CLAB unloading is done under water after cooling of the cask and fuel. The pool system consists of two pools, one clean where the cask is set down then the cask is docked to the bottom of the next pool where the fuel is unloaded. This eliminates the risk of contaminating the cask at CLAB but a protective/cooling skirt is fitted as well.
Cask maintenance
Each cask has now made approximately 130 transports and they are still in very good condition. Casks not in use are stored indoors at CLAB and the work crews responsible for handling casks at the power stations and CLAB by now have long experience of this work. This helps keeping wear and damage at a low level.
Maintenance is carried out in accordance with the safety report. Basic maintenance is carried out every 15 transports or 3 years, and a major maintenance every 60 transports or 6 years. This is done in our
maintenance shop at CLAB where we have the special equipment necessary for this. Maintenance personnel follow a computerized checklist describing the different operations to be performed. Each maintenance is documented.
At maintenance all o-rings are replaced and we leak test the cask according to the same acceptance criteria as at manufacture. We do not see any trend towards deteriorated functions, which would show in difficulties in reaching acceptable results in leak tests.
Another thing we spend some time on is restoring outside surfaces, trying to keep them as smooth as possible to make decontamination easier. Since 1998, when the problem with casks being contaminated on the outside was brought into focus, a lot of energy is being spent on cleaning the cask before transport.
At CLAB we also have equipment for cleaning (brushing) the cooling fins on the outside of the cask.
This is now done regularly in connection with maintenance.
These measures have helped bringing down the frequency of contamination found on the outside of the cask at receiving control at CLAB.
Each maintenance and transport is logged in a data-base at the CLAB facility. This data-base contains information of the type :
• Spare part replacement,
• Deviations from the normal on site or during transport,
• Remarks from turn around maintenance at CLAB,
• Cask contamination on site and at CLAB.
This data-base has over the years proved useful in providing statistics enabling us to study the frequency of spare part replacement and / or repairs to the cask.
We do not as yet see any alarming trends for any of the replaceable parts, although some have a rising frequency for replacement. For those we have started looking at possible repair methods to restore functions to required standard.
Cask repairs
Through the years a couple of more complicated repairs and/or modifications have been carried out.
In 1994 we had a case where the stainless cladding inside one cask was separating from the base material. All the cladding was then machined out and new cladding was welded on and remachined to size. Notable from this operation was, how little of the material that was contaminated. Only the surface was radioactive, after that the material was clean.
In 1995 extra neutron shielding was applied to two casks. The existing silicone layer was torn of and the extra layer of neutron shield was poured around the cooling fins. Finally a new layer of silicon was poured on top of the neutron shield.
Both of the operations were successfully performed at the CLAB facility.
We now plan to replace the outer silicone layer on eight casks. This will be done next spring at CLAB.
The reason for this is that we want to make sure no contamination is hiding behind the old silicone layer.
These examples show that major repair / maintenance operations are possible as well and we expect to be able to keep our casks in service for a long time .