The BR3 reactor was the first PWR (pressurised water reactor) in Western Europe and it is also the first to be decommissioned. BR3 was a demonstration unit of an industrial power station and served as a test reactor for prototype nuclear fuels. It was also an education centre for the operating personnel of the nuclear power plants expected at that time in Belgium.
BR3 became critical for the first time on August 19, 1962 (critical = reactor in working in which an itself maintaining chain reaction takes place). On October 25 of the same year, BR3 was connected to the electricity grid. On June 30, 1987, the BR3 reactor was the first pressurised water reactor in Europe to be shut down.
Within the framework of the European five-year programme for research and technological development for the decommissioning of nuclear installations, BR3 was chosen, next to three other European installations, as a pilot project for the demonstration of the decommissioning of PWR plants. These projects aim to develop the necessary scientific and technical knowledge for decommissioning projects in real conditions.
According to plan, the plant will be completely decommissioned by the end of 2013.
The decommissioning strategy is described in a legally required decommissioning plan. This plan mentions, among other things, the amount of released materials and their radioactivity, the big steps of the demolition process, the waste treatment and a description of the final treatment of the dismantled material (ways of evacuation). Because the legal framework concerning decommissioning plans became effective in 2001, the decommissioning plan for the BR3 reactor was only introduced in 2002.
Firstly, the primary circuit was decontaminated. This reduced the radioactive radiation around the primary circuit with a factor 10 and allowed the operators to continue the decommissioning activities at a lower radiation level, causing lower dose loads. A second phase included the cutting up of the highly activated metal reactor components. For this purpose, SCK•CEN tested a number of cutting techniques (band saw, circular cutter, spark erosion and plasma torch). In 1991, a first demonstration was given of the cutting up of the highly active thermal reactor vessel shield, a 2.2 m high cylinder in stainless steel weighing 5 tons. Because water offers superior protection against radiation and, at the same time, allows a good and direct view on the work, these cutting up activities were carried out under water via remote control.
Furthermore, techniques were developed and applied to cut up, remove and store all other activated internal components like internal structures of the reactor vessel that support the reactor core and instrumentation. These are highly radioactive and have a complex geometry. This phase is particularly interesting because the geometry and the composition of the components correspond to those of large power reactors. In 1999, the 29 tons heavy reactor vessel was pulled out of the BR3 in order to be decommissioned in the same way. This event was a scoop for Europe. At present, the final tests are running for the cutting up of the last activated structure, the Neutron Shield Tank (NST). This is a double-walled cylindrical tank situated around the reactor vessel that served to stop the radioactive radiation as much as possible. Because this tank has a very complicated structure and it can only be decommissioned in situ, the HPWJC or High Pressure Water Jet Cutting technique was used, placing the cutting mechanism on a robot arm.
During the decommissioning of the highly activated components, pioneering work was done in the field of the development of robots and remotely controlled instruments, in order to keep the radiation exposure of the employees as low as possible.
However, the project was not limited to the decommissioning of the highly activated reactor components. In 1992, we also started the decommissioning of the different contaminated circuits. At present, all circuits have been decommissioned, except for the utilities that are necessary for the decommissioning like the circuits for compressed air, water, ventilation and discharge of radioactive effluents.
We not only carried out research on different cutting techniques and their practical use during the decommissioning activities. Together with the cutting up of the activated structures, different decontamination methods were examined.
This has led to the development and construction of the sandblast unit ZOE and the chemical decontamination unit MEDOC®, both for the treatment of contaminated metals. The sandblast unit ZOE uses the combined projection of grit and pressurised water for the decontamination of rusted or painted steel with an easy reachable surface. The chemical decontamination unit MEDOC® uses specific chemicals to attack the fixed contamination on (mainly) stainless steel with all types of geometry. The combined use of both techniques – depending of the material, geometry and type of contamination - allows for the unconditional clearance of most of the contaminated metals coming from the dismantling of the plant.
The decommissioning of nearly all circuits brings us to a next important phase of the decommissioning project, namely the decontamination and demolition of the concrete infrastructure. Because the concrete represents the largest amount of the existing materials, a good determination of the contaminated part is very important. For this purpose, special measurement methodologies were developed in order to minimise the amount of radioactive waste and limit the waste cost. An own liberation methodology for the concrete infrastructure was developed in collaboration with the Internal Service for Prevention and Protection at Work.
At the beginning of 2008, this new liberation methodology was introduced to the Belgian safety authorities in order to obtain an approval.
When decontaminating building structures, mechanical surface removal techniques are a primary consideration. Surface removal techniques are used when land use scenarios include reuse or to minimize waste volumes (reducing the amount of contaminated material for disposal by removing surface contamination of varying depth). Depending on the surface to be treated (ceiling, wall, floor,…), different techniques can be used. Examples of used mechanical techniques are scabbling, hammering, needle scaling, shaving, abrasive blasting etc.
Also other (non mechanical) decontamination techniques have been tested and evaluated intensively throughout the BR3 dismantling and decontamination project, mostly resulting in lower production rates, higher production costs and/or high volume of secondary waste produced (compared to mechanical techniques). Examples are sponge jet, laser ablation etc.
Removing contaminated tubes embedded in walls, floors etc. has proved to be a very time and labour intensive activity. Used techniques are hammering, diamond wire sawing, core drilling etc.
Close to and in the proximity of the reactor vessel, during active lifetime of the reactor, the surrounding concrete infrastructure has been in the line of sight of neutrones, resulting in activated concrete and reinforcement bars. The activated part can take up a significant part of wall thickness. Building stability therefor becomes an important issue as well. Demolition techniques to be used include hammering, circular sawing, diamond wire sawing etc.
Radiological characterization & measurements
SCK•CEN has built up a broad experience regarding the radiological characterziation in all stages of the decommissioning process, covering measurements in view of:
- the initial inventarization of a nuclear facility during the pre-decommissioning phase;
- the mapping of contamination & activation distribution in buildings and soils;
- the control of all the material flows arising from dismantling activities including the unconditional release of materials in accordance with the Belgian regulation and the radioactive waste acceptance in accordance with the criteria defined by NIRAS/ONDRAF;
- the clearance of buildings and sites.
During its operation, the BR3 reactor was loaded eleven times. The first and second load consisted of 32 elements, the next loads all contained 73 elements. The fuel of the first and the second load was reprocessed in the Eurochemic factory, active at that moment. In 2002, the remaining fuel that was stored in the deactivation dock at BR3 was transported to Belgoprocess in Castor BR3 containers. The Castor BR3 is a container in which the fuel, surrounded by a dry inert gas (He), can be stored for at least 50 years. Afterwards, the fuel will be repacked to enable underground disposal, analogous to the commercial fuel of the Belgian nuclear power plants.
During the decommissioning of the BR3 reactor, extra attention was paid to the radiation protection of the people who sometimes had to work in complex nuclear environments. It is an essential part of the safety culture. According to the ALARA principle, i.e. As Low As Reasonably Achievable, dose control is very important. It is therefore recommended that the possible doses can be predicted and analysed in advance. The geometry of the workplace, the type of existing materials and the radioactive sources are often variable data. In order to make the best predictions possible, SCK•CEN developed the VISIPLAN 3D ALARA planning tool, which has been spread in a lot of European countries and was also used during the decommissioning of BR3.
All data on radiation doses, waste produced and decommissioning costs have been accurately kept up to date in a database. Later on, this database will serve as a basis to predict the economic and radiological cost of new decommissioning projects.
Contact: Dadoumont Jérôme