Underground movements

In 1676 a Swedish warship, the Kronan, sank in the Baltic off the coast of the island of Öland. A bronze cannon sank vertically into the sand and clay sediment and remained undisturbed for 310 years. In 1986 the Swedish Nuclear Fuel and Management Company (SKB) financed marine archeologists to excavate the cannon and return it to land for scientific analysis.

The cannon was made from a copper rich bronze, and showed little sign of corrosion following more than three centuries in the sea bed. The fact that the copper rich metal corroded so little in such an environment provides clues to the rate at which copper canisters containing radioactive waste would perform in a deep waste repository. The conclusion from the cannon study is that the rate of corrosion for copper is less than 10 mm in 100 000 years, the length of time that spent fuel must remain isolated from the environment.

The Swedish nuclear industry is relatively advanced in addressing the question of how to deal with spent fuel. Nuclear power in Sweden carries a tariff of 0.01Kr/kWh (£0.001/kWh), which is then put aside to finance waste disposal. The fund now contains Kr21 billion. The resources are therefore in place to develop the processes necessary to encapsulate spent fuel in a stable rock formation deep underground.

As part of the process of development towards the final objective of deep disposal, SKB has built a canister laboratory in the town of Oskarshamn. Near the town is the Oskarshamn nuclear power station, the Central Storage Facility (CLAB) where spent fuel from the Swedish programme is taken for temporary storage and the SKB Äspö Hard Rock Laboratory (HRL) where experimental work on the geological issues surrounding deep disposal is carried out.

“The main reason for building the canister laboratory is to develop the technique for welding the lids to the canisters once spent fuel has been inserted, in a process rather than a laboratory environment,” said Henry Gustafsson, the canister laboratory site manager.

The CLAB can currently store about 5000 tons of spent fuel, but an expansion programme means by 2004 it will have the capicity for 8000 tons, large enough to hold all spent fuel up to 2010.

The canister laboratory was designed by BNFL Engineering, which has been closely involved in the project for five years. Richard Acton, BNFL Engineering’s project and business development manager for SKB, described BNFL Engineering’s role.

“The construction of the canister laboratory is an important milestone in this project,” says Acton. “The main process stations and equipment in the process have been fully designed by BNFL Engineering and most of the equipment is constructed to allow reuse. We’ve used BNFL Engineering’s experience of remote spent fuel handling to produce an integrated process design which also incorporates special electron beam welding and X-ray techniques. With SKB personnel, we have used safety analysis techniques including HAZOP studies, to produce the basis for SKB’s preliminary safety report. This will be developed to incorporate the experience from the canister laboratory for submission to the Swedish regulatory authorities.”

The process of encapsulation being developed involves retrieving spent fuel from the CLAB facility after 30-40 years, placing the fuel assemblies within a steel matrix which is then placed within a copper outer canister, sealing of the canister using electron beam welding and inspection of the completed canister. The canister laboratory is designed to develop the remote handling and welding processes. BNFL Engineering has been involved in providing the welding equipment in collaboration with The Welding Institute, based in Cambridge UK. BNFL Engineering took responsibility for the design of special purpose equipment including one of the largest vacuum chambers in the world and a jacking frame designed to transport the 25 ton canister to the process stations where welding and testing will take place.

Between now and 2001 SKB will develop the encapsulation process, in order to have an industrial scale system ready for government approval at that time. A key issue is the quality of the weld between the canister and the lid. The electron beam welding machine is designed to penetrate 50 mm of copper, the same as the thickness of the copper surrounding the fuel and steel matrix. The jacking frame will manoeuvre the canister into the vacuum chamber where the weld will take place. The canister will rotate through 360? over a 20 minute period. A number of different lid designs will be tested. The quality of the weld has to be of such a high standard, the process BNFL Engineering and SKB are developing is likely to involve the most sophisticated welding ever attempted.

Once the weld is complete the canisters will be moved remotely for X-ray and ultrasonic inspection. The X-ray machine has a power of 9 MeV, compared with the 300 keV of a normal unit. Through computer tracking it will be possible to locate flaws as small as 1 mm in the weld.

SKB plans to build the encapsulation plant next to the CLAB facility. It will have a throughput of one canister per working day, equivalent to 210 in a year. From the encapsulation plant the canisters will go for final disposal in a deep hard rock repository. There they will be lowered into a deposition hole and surrounded with bentonite, a form of clay. The disposal canister has to seal the spent fuel from the surrounding environment for 100 000 years, the time it will take for the material to decay to natural background levels of radiation, and it is this design criteria which is leading to the development of such advanced engineering.

In the environment in which the canisters are placed there is no free oxygen as micro-organisms within the rock reduce any free oxygen within the ground water. As a result copper will behave almost is if it were a noble metal, with very slow corrosion rates, as the excavated cannon shows. The steel matrix will provide the strength necessary to withstand the pressures of earthquakes and glacial movement likely to impact on the canisters over a 100,000 year period.

Final disposal

At the same time as research is taking place at the canister laboratory into the encapsulation process, work is ongoing at the Äspö HRL into the development of the final repository. Dropping to over 500 metres below ground level, the HRL offers the opportunity for research, technical development and demonstration within a realistic setting.

“One of the major aims was to locate the main fracture zones within the rock,” said Olle Olsson, the Äspö laboratory manager. “Models of the site proved relatively accurate and this suggests we can predict fracture zones elsewhere. We had similar success in predicting the height of the groundwater table.”

SKB has ruled out areas of sedimentary rocks for the final repository as they are generally too porous and allow too rapid a flow of ground water. However, much of Sweden is made of granite, part of a large geological feature known as the Baltic Shield, and it is within this structure that the facility will be built. The Baltic Shield is geologically stable and the tectonic pressures the disposed canisters would be likely to experience, even if a large earthquake were to occur, are unlikely to involve movements greater than 1 cm. Another criterion for the final repository is that there should be no ore bearing minerals in the vecinity in case future generations prospect in the area.

Much of the geology of Sweden is suitable for the purposes of deep disposal and the final decision as to the location of the repository is likely to be more of a political than a technical exercise. Any final repository will only be built with the support of the local population and SKB is working on feasibility studies for four possible sites at Tierp, Östhammer, Nyköping and Oskarshamn. The sites are in southern Sweden and three are nuclear communities. Two possible sites in the north of the country have already been rejected following local referenda.

The feasibility studies and the experiments currently underway at Äspö should be completed by 2001, when two sites will be chosen for investigation. SKB must obtain local acceptance at two sites before drilling can take place. Following the development of a site investigation programme and feasibility studies SKB will then apply for a licence and start construction.

Once the final site is chosen SKB will then apply for another licence to carry out a demonstration deposition of around 400 canisters, 10 % of the anticipated final quantity of around 4000 canisters. This is likely to take place around 2015. There will then be a two year evaluation period and finally the government should issue a licence to place the rest of the canisters in the repository. The final canister is likely to be put into deep disposal some time between 2050 and 2060.

The repository will be designed so that it is possible to retrieve the canisters should a decision be made to do so. The tunnels will be backfilled once the canisters are in place, but the main tunnel will remain open. One of the experiments being planned at Äspö is designed to ensure that retrieval is possible.

“We need a test facility to provide input to performance assessments under realistic conditions,” said Ollson. “We can develop, test and evaluate methods for investigation, construction and deposition. It will give us a good feeling for the parameters to which we can work.”

Other experiments include analyses of the movement of radioisotopes such as strontium-137, technecium-99 and cobalt-60. Model predictions for the movement of strontium have proved very accurate when compared with experimental data. Generally radioactive nucleides bond strongly to rock as the rock surfaces are negatively charged and the radionuclides positive. Combined with the very slow movement of ground water in the granite means the likelihood of radioactivity being released into the environment from a waste repository is extremely low.

Another area of research is looking at the impacts of drilling, blasting and boring on the rock structure. From work already completed the zone of increased permeability, where fractures form in the rock structure, is limited to 0.8 metres. Blasting techniques should be refined to reduce this distance.

SKB plans to build six deposition channels at Äspö. Sample canisters will be electrically heated to 90?C, the maximum temperature which will be acceptable when they contain waste.

“The aim is to have a repository replica working for 20 years when we do a final evaluation,” says Ollson.

Sweden rejected the reprocessing method of dealing with spent fuel a number of years ago and instead has concentrated on deep disposal. As a result it now has a disposal programme more advanced than almost any other country in the world. Sweden enjoys certain advantages over other countries in that it has suitable geology and a relatively low density population. But should deep disposal become the method by which the world deals with high level waste the expertise SKB and BNFL Engineering is developing is likely to be of great value in years to come.