Decontamination & decommissioning | France
Breaking up Brennilis1 August 2012
Remote technology will be used to dismantle Brennilis, a unique experimental heavy water reactor built in France more than 50 years ago. It is one of the first heavy-water reactors to be decommissioned. By Thierry André and Werner Botzem
The nuclear power plant located in Monts d’Arrée, Brennilis, northwestern France, is a unique 75 MW (250 MWt) gas-cooled heavy water reactor (GCHWR) that was developed during the 1950s and 1960s. The reactor, also known as EL 4, operated for nine years between 1972 and 1981, before being permanently shut down in July 1985. The reactor was defuelled and dewatered by 1992.
Partial decommissioning operations were carried out from 1997 to mid-2007 (plugging of circuits, decommissioning of certain heavy water and carbon dioxide circuits and electromechanical components, demolition of non-nuclear buildings, and so on.) However, the process was interrupted when a 2006 decommissioning licence was cancelled by the French Council of State in June 2007. It was cancelled because an environmental impact study of the project was not published before the licence was granted, according to owner and operator EDF. However, a new decree authorizing decommissioning of the reactor block is expected in 2013.
Work is now underway to prepare for dismantling of the plant by the end of this decade. EDF commissioned a consortium of Onet Technologies Grands Projets (France) and Nukem Technologies (Germany) with this work in 2009. In addition to complete dismantling, the scope of the contract also includes requirements to meet safety, environmental, waste management and radiological protection regulations.
As with most first-generation nuclear power plants, requirements for dismantling were not taken into account during the construction process. Consequently, the reactor pressure vessel at Brennilis is equipped with a complex piping system on all sides, which makes it difficult to freely access the core and to dismantle the reactor. In addition, the dewatered reactor is now completely dry. Even after 30 years, the high local dose rate in the area of the reactor pressure vessel, a maximum of 70 Sv/hr (7x106 mrem/hr), makes manual work impossible, so remote dismantling techniques have to be used. The large amount of repetition expected during the dismantling will put high demands on the capacity and reliability of remote devices, so they need to be tested in simulated conditions prior to the project.
The Brennilis dismantling project has five main milestones, ranging from initial planning to project execution. Work to consolidate the dismantling scenarios, as well as the design and qualification of initial equipment (zircaloy pipes and sampling processes) was carried out from 2009 to 2011. Until dismantling begins in 2014, qualification of the remaining equipment, site preparation and commissioning of tools will take place. Dismantling works and the production of waste containers is expected over a five-year period between 2014 and 2019.
The ultimate aim of the project is to dismantle the large elements within the Brennilis reactor block that have been activated and contaminated during operation of the plant.
Special attention has been placed on the minimization of radioactive waste, and waste packages must meet the terms of acceptance of the French national radioactive waste management agency, Andra. The project must also comply with French nuclear regulation as well as environmental, industrial safety and radiological protection requirements.
Organizational and technical measures have been taken to reduce the collective dose rate to 900 mSv for workers over the whole five years of site operation, and to minimize the risks of staff exposure as much as possible.
Project responsibilities are split between the consortium partners. As the leader, Onet Technologies is responsible for the general dismantling scenario and coordinating the project. It will also carry out removal of the reactor vessel internals and dismantling of fuel channels, zircaloy pipes, guide tubes and control rods. Nukem’s role will be to dismantle the reactor pressure vessel, lateral axial shielding, and much of the primary circuit using mechanical and plasma cutting tools. Each company is responsible for the design, manufacturing, qualification and operation of the process equipment required for its own systems.
The central element of the Brennilis plant is the cylindrical reactor pressure vessel (diameter 4.8 m; length 4.3 m), originally filled with heavy water, but now dry. Pressure tubes (216 total) made from zircaloy run through the interior of the RPV, and fuel assemblies inside the pressure tubes would transfer nuclear heat to carbon dioxide gas flowing around them. Each end of the 216 fuel element channels is connected to a pipe that directs the heat transfer gas to a header mounted in the upper part of the reactor block. Here, the gas is collected and directed to the heat exchangers. Cooled gas is then directed to a cold header and fed back into the reactor. Control rods (approximately 30) are introduced vertically into the reactor.
A maze of pipes surrounds the Brennilis RPV on all sides, which makes it difficult to freely access the core area of the reactor block. Axial and lateral neutron shields are also situated in close proximity to the reactor, as well as the biological shield, which protects from ionizing radiation originating from the RPV.
For dismantling of internal parts inside the reactor vessel, remote-controlled cutting tools are used.
The horizontal fuel channel is composed of two concentric zircaloy pipes. The inner one is for guiding the fuel elements and the outside one is a thicker pressure tube. There is a thermal isolating layer between the concentric pipes. For both of these pipes, novel cold cutting techniques using thumbwheels have been chosen to reduce the number of particles released.
The cutting operations are performed individually on each of the 216 channels of the reactor, with each tube taking one eight-hour shift to cut completely. The first stage involves using a swaging head to compress the guide tube against the pressure pipe, an operation that is carried out in succession seven times per channel. For the second stage, the cutting head is positioned on the imprints left during the first stage, and seven cuts will be made. The resulting six 700 mm pipe sections will drop down into the bottom of the vessel for removal at a later stage.
Due to the pyrophoric properties of the material, the zircaloy particles must not be heated above a certain temperature (around 200°C for ‘fines’ or small particles), to prevent ignition. Onet has therefore incorporated several safety measures into the cutting system to prevent the zircaloy temperature exceeding 150°C. As a first step, the system stops the cutting process. Then, if the temperature continues to increase, an inert gas is injected into the cutting area, and subsequently into the whole reactor vessel. Preliminary results from ongoing qualification tests on cold zircaloy pipes suggest that the local temperature remains significantly lower that the first security threshold.
The cutting process is carried out under visual controls and a vacuum system is used to collect zircaloy particles during the cutting operation.
Prior to the horizontal cutting operations, the 64 vertical guide tubes and 30 control rods, which have settled between the horizontal channels, must be cut and removed to prevent interference with horizontal tube cutting operations.
Mechanical orbital cutting machines will be used for the cutting operations and the tubes will be removed vertically from the top of the reactor block using a flask-shielded hood device. This flask will be installed on the top of the tubular reactor block and can use grippers and cutting tools.
Operating the cutting tools will be standardized industrial robots known from markets such as clean room technology or automotive manufacturing. Upgraded industrial robots, rather than special manipulators, have been chosen because of their ubiquity in industry. This brings with it high levels of availability, low hardware costs, simple maintenance operations and the ability to carry out rapid motion sequences of work. A combination of individual joystick control and basic installed programs permits both highly-flexible work and high handling speeds. At the same time, an integrated collision prevention system ensures that human error in handling cannot damage the robots. Moreover, the robots are installed on a rail-mounted, motorized drive unit, which allows for a correspondingly high repeatability of the driving movements.
The robots are ‘nuclearized’ for their work in the controlled-access area, which involves hardening of electronics, provision of back-up in case of mechanical failure, and facilitation of decontamination of components.
To access to the interior of the reactor, a 5m-high working platform will be attached to the reactor block in the northern part of the plant. A standing airtight steel plate caisson on the platform forms a walk-in room between the reactor building and the reactor block. The function of the caisson is to arrange an airlock between the reactor block room (where radiation levels, which range from 2-100mSv/hr, and higher, prevent human access) and the reactor hall (where radiation levels, of less than 25 µSv/hr, make human access possible, but require it to be controlled).
In addition to the robot system itself, it is also necessary to develop and build a corresponding infrastructure. The caisson is at the heart of this infrastructure. This steel structure is some 6 m wide, around 24 m long, stands on supports approximately 5.2 m high and is installed inside the reactor building between the existing building structures. The caisson is the base for the robot system, includes parking and service equipment and automatic tool change systems for robots, as well as the peripheral technologies such as exhaust equipment and ventilation. In addition to this, the caisson provides access to the reactor chamber via suitable access openings, cut through what is sometimes a metre-thick reinforced concrete construction and then closed again with bulkhead doors. The caisson also serves as a sort of two-door interlock through which the decommissioned reactor structures are transported to a sorting cell approximately 16.5 metres below.
Coolant piping/axial shield
At the beginning of the remote dismantling process, the reactor corridors will be filled with coolant piping with a pipe diameter of 114-133 mm and a wall thickness of 5-8 mm that blocks access to the reactor pressure vessel lying behind the corridors.
Robots will use several orbital machining tools, including inner pipe cutters, to cut the CO2 piping, gradually working their way in the direction of the reactor on purpose-built rails. The rails will be extended at regular intervals as work progresses. Once all 216 pipes (average length 13m) are cut, the reactor and the axial shield will be accessible.
The shields, which consist of elements built of water tanks, will be thermally cut to create additional room for the dismantling of the reactor.
The reactor will be dismantled from the inside to the outside. The first step involves creating two 3m x 1.2m openings in the reactor wall. Sawing and drilling tools will be used for cutting the concrete, as well as wire-cutting techniques.
The temperature of the cutting zone is monitored by a temperature sensor, which would automatically switch off the circular saw if a certain temperature limit is exceeded, to prevent the risk of the zircaloy pressure tubes catching fire.
The pressure tubes segmented in an earlier stage are collected and transported to a conditioning cell. After removal of all elements containing zircaloy, thermal (plasma) cutting techniques can again be applied to cut the reactor and the shield therein from the inside to the outside. The resulting waste segments will also be transported to the conditioning cell to be processed for final disposal.
The conditioning cell deals with both short and long-lived low-level waste and long-lived intermediate-level waste. Sorting, characterization, container-filling operations are all carried out inside the cell. To the left of the conditioning cell there is a storage area for activated waste, stored in concrete casks.
Solely mechanical techniques (no chemical process) are envisaged for dismantling the activated reactor components at Brennilis. The main requirements for the project are for cold cutting techniques to avoid the risk of heating and burning, and for remote-controlled devices due to the high radiation levels of the source terms.
Orbital machining techniques will be used for cutting the guiding tubes and rods while mechanical sawing techniques will be used for the CO2 (coolant) pipes. A novel application of thumbwheel cutting technology has been specifically developed for cutting the zircaloy pipes. This experience could be used for cutting similar kinds of channels in other reactors. Finally the use of robots in such an environment is also new.
The Brennilis NPP is an unique industrial example of a French heavy water reactor. The specific dismantling operations, which will be carried out under dry conditions, cannot be reproduced for decommissioning any of France’s pressurized water reactors (dismantled underwater). However the project still provides useful nuclear safety and radiation protection experience.
Thierry André, Onet Technologies, Marseille, France and Werner Botzem, Nukem Technologies, Alzenaü, Germany
This article is based on a paper presented at the ASME 14th International Conference on Environmental Remediation and Radioactive Waste Management, September 25-29, 2011, Reims, France.
This article was first published in the July 2012 issue of Nuclear Engineering International.Related ArticlesExcavating Chooz A
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