Dismantling of Hot Cell n°41 at SCK·CEN by using the Alara Visiplan Planning Tool

1. Introduction
The BR3 (for Belgian Reactor n° 3) was the first PWR installed in Europe.
The Laboratories of High and Medium Activities at SCK·CEN allow to manipulate, in several series of hot cells, high radioactive sources in the framework of programmes for the follow up of reactor vessel steels or concerning fuel.

2. Cell description
Cell 41 was already several years out of operation when, in 1999, the decision was taken to dismantle its content in order to fit it out for another programme for the re-instrumentation of irradiated fuel.
From an historical point of view, since the sixties this cell was used successively for the reprocessing of ceramic fuel, the manipulation of Pu fuel and, finally, for the reprocessing of irradiated fuel highly enriched with uranium, by using chemical extraction techniques. The last manipulations in this cell took place in October 1990.

During the time the cell was used, several interventions were done in order to modify the installations and to maintain them. In 1999, it was thus several years that the cell was in the same condition. It comprised the following elements : sections in stainless steel supporting a worktable and a vertical wall, several tanks, lead protection walls, a robot arm inside the cell that was out of order, an overhead crane at the ceiling and various waste from the last interventions.

3. Alara study - Use of Visiplan
3.1.  Measurement campaign

Before starting the practical dismantling of the cell, a thorough Alara study was necessary. The first step was to carry out a radiation prospection campaign inside the cell.
TLD dosimeters (thermoluminescent) were used to obtain the dose rate values. Smear tests, taken on several representative places in the hot cell, were measured afterwards to have an estimation of the transferable contamination.
Various smear tests were also measured by gamma spectrometry in order to have an estimation of the isotopic composition of the contaminants.
This first campaign gave the following results :
  • the vertical wall of the work surface presented a dose rate of 15 mSv/h  
  • the dose rate on the work surface varied from 1 to 3 mSv/h with hot spots of 4 and 20 mSv/h  
  • the bottom of the α box of the cell gave an exposition of 3 to 6 mSv/h  
  • the cylindrical shaped tanks presented a dose rate of 1 mSv/h  
  • the isotopes the most present, resulting from the gammaspectroscopy measurements were 137Cs (62%), 241Am (16%), 106Ru (15%) and also 60Co, 239Pu, 144Ce ...

    3.2.  Simulation with Visiplan

    Based on the results mentioned above, a thorough Alara study was carried out with the Visiplan software, developed at SCK·CEN. This software runs on a personal computer. Visiplan allows to plan a work in a 3D virtual environment, based on information about the geometry, the nature of the material and the radiation field.
    The Visiplan methodology is characterized by different steps :  
  • gathering the information and the construction of the 3D model presenting the work location,  
  • the general analysis step,  
  • the detailed analysis step following the work planning and  
  • the follow up step.
    In the first step the model of cell 41 was built based on the plans of the cell and the nature of the different materials present. When the sources and their activity are known, a calculation of the radiation field can be done immediately. In our case, a supplementary step had to be done. Based on the nature of the isotopes determined by gamma spectrometry and the dose rate measurements inside the cell, an algorithm of Visiplan allowed to calculate the average activity of the different elements.
    In the general analysis step, the calculated field is studied and suggestions concerning the possibilities to reduce the dose are tested (i.e. use of shielding) by recalculating the values of the radiation field for each option. In the case of cell 41, different options for the shielding of the hot sports were studied.

    Fig.1. :  Model Visiplan cell 41 Fig.2. :  Estimation of dose rates by means of a grid
       

    In the detailed analaysis step, a dose calculation is carried out following a trajectory that is constituted of a series of tasks, each characterized by a position, a task description and a work duration.
    For cell 41, task examples are the installation of the material in the cell, the cutting of each part of the vertical wall, the cutting of the different parts of the work surface, the cutting of tanks... The software takes into account the activity having disappeared as result of the evacuation of cut pieces.

    Fig.3. :  VRML conversion of one of the trajectories Fig.4. :  Distribution of the collective dose between the different trajectories
     

    Based on different calculated trajectories, a scenario can be set up allowing to have an overview of the different operations and the dose levels for the operators. Based on the information available at the time of the calculations, a collective dose of 21 man*mSv was foreseen (taking into account the uncertainties defined in the system, between 15 and 28 man*mSv).

    3.3.  Procedures

    Before starting the intervention, different procedures have to be written. They concern, among others, the installation of an intervention zone in front of the cell, the waste evacution ways, cutting tools, equipment to protect the personnel ... The intervention zone in front of the cell is built up of different parts : a work area and a "waste" lock composed of wooden panels covered by plastic, and a "personnel" lock.
    As the risk for contamination is high (also α), the personnel had to wear ventilated suits in overpressure and an independent mask with a P3 type filter. In order to control the internal contamination risk of the different operators, a nose-blow of each person was taken at the end of the intervention. One person underwent also a "Whole Body Counter". It was also foreseen that, in case of an incident (i.e. tearing of the ventilated suit), the concerned person would leave immediately the cell for control by nose-blow and direct measurement at the "Whole Body Counter".
    Each operator had a TLD dosimeter, a wrist dosimeter of the TLD type, an electronic Siemens EPD dosimeter and an electronic dosimeter of the Xetec type that could be read from a distance. In this way the radiation protection officer, outside the cell, was able to have a direct follow-up of the dose uptake.
    Before starting the operations, the operators disposed of dosimetric cards in 3D. This also allowed to improve the communication and to highlight the Alara problems.

    4. Dismantling
    4.1.  Difficulties

    Various difficulties were encountered mainly at the beginning of the cutting operations.
    Before opening the cell, there were many uncertainties on the nature of the materials present inside. Indeed, as the cell was out of operation during several years, the available information was scarce. This had an influence on the choice of the dismantling techniques. Originally, a circular saw with two contrarotative saw blades was chosen. The goal was to cut large surfaces first inside the cell, then to take the parts outside to cut in smaller pieces. However, at the moment of the first cuts, it appeared that the structures to be dismantled were much more resistant than expected. The cutting technique had to be revised and a disk grinder was selected. This technique, allowing to cut larger profiles, presented the disadvantage of producing sparks, so the fire risk in the work area increased. The cutting of the parts in smaller pieces had therefore to be done inside the cell where the dose rate was more important.

    4.2.  Dismantling phases

    In general, there are three large phases in the dismantling of cell 41 (between February 7 and October 31, 2000) :
    1.  cutting of the worktable, the profiles, the tanks, the lead castle
    2.  decontamination of the cell, dismantling of the robot arm, the bridge and the rest of the lead wall
    3.  different phases of decontamination and fixation of the contamination by several coats of paint.

    The collective dose resulting from the different operations amounted to 26,4 man*mSv (for 19 persons). This dose is distributed as follows :
      - 19,4 man-mSv for the first phase,
      - 5,3 man-mSv for the second and
      - 1,7 man-mSv for the third one.

    5. Conclusion
    The content of cell 41 of the installation LHMA from the Belgian Nuclear Research Centre SCK·CEN has been dismantled with success on a period of one and half year.
    The VISIPLAN software, developed at the SCK·CEN has allowed to carry out an ALARA optimization of the doses and to improve the communication towards the operators in charge with the dismantling. This software allows to simulate a 3D working environment in presence of several sources.
    A collective dose of 26,4 man*mSv, for an evaluated one of 21 ± 7 man*mSv, was received from the beginning of the dismantling up to the last stage consisting in the fixation of the individual contamination.
  •