Dose Assessment and ALARA calculations with the VISIPLAN 3D ALARA planning tool


1. Introduction
The assessment of gamma radiation shielding is an important task in and near nuclear facilities in order to protect the personnel and the members of the public from exposure to radiation.

The assessment must be fast and reliable. In order to perform this task a flexible calculational tool is needed that makes it possible to quickly estimate the exposure to gamma radiation for a variety of sources, shielding geometries and shielding materials. A valuable calculational tool that greatly fulfils these needs is the program VISIPLAN 3D ALARA planning tool.

In the following, it will be shown how the programme can be used for tasks such as shield design, personnel dose assessment and personnel dose planning during operations and maintenance.

Firstly, we refer the reader to the basics in physics for notions on gamma radiation and gamma radiation shielding calculations used in the programme.
Secondly we introduce the general methodology in treating ALARA dose assessments with the use of VISIPLAN.
Thirdly a detailed description is given of the geometry definitions used in VISIPLAN. This is followed by a description of the Take geometry and source definition.
The last section deals with the creation of scenarios for an integrated dose assessment of a work description.

This document together with the VISIPLAN user's guide provides the information for an effective use of the VISIPLAN 3D ALARA planning tool.

2. VISIPLAN Methodology
ALARA dose assessment for work planning in complex nuclear installations is difficult. The aspects of geometry, source distribution and shield geometry play an important role in the dose prognoses. Also work organization, type and work duration are non-negligible aspects in ALARA considerations.

The VISIPLAN 3D-ALARA planning tool streamlines this information and keeps track of the different assumptions that were made during the ALARA assessment. This PC-based tool calculates a detailed dose account for different work scenarii defined by the ALARA analyst, taking into account worker position, work duration and subsequent geometry and source distribution changes.
In the following we describe the VISIPLAN 3D ALARA planning tool strategy in dealing with an ALARA assessment.
The VISIPLAN methodology consists of four steps
  • Model Building
  • General analysis
  • Detailed planning
  • Follow up

            Fig.1. : The four main stages in the VISIPLAN methodology
  •  


    2.1.   Model building stage

    The first step in the analysis is the characterization of the site or work area. This geometrical and materials information can be derived from technical drawings or survey techniques. Site experts are also a valuable source of information. The radiological input can be determined from dose rate measurements, gamma spectroscopy, site history, technical data, ...

            Fig.2. : The composition of a Take definition in VISIPLAN
     
     
  • A set of model building tools is provided to translate the geometrical model and associated materials information of the work area into a VISIPLAN model by using primitive volumes such as boxes, spheres, cylinders and tubes.  
  • The material information is entered in the model as standard materials such as concrete, water, iron ... and is attributed to the different volumes. The density of these materials can be changed according to the model needs. Mixtures of materials can also be attributed to a volume in order to simulate the attenuation by complex internal structures.
  • Source position, source strength, source geometry and source composition can be entered directly in the model. The source spectrum is selected from an isotope list or can be defined by the user.

            Fig.3. : Screen interfaces for geometry and source building
  •  


    2.2.   General analysis stage

    Once the model is defined tools become available for the general analysis stage. They involve the calculation of dose maps of the working areas. The dose rates can be displayed as contours or as colour patterns on grids perpendicular to the x-, y- and z-axes of the model. This allows a quick detection of the high dose rate areas.

            Fig.4. : Screen interfaces for grid definition, calculation and results
     

    A graphical interface is provided to display the contribution of each source to the dose at each location on a grid. This tool helps the analyst to suggest and test possible shielding before going to the detailed work planning.

            Fig.5. : Screen interfaces for grid definition, calculation and results
     
    2.3.   General analysis stage

    The tools available for the detailed planning phase involve a trajectory calculation and a scenario building tool. A trajectory is defined as a sequence of tasks to be performed in a fixed geometry and source distribution. These trajectories contain information involving the task description, the location and the duration of the sequential tasks to be performed. The dose account is then calculated for the trajectory based on the radiological and geometrical information of the model. Uncertainties on the work duration can be taken into account making it possible to calculate an upper and lower limit for the acquired doses. A calculated trajectory contains information on the accumulated dose versus time, the dose rate and the dose per task, per task details are given on the contribution of each source to the accumulated dose. This information supports the analyst in the decision to introduce new shielding solutions or to reduce the source strength by other techniques.

            Fig.6. : Screen interfaces for Trajectory definition, calculation and result
     
    From a set of trajectories the analyst can build a scenario. The scenario is defined by selecting a set of calculated trajectories and associating each to a worker or a group of workers. The scenario results include collective dose for the work as well as the individual dose specified for each worker. The comparison of different scenarii then leads to the selection of an optimal work scenario.

            Fig.7. : Screen interfaces for scenario definition and analysis
     
    2.4.   Follow-up stage

    The graphs and task lists produced in the detailed planning stage make it possible to perform a thorough follow up of the dose account during the work. This is achieved trough comparison of the predicted and the received dose. Large deviations between both are an indication that risks which where not foreseen in the planning stage are present on the work floor. An appropriate answer and new prognoses can then be formulated based on new measurements and an adaptation of the model including the detected risks. This approach makes it possible to update the model during the work progression and to suggest scenarios with a lower dose account for future activities.

    3. Geometry definitions in VISIPLAN
    The primitive volume box, cylinder, tube, sphere and hollow sphere define the geometry in VISIPLAN. The data of these volumes is stored in an MS-Access table describing the volumes of a "Take".
    A "Take" is defined as a fixed set of volumes with their materials information.

    3.1.   Take volume table

    The structure of the Access table containing the volumes of a Take is shown here below.

            Fig.8. : Take volume table in the VISIPLAN database
     
    The defined fields are :
    Volumenr : contains a sequential number and is the key of the table
    Naam : contains the name of the volume
    Groep : contains the group name of the volume
    x0 : x-co-ordinate of a vertex point (determining position)
    y0 : y-co-ordinate of a vertex point (determining position)
    z0 : z-co-ordinate of a vertex point (determining position)
    W : parameter of the volume
    L : parameter of the volume
    H : parameter of the volume
    Orientatiex : orientation parameter x-component
    Orientatiey : orientation parameter y-component
    Orientatiez : orientation parameter z-component
    Res : resolution for graphic representation of cylinders and spheres
    Density : material density
    Mat : type of material
    Type : volume type

    The volumes are defined by "Type" and selected from the following list :

        Table 1. : Available primitive volumes

    Box Standard box with the sides oriented parallel with the co-ordinate axis
    Tbox Standard box transformed by a rotation round the z-axis (horizontal angle) through the point x0, y0, z0 : axes become then (x',y',z), followed by a rotation round the y'-axis through x0, y0, z0 (vertical angle)
    Cyl x Cylinder defined along the x-axis
    Cyl y Cylinder defined along the y-axis
    Cyl z Cylinder defined along the z-axis
    Cyl Cylinder defined along a freely chosen axis
    Tube x Tube defined along the x-axis
    Tube y Tube defined along the y-axis
    Tube z Tube defined along the z-axis
    Tube Tube defined along a freely chosen axis
    Sfeer Sphere
    Hsfeer Hollow sphere

    Materials can be chosen from :

        Table 2. : Available standard materials

      Air   Aluminum   Carbon   Concrete   Iron   Lead   Leadglass
      Nickel   Tin   Tungsten   Uranium   Water   Zirconium    

    Density :
      The density field contains the multiplication factor for the density of the materials.
    The density of a material can thus be adapted without redefining the material specifications.

    VISIPLAN makes use of a left-handed reference frame (historic development).
    The transformation to change from a right-handed reference frame is given below :

            Fig.9. : Left-handed reference frame used in VISIPLAN
     

    3.2.   Box volumes

    Defined by type "Box"
    Parameters :
    x0, y0, z0 : The co-ordinates of a corner point (cm)
    W, L, H : Dimensions along the x-, y- and z-axis (cm)

    Given below is a line from the database describing a box volume at with the corner at position 10,20,30 cm with dimensions along x-, y- and z-axis of respectively 50, 100 and 150 cm.
    The material is concrete with density 1 times the density of the standard concrete in the VISIPLAN materials file.

            Fig.10. : Box volume representation in VISIPLAN and VISIPLAN-VRML
     

            Fig.11. : Box parameter definition
     

    3.3.   Cylinder volumes

    Cylinder along the co-ordinate axis defined by Type Cylx, Cyly, Cylz
    Parameters :
    x0, y0, z0 : centre point on the bottom surface (cm)
    W : radius of the cylinder (cm)
    H : length (height) of the cylinder (cm)

    Cylinder along a freely chosen axis defined by Type Cyl
    Parameters :
    x0, y0, z0 : centre point on the bottom surface (cm)
    W : radius of the cylinder (cm)
    H : length (height) of the cylinder (cm)
    Orientatiex :
    Orientatiey :
    Orientatiez :
    direction vector of the freely chosen cylinder axis

    Lines from the volumes tables containing the description of 4 cylinders are given below :
    1 . Cylinder oriented along the x-axis starting at position 100, 50, 50 cm of radius 5 and length 100 cm
    2 . Cylinder oriented along the x-axis starting at position 50, 100, 50 cm of radius 5 and length 100 cm
    3 . Cylinder oriented along the x-axis starting at position 50, 50, 100 cm of radius 5and length 100 cm
    4 . Cylinder oriented along the diagonal axis of the co-ordinate system at position 100, 100, 100 cm with radius 5 and length 100 cm

            Fig.12. : Cylinder representation in VISIPLAN

     

    Fig.13. : Parameter definition for a cylinder along the x-,y- or z-axis Fig.14. : Parameter definition for a freely oriented cylinder
       

    3.4.   Cylinder volumes

    Two kind of tubes are available, tubes along a co-ordinate axis or a freely oriented tube.
    Tube along the co-ordinate axis defined by Type Tubex, Tubey, Tubez
    Parameters :
    x0, y0, z0 : centre point on the tube axis at the bottom surface (cm)
    W : outer radius of the tube (cm)
    L : inner radius of the tube (cm)
    H : length of the tube (cm)

    Tube along a freely chosen axis defined by Type Tube
    Parameters :
    x0, y0, z0 : centre point on the bottom surface (cm)
    W : outer radius of the cylinder (cm)
    L : inner radius of the cylinder (cm)
    H : length of the cylinder (cm)
    Orientatiex :
    Orientatiey :
    Orientatiez :
    direction vector of the freely chosen cylinder axis

    Given below are lines from the database defining the volumes in the following figures.         Fig.15. : Tube representation in VISIPLAN

     

    Fig.16. : Parameter definition for a tube along the x-, y- or z-axis Fig.17. : Parameter definition for a freely oriented tube
       


    3.5.   Sphere and Hollow Sphere

    Sphere defined by Type Sfeer.
    Parameters :
    x0, y0, z0 : centre point on of the sphere (cm)
    W : radius of the sphere (cm)

    Hollow Sphere defined by Type Hsfeer.
    Parameters :
    x0, y0, z0 : centre point on of the sphere (cm)
    W : outer radius of the hollow sphere (cm)
    L : inner radius of the hollow sphere (cm)

    Given below are lines from the database defining the volumes in the following figure.

            Fig.18. : Sphere and hollow sphere representation in VISIPLAN

     

    3.6.   Tilted Box

    Tilted box defined by Type Tbox.
    Parameters :
    x0, y0, z0 : corner point of the box (cm)
    W, L, H : dimensions of the box (cm) along x-, y- and z-axis before tilting operation
    Orientatiex : rotation angle in degrees round the z-axis through the corner point
    Orientatiey : rotation angle in degrees round y'-axis through the corner point

    Given below is a line from the database describing the volume in the following figure.

            Fig.19. : Tilted box representation in VISIPLAN

     
            Fig.20. : Parameters defining the tilted box

     

    4. The Take definition in VISIPLAN
    The Take is defined as a fixed selection of geometry, material and source data. The Take scheme was introduced to avoid the association of results to the wrong geometry. Calculations can only be performed on a take when the Take is saved. The calculations can be performed on trajectories, grids and measured dose rate sets (MDR sets).

            Fig.21. : Take definition and calculations performed on Trajectories, Grids and MDR sets

     
    A Take can be loaded in the system in different ways. A Take loaded in the VISIPLAN software is called the current or the active Take. A current Take can have the status saved or unsaved.


    4.1.   Create a new take

    In the following diagram a flow diagram is given for the creation of a new Take. Dose calculations are only possible when the Save Take operation is performed.
    A modification of Take 1 is not possible.
            Fig.22. : Create new Take flow diagram

     
    A Take can be loaded in the system in different ways. A Take loaded in the VISIPLAN software is called the current or the active Take. A current Take can have the status saved or unsaved.

    4.2.   Load Take without updating

    The flow diagram below shows the loading of a Take without updating. Dose calculations are allowed since the Take is saved.
    No modification can be made to the saved Take.

            Fig.23. : Load Take without updating flow diagram

     

    4.3.   Load Take with updating

    The flow diagram below shows the loading of the Take with updating options. The volumes, materials and sources can be changed before the Save Take operation.
    No dose calculations are allowed until the updated Take has been saved.

            Fig.24. : Load Take with updating flow diagram

     


    5. The Edit Scenario operation
    The flow diagram below shows the creation of a scenario from the Take results of different Takes. This gives you the possibility of assessing the dose in an evolving environment such as a decommissioning site or a site with changing geometry and sources.

            Fig.25. : Scenario creation flow diagram