Dismantling tasks at BR3: An example of optimization of the dose uptake using Visiplan


1. Introduction
The BR3 (for Belgian Reactor n° 3) was the first PWR installed in Europe.
In service since 1962, it was shutdown in 1987 after 25 years of operation and 11 campaigns. It is a quite small reactor with a net electrical power of 10.5 MWe or a thermal power of 40.9 MWth. At the end of its operating life, the European Commission, in the framework of its five-year plan of RTD selected in 1989, the BR3 as one of the four pilot projects on decommissioning of nuclear installations.

The main steps of the decommissioning programme up to now were :  
  • Full System Decontamination of the primary circuit in 1991 (the CORD® process),
  • Dismantling of the high active thermal shield with three different techniques, of the Vulcain internals and of the first set of Westinghouse internals (30 years decay time) by mechanical cutting up to 1996,
  • The dismantling of the auxiliary circuits started in 1995 and is still going on, with at this moment 98% of the secondary and tertiary loops, and more or less 60% of the loops in the controlled area.
  • The dismantling of the reactor pressure vessel ended in 2000 after its removal in one piece ("world première"),
  • The construction and start of the exploitation of thorough decontamination processes for dismantled pieces in the period 1996 to 1999 (the MEDOC installation).

    For the dismantling of the primary loop, we decided to reduce the dose rate by chemical decontamination, to dismantle the pieces and to sort them following their specific evacuation route, and finally to decontaminate the pieces with the objective to minimise the amount of materials disposed as radioactive waste.

  • 2. Dismantling works in the Primary Containment
    The primary containment contains the whole primary loop and the associated purification and safety loops.
    Before starting the dismantling of the large components (steam generator, pressuriser, primary pumps, large tanks...), it was necessary to remove all the piping with their associated auxiliaries (instrumentation, electric cables..). In total 20 loops were dismantled or adapted corresponding to a removal of about 40 tons of materials.

            Fig.1. : Overview of the area "onder the Operating Deck"
     

    Due to the complexity of the environment, a detailed organisation had to be set up comprising :  
  • the preparation of a general strategy with the help of an Alara planning software,
  • modifications to a number of loops which have to be maintained for further use and need,
  • dismantling of light accessible pieces to allow the installation of scaffoldings and a new transport rail,
  • dismantling of the remaining loops and of the primary piping.
    The general strategy developed considered the following aspects :
  • The detailed elaboration of the different tasks; for each task, an estimation of the manpower, the technical issues, the classical and radiological hazards.
  • The planning must fit into the general D&D planning. The evacuation of the active pieces from the dismantling of the RPV in a shielded container through the primary containment was the main conflicting task because it occurred in the middle of the D&D dismantling work.  
  • The optimisation of the D&D work to minimise the dose uptake. Due to the complex environment, we used the Visiplan software. This allowed us to estimate and compare the collective dose for different dismantling scenario and to optimise the work organisation. Comparison has also been made with a "manual" estimation.  
  • The size reduction of the dismantled pieces, the sorting and evacuation of the pieces according to their specific evacuation route is often the bottleneck in a dismantling work. We had to organise the work to avoid the accumulation of cut pieces at the outlet of the dismantling yard.  
  • A detailed Alara study was necessary and requested by the Health Physics department due to the foreseen length of the work (about 6 months), the number of operators involved (up to 4), and the dose rate (foreseen up to 11 mSv/h).

    A detailed survey was done revealing the presence of important hot spots (about 11 hot spots with dose rates from 0.1 to 11 mSv/h were identified), as shown in figure 3.
    The Alara study confirmed that the removal of hot spots was beneficial for a reduction of the collective dose. Nevertheless, it was not always technically feasible. We first removed the "easiest" hot spots at the ground level and then we removed progressively the hot spots starting at the + 7 m level down to the - 4.8 m level.
    The following figure gives an overview of the dose rate in the vicinity of the foreseen dismantling tasks.

            Fig.2. : Radiological mapping before the dismantling
  •  

    The dismantling operations were performed in a sequence taking into account the Alara aspects and the classical safety aspects.
    The sequence selected is given hereafter :
  •   Preparation phase
     
  • Installation of a confinement for the whole yard and of a ventilation system with pre- and absolute filters.
     
  • Installation of a cutting workshop at the ground level containment outlet; this zone was mainly equipped with a band saw machine.
     
  • Clear identification of the piping by markers to facilitate the materials follow up (facilitating the sorting).
     
  • Modification of some vital loops : compressed air, treated water, and neutron shield tank loop (cooling with chromated water).
  •   Dismantling of some hot spots at the ground level : coolant sampling, heat exchangers, drain collectors, ...
  •   Cutting and evacuation of piping around several equipments, electric cables and their supports, small tanks, evacuation of the rotors of the primary pumps, in order to allow the placement of scaffoldings and a transport rail with pneumatic cranes, evacuation of the main steam and feed water piping using an oxyacetylene burner.
  •   Systematic dismantling of the remaining.
  •   General water cleaning of the yard to allow the removal of the loose contamination and the further working without masks.
  •   Cutting of the primary pipes: in-situ with a lathe rotating cutter for the segmentation in pieces of several m long, size reduced to max 1 m long with a band saw in a cutting stand at the containment outlet.

    The collective dose, total workload and material dismantled for these operations are respectively : 11.95 man.mSv - 4124 man.hours - ~ 40 t.
    Up to now, for the whole dismantling works realised under the operating deck, the total collective dose amounts only to 49.85 man.mSv; besides what mentioned above, it includes :  
  • Dismantling Regenerative heat exchanger  
  • Removal of loops in the Siba room and the cutting of the metal sheet of the containment  
  • First opening in the Siba room and removal of piping there  
  • Removal of the contaminated asbestos  
  • Dismantling of Safety Injection loop and other piping  
  • Final cutting of the concrete for the new evacuation way  
  • Partial Removal of several loops  
  • Size reduction in cutting workshop  
  • First cutting of the primary pipes at the reactor bioshield outlet  
  • Dismantling component cooling, pressuriser surge line, primary pumps

    Without decontamination, the exposure would have been much higher. The installation of shielding's or the remote dismantling in such a confined environment would have been very difficult so that we can assume that the exposure would have been about 10 times higher i.e. about 500 man.mSv.

  • 3. The same dismantling tasks using Visiplan
    The analysis is built in three steps as follows :  
  • the dose measurement on site followed by the calculation of the main radiating sources (as shown on the figure here above)  
  • then the geometrical model of the working area is built (this model building was made using simple geometrical building blocks, like cylinders, sphere, cubes, etc)  
  • then, one build the scenarios, based on the operations to be carried out, and the potential alternatives to be analyzed  
  • the calculation is then performed, and allows to compare the various alternatives, and to refine some of the selected scenarios  
  • the analysis should then be completed by a follow up of the dose uptake and of the workload during and after the work has been carried out.
    Some of these steps are explained a little bit hereafter, for a small part of the operation, to show how the approach is made and how the optimization is carried out.

    The complete operation took into account the optimisation scenarios calculated with the same system as explained above. For instance, most of the hot spots were located and removed early in the dismantling tasks.
    In order to give almost virtual reality 3D screens to better visualize the work and results of the calculation, the calculation made use of an advanced version of Visiplan.


  • Fig.3. :  Modelisation of the working area in Visiplan using standard shapes Fig.4. :  3D visualisation of the work site, including indications of the local dose rate
       

    The global estimate of the operation duration and of the dose uptake is given in table 1. It takes already into account the optimization recommended by the analysis done for each of the main phases.
    It should be noted that the estimated hours concerns only the operational hours and do not cover the mobilisation, entrance and exit time, which represents commonly around 40% of the actual spent time, but which does not contribute to the dose uptake.
    Another aspect is the occupational factor. This factor is really difficult to estimate but is well known from radio protectionist.
    In fact when operators are busy in a certain area, they are not remaining all the time at the same place, and are often removed from the high radiation areas (this is still more the case since the use of portable electronic dosimeters with alarms). Currently, in the decommissioning project of BR3 we have deduced from former experience that the occupational factor is situated between 0.6 and 0.7 (i.e. the actual exposure to the radiation field amounts to 60 to 70% of the calculated exposure if the operator was still standing at the allocated position).
    This has been applied for calculating the values in the last column to give a corrected estimate of the dose uptake.
    Finally, the actual results of the dosimeters measurements have allowed us to add two columns to the table, taking into account the correction due to the entrance and exit time periods to the controlled area, reducing then the "effective time in operation".

    Table 1 : Overall dose estimate for the whole operation (excluding preparation and post-operational phases)
     
    Operation Rough estimate Visiplan estimate Actual With occupational factor
    man*mSv man*hours man*mSv man*hours man*mSv man*hours man*hours man*mSv
    SOD 1 5.90 498 5,00 400 3.33 3.74 959 575
    SOD 2 7.90 500 8,00 900 5.33 1.77 784 470
    SOD 3 3.62 422 1.50 250 1,00 3.05 1135 681
    Total 17.42 1420 14.50 1550 9.66 8.57 2878 1726