Rokkasho-Mura: from plans to reality27 August 1999
The final destination of vitrified high level waste shipped to Japan from Europe is Rokkasho-Mura in Aomori, which has been called Japan’s “nuclear prefecture”. Besides the HLW storage facility at Rokkasho-Mura, JNFL (Japan Nuclear Fuel Ltd) operates a uranium enrichment plant, low-level radioactive waste disposal centre and a vitrified high level waste storage facility, and is constructing a reprocessing plant scheduled for completion in 2003.
An article in the October 1991 issue of Nuclear Engineering International described the many nuclear projects for Aomori prefecture. At that time, most of the nuclear facilities were in the planning stage and the most newsworthy project was the nuclear ship Mutsu which had finally begun sea trials after an 18 year delay. The Mutsu completed its trials successfully and has since been decommissioned but, in the mean time, progress has been made with other projects.
THE ROKKASHO SITE
The fuel cycle plants at the Rokkasho-Mura site have made the most progress. The two companies which were set up to run the Rokkasho facilities, Japan Nuclear Fuel Industries Co and Japan Nuclear Fuel Services Co, have since merged to form Japan Nuclear Fuel Limited, JNFL. Unlike BNFL, the British company, JNFL is entirely privately owned with the nine utilities having more than 70% of the shareholding.
Construction of the centrifuge enrichment plant was started in 1988 and the first stage, with a capacity of 150 SWU/year went into operation in 1992. JNFL has been adding additional stages, each of 150 SWU/year, at approximately yearly intervals, so that today the total capacity is 1050 SWU/year. Additional stages will be built until the total capacity of 1500 SWU/year is reached. This is roughly sufficient to provide fuel for only 13-15 1000 MWe class LWRs and so Japan will continue to use overseas providers of enrichment services for most of its requirements for the foreseeable future.
The plant’s centrifuge technology was developed by PNC (now JNC) and Japanese vendors. The construction cost of the whole facility will be about ¥250 billion ($1.9 billion).
Low level waste disposal centre
Sufficient space has been provided at the LLW facility to permanently dispose of 3 million drums. At present, the centre is licensed for 200 000 drums. An area required to accommodate this number of drums has been excavated to a depth of approximately 12 m and a series of reinforced concrete vaults constructed. Each vault is 24 m square and 6 m high and is divided into 16 compartments.
A specially designed, enclosed gantry crane is positioned over a vault and the 200 litre drums of waste, which have previously been inspected, are loaded eight at a time into the vault. When a compartment is full a temporary cover is placed over the drums and the gaps between the drums are filled with a cement-based grout to form a monolithic mass. Each compartment holds 320 drums (eight layers, each of 40 drums), so that one vault holds 5120 drums. After a vault has been completely filled, the crane is moved to the next vault and the full vault is covered with a concrete slab.
Each compartment is surrounded with a 10 cm layer of porous concrete so that if any moisture should leak into the vault, it will flow through the porous layer and be taken by a drainage system to an inspection tunnel, instead of penetrating the drum storage area.
When sufficient vaults have been filled, the spaces between the vaults will be filled with a waterproof bentonite-sand mixture and a 2 m deep layer of the same material will be placed on top of the vaults. Finally, the whole area will be covered by 4 m of earth. The site will be managed for 300 years allowing the radioactivity to decay to negligible levels.
As of last April, 123 000 drums had been received at the centre. To put this into perspective, Japanese nuclear plants have produced a total of about 600 000 drums of low level waste which have been stored, without any problems, at the power station sites. This waste is gradually being moved to Rokkasho, by sea, at a rate of about 16 000 drums/year.
The cost of developing the centre to accommodate 1 million drums is approximately ¥160 billion ($1.3 billion).
High level waste storage facility
Almost all of the spent fuel from Japan’s LWRs has been sent to Europe for reprocessing by Cogema and BNFL. The reprocessing contracts require that the high level waste containing the fission products should be returned to Japan and this is done in the form of canisters of vitrified waste, each canister being 40 cm in diameter and 1.3 m high.
When the canisters arrive in Japan they are at a temperature of 300°C to 400°C and must be placed in a temporary storage building for 30 to 50 years, by which time they will have cooled to 100°C and can be placed in a deep underground repository. Construction of the vitrified HLW storage facility began in 1992 and it was operational in 1995. It has the capability of storing 1440 canisters but will be extended in future to hold a total of 3000 – 4000 canisters. At present it contains 128 canisters with an additional 40 received at Rokkasho in April.
The facility follows the well-known dry storage design in which the heat is removed by natural convection air-cooling. The storage building has 160 storage locations, each provided with a steel “thimble”. The canisters are stacked nine high inside the thimble and the cooling air passes through the annular space around the thimble. The building has a tall discharge air duct to give the required natural convection flow.
When a transport cask arrives at the port it is transferred by a special road vehicle to the receiving building where it is off loaded and lowered into a part of the building below ground level. There the cask is up-ended and moved through an underground passage to the inspection area in the storage building where the head is removed and the canisters are taken into the inspection room. They are then checked dimensionally and the heat rate, radioactivity and surface contamination are measured. After that the canisters are raised through a hole in the ceiling of the inspection room by a special shielded crane which transfers them to the storage locations.
The walls of the storage and inspection areas are made of reinforced concrete, 1.9 m thick, which provide radiation shielding and protection against aircraft impact and earthquake damage. The facility, with its present capacity of 1440 canisters, cost ¥80 billion ($640 million).
The reprocessing plant
The spent fuel reprocessing plant is the largest and, at ¥1880 billion ($15 billion), by far the most expensive of the JNFL projects. The plant will have the capacity to reprocess 800 t of uranium per year and is based on technology used in Cogema’s plants at La Hague. However the design has been extensively re-engineered to suit Japanese conditions, including the need to protect against earthquakes, and includes some technology from Japanese and UK sources.
Construction started in 1993 but has been delayed several times. However 90% of the necessary approvals have been obtained and construction is proceeding well. JNFL is confident that reprocessing will start in January 2003.
The spent fuel receiving facilities and spent fuel holding pools are already finished. Spent fuel casks will be brought to Rokkasho by ship and moved into the receiving area by special road vehicles. They will then be taken into a temporary storage building capable of holding 20 or 30 casks. The fuel assemblies will be removed from the casks in special unloading pools and transferred underwater to the spent fuel holding pools. There are three of these pools, each capable of holding 1000 t of fuel. They are 27 m long, 11 m wide and 12 m deep. One will be reserved for PWR fuel, one for BWR fuel and the third will be for both types.
Fuel will be transferred from these pools to the head-end process building through a 100 m long underground water channel. In October last year, trial shipments of BWR and PWR fuel were made from Fukushima Daini, Sendai and Ikata nuclear power stations. As well as proving the transportation process, these fuel assemblies will be used to check the burn up measurement instrumentation at Rokkasho.
A 800 t/year reprocessing plant can handle the fuel from about thirty 1000 MW class reactors and so the Rokkasho plant will not be able to reprocess all of the spent fuel from Japanese nuclear power stations. The remaining spent fuel will be stored until decisions have been made about future reprocessing or disposal.