Decontamination & decommissioning
Excavating Chooz A6 November 2010
Modern nuclear reactors are designed with decommissioning in mind. Earlier reactors were not, so a current industry challenge is to demonstrate that existing reactors can be decommissioned and decontaminated in a timely, cost effective and efficient way. Just such a test case is beginning in France, where, earlier this year, French utility EDF awarded Westinghouse a contract for decommissioning of the first of its fleet of 59 commercial PWRs. By Penny Hitchin
French utility EDF operates more nuclear reactors than any other utility worldwide. Pressurized water reactor technology (PWR) forms the mainstay of EDF’s French fleet of 58 operational reactors spread across 19 sites. The utility’s prototype PWR was a 310MW reactor at Chooz A which supplied electricity to both France and Belgium from 1967 to 1991. Chooz A is also to be the French prototype PWR decommissioning site, and EDF has set an ambitious target of completion by 2016.
EDF’s strategy now is to aim for ‘immediate’ decommissioning of reactors which have ceased generating rather than deferring the clean up work. Work is now under way to totally dismantle the nine oldest French nuclear reactors which started generating in the 1960s and 1970s. These include the heavy water reactor at Brennilis, the fast breeder reactor at Cresy-Malville, and six natural uranium gas-cooled reactors (similar to the UK’s Magnox reactors); three at Chinon, two at St Laurent and one at Bugey. Chooz A is the first commercial PWR to be decommissioned in France and EDF is looking for the project to provide experience and feedback for the decommissioning of its other 58 units in due course as they come to the end of their generating lives. It hopes to gain expertise in terms of schedule, cost, worker exposure, safety and waste management.
First for Europe
Chooz A was the first commercial PWR built in Europe. It was commissioned by the Franco-Belgian Nuclear Energy Company of the Ardennes (SENA), a joint venture created in 1960 by EDF and a group of Belgian utilities, to construct and operate a 240MW PWR (subsequently increased to 305MW) to provide electricity to both France and Belgium. At that time, France was operating graphite moderated gas cooled reactors, but EDF, then a state-owned industrial and commercial entity, was interested in trialling the PWR technology that the Belgians were keen to develop. Belgium was at that time building its 11MW BR3 demonstration PWR at Mol. (BR3, the first PWR in Western Europe was connected to the grid in 1962 and shut down five years later. It has subsequently been decommissioned.)
Chooz is 10km from the Belgian border and its proximity to high-voltage transmission lines meant it could provide both French and Belgian companies with power. The reactor was based on the design for the 185-megawatt synchronized PWR Yankee Rowe plant in Massachusetts, which operated from 1961 until 1992, and has now been decommissioned.
The location of the reactor at Chooz A is unique. The plant was installed deep inside the bedrock of a hill in the Ardennes in two caverns excavated specifically to accommodate the reactor and auxiliary installations. Buildings at the top of the hill housed the emergency core cooling system and the ventilation stack. The access galleries and the caverns inside the hill are kept at a lower pressure than the world outside, ensuring that no nuclear material escapes. A pressure lock separates the subterranean system and entry is through a pressurized antechamber. A steep, wide access gallery containing pipework connects those buildings to the reactor cavern and the nuclear auxiliary cavern 200 metres below, 60 metres from the side of the hill. A drainage system collects water percolating through the hillside in case of potential contamination.
Reinforced concrete is the standard method of containing a PWR but at Chooz A the designers decided to house the reactor underground to utilize the containment characteristics and pressure resistance of the granite-like blue schist rock.
The nuclear steam supply system was designed by Westinghouse and the equipment built in France and Belgium. The reactor cooling system was a four loop circuit including four steam generators, four primary pumps and one isolation valve.
Construction at Chooz A started in 1961 and generation commenced in 1967.
Chooz A was also the site of the first French experiments in burning mixed-oxide fuel. Four MOX elements were loaded in 1974, two in 1979, and eight in November 1987. In its 24 years of generation, Chooz A produced 38TWh net of electricity (40.3TWh gross).
More conventional techniques have since been used by EDF to build two other reactors on the surface at Chooz. Two modern 1450MW PWRs Chooz B1 and Chooz B2 started operating in 2000 and 2002, respectively.
Chooz A was shut down in 1991, and the liability for decommissioning transferred from SENA to EDF, which at that time had adopted a deferred dismantling strategy.
By December 1995 all the fuel had been removed and dispatched to the then Cogema (now Areva) fuel-reprocessing centre in La Hague. By 1999 the circuits had been drained, industrial waste removed and the turbine hall equipment dismantled. EDF estimates that defuelling and drainage of circuits eliminated 99.9% of the plant’s radioactivity.
By the turn of this century EDF’s decommissioning strategy had evolved from deferred to immediate decommissioning and plans for the plant were accelerated, reducing the safe enclosure period to only a few years. EDF created CIDEN (The Engineering Centre for Dismantling and the Environment) in 2001 to focus on the back-end of power operations and environmental safety. CIDEN provides engineering consultancy for decommissioning operations, waste management advice, and advice on environmental impact and decontamination. CIDEN employs 540 people, over two thirds at the Villeurbanne head office and 200 working across six nuclear sites. EDF cites the advantages of immediate decommissioning as preserving jobs, technical knowledge, and avoiding passing the problem on to subsequent generations. Immediate decommissioning is now recommended by the French nuclear regulatory authority (ASN dismantling policy, February 2008) and is the preferred option of the International Atomic Energy Agency (IAEA).
At Chooz A, dismantling, clean-up and demolition of nuclear buildings on the hillside, refurbishment of changing rooms, dismantling of service tunnel facilities (including the tunnel linking the cavern bottom to the facilities on the hill) all took place before 2008.
A public enquiry was carried out in 2006, and then in September 2007 a decommissioning licence was published authorizing total dismantling of the plant.
During the next six years of decommissioning at Chooz A, all the equipment within the caves will be dismantled except the apparatus needed to collect, monitor and release the drained water from within the hill.
The major activities that will be implemented include:
- Dismantling of the electromechanical equipment in the auxiliary cave and the reactor cave. All components in the nuclear auxiliary cavern will be dismantled, with the exception of those needed for monitoring the radioactivity of infiltrated water percolating through the hillside.
- Chemical decontamination of the steam generators in order to decategorise from low level waste to very low level waste (VLLW).
- Steam generators to be removed in one piece following decontamination, and stored in one piece in a VLLW repository (operated by ANDRA since 2003).
- Reactor pressure vessel (RPV) and reactor vessel internals (RVI) to be dismantled underwater.
At the end of operations, the plant will remain under surveillance for several years. Water from inside the caverns will be collected and tested until the radioactivity levels (caused notably by the presence of tritium) drop sufficiently to enable discharge to take place without tests. Once these levels are reached, the final components remaining in the nuclear auxiliary cavern and the waste treatment facility will be dismantled. Buildings forming part of the outside platform will then be decontaminated and demolished, and the site will be completely regenerated.
Decommissioning operations are expected to be completed by 2020-2025. Once the plant has been completely dismantled, underground structures will be emptied of all electro-mechanical components, and their civil engineering structures will be decontaminated.
The caverns will be partially filled in to avoid the risk of hill subsidence, while the buildings located on the site’s outside platform will be demolished. The plant will be downgraded and the site will remain the property of EDF.
Arguably the main technical issue for the accelerated decommissioning is the segmentation of the RPV and reactor vessel internals.
The scheme for segmentation is dependent on, amongst other things, accurate characterization of the radioactive elements, so that the dismantled items can be packed into containers whose radiochemistry profiles are compliant with regulatory specifications.
EDF’s decommissioning arm CIDEN had set out an initial dismantling scenario for RPV and RVI dismantling in 2004 in the framework of the application for the decommissioning licence. This scenario was described in the safety report and had been reviewed by the French nuclear regulatory body. This dismantling scenario was also reviewed by EWN, the German company responsible for decommissioning Greifswald and Rheinsberg nuclear power plants.
In 2006 EDF arranged a seminar in the United States to draw on the experience of US decommissioning experts and specialists in the characterization of RPVs. During the seminar, experts presented different segmentation approaches they had used to dismantle reactor vessels and internals in the USA. These ranged from no segmentation at Trojan, through a minimal segmentation approach at Maine Yankee, to the full segmentation approach at SONGS 1 reactors. However, differences between waste definitions, waste packaging and acceptance regulations in the USA and France meant that some of the US experience could not be directly applied to the proposed RPV and RVI dismantling at Chooz A.
The collaborations with German and American decommissioning experts helped EDF draft the technical specifications for the Chooz A RPV and RVI dismantling contract. EDF decided that while it would set out the dismantling scenario, it would leave bidders to specify the selection of the segmentation process as part of the tender.
The contract would be awarded on the basis of the total cost for RPV and RVI dismantling including the price of the contract and the waste disposal cost. Waste disposal costs were also given to the bidders to help them select the most appropriate segmentation process.
Westinghouse wins contract
In April 2010, CIDEN awarded Westinghouse Electric Company the contract to dismantle the Chooz A rector vessel and its internals. Westinghouse will be the lead in a consortium with Nuvia France. The project is expected to take six and a half years to complete. Engineering studies will take place from January 2010 to June 2011, qualification from June 2011 until January 2013 and site work from the beginning of 2013 until June 2016, according to Westinghouse’s approximate schedule.
The scope of the contract includes overall project management, RV and RVI segmentation, reactor nozzle cutting, dismantling of the RV thermal insulation, performing ALARA (As Low As Reasonably Achievable) forecast to ensure acceptable doses for the personnel, and providing complementary water filtration system to maintain water clarity during the segmentation work.
The first step will be to devise a segmentation plan that will determine the step-by-step approach for cutting and packaging. All reports will need verification and approval by EDF and ASN in order to authorize the site work.
Joseph Bocau, director of Westinghouse global decommissioning & dismantling, is leading the project. He explained to NEI that Westinghouse also has experience with large modernisation programmes requiring reactor internals replacements and dismantling of retired components at Forsmark 1, 2 & 3 and Oskarshamn 1, 2 & 3 in Sweden, and Olkiluoto 1 & 2 in Finland.
The dismantling of the RV and RVI will take place in the water-filled reactor cavity. EDF will provide a brand new water filtration system to ensure it will be operational during the site activities and later during temporary underwater storage of the long-lived low and intermediate level waste (LILW).
Bocau said:“Our preliminary segmentation and packaging plan determines the strategy for separating the highly activated components from the less activated material, so that they can be disposed of in the most cost-effective manner. Highly activated components cannot be typically shipped off-site, so they must be packaged such that they can be dry stored with the spent fuel in an independent spent fuel storage installation (ISFSI). Less activated components can be shipped to an off-site LILW disposal facility.”
There is currently no intermediate storage facility for LILW. EDF plans to construct an interim storage facility on another site but it will not be available until after the segmentation work is completed. In the meantime, waste will be temporarily stored in a pool located in another cave, which has the disadvantage of lots of waste transportation through an access gallery.
Westinghouse will use mechanical cutting methods rather than abrasive and thermal methods. The company says that it has found that the reduction in cutting speed is more than made up for by savings in cost and schedule from eliminating the need for special waste handling equipment and processes required to handle the large volumes of secondary waste created by abrasive and thermal techniques.
Bocau is relaxed about the constraints imposed by working in caves, saying that the working conditions compare to a classical containment, except that access to the reactor building takes a bit longer. He added, “The layout is of course different and that will require some verifications and reinforcement of the civil structures for installing some heavy equipment like hot cells for waste characterization and container filling.”
Reflecting on the reactor design, he says, “To my knowledge this was a unique design for a commercial reactor. However, it provided some nice features during plant operation, like passive safeguards systems. The refuelling water storage tanks were indeed located at the top of the hill at about 200m high providing a pressure drop of 20 bars with a very reliable emergency core cooling and spray system supplied by gravity. Those passive system features can now be found in the Westinghouse AP1000 design!”
Bocau worries that the condition of the outer insulation of the reactor vessel is difficult to predict after so many years of irradiation. If it has disintegrated, component rock wool particles could disperse, causing some visibility problems in the pool.
Asked about technical working constraints on the project, Bocau says that compared to other reactor dismantling projects, Westinghouse will have the responsibility to bring all the equipment and some of the services required for the job. This includes providing the power supply, compressed air, water, ventilation, fire detection, centralized control in control room, lighting, health physics equipment, waste characterization monitors and scaffolding.
The scope for consortium partners Nuvia France covers waste management, dismantling of concrete structures, dismantling of auxiliary equipments, waste container handling and conditioning, waste characterization and sorting in hot cells, and site ALARA.
Although the subterranean PWR at Chooz A is unique, its dismantling and decommissioning is being carefully studied by CIDEN, which is looking to apply the benefits of this experience to its fleet of PWRs when their working lives are completed. ¦
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|Chooz A decommissioning schedule:|
2007â€“2008: Planning work (ventilation, waste removal, and other work).