Practice of the ALARA philosophy

1. The optimization principle
The last decades the optimization principle has become a key element in the radiation protection system, recommended by the ICRP (International Commission on Radiation Protection).
One presumes that the optimization principle originates in the fact that no undeniable scientific basis can be found for the whether or not existence of a threshold for the origin of stochastic effects. The members of the ICRP therefore decided to be very cautious. This is translated in the linear dose-effect approach without threshold. Starting from this hypothesis, the minimization of the exposure has become the cornerstone of radioprotection.

The objective of a zero risk is, from an economic or ethic point-of-view, not defensible. Therefore, one choses for the ALARA approach "As Low As Reasonably Achievable", as low as reasonably possible, taking into account economic and social aspects.

The wording for the ALARA principle becomes then : "Each act or a whole of acts, will be carried out in such a way that the doses will be kept as low as reasonably possible, taking into account all economic and social factors".

The relation between the limits and optimization of radiation protection is described by the ICRP in publication 60 in the so-called model of the "tolerated risk". The exposure limits are the boundaries between unacceptable and tolerable. Respecting the limits guarantees the individual that he will not suffer deterministic effects, but also that the chance of developing a radiation induced cancer is small compared to other industrial and technological risks to which he is exposed. So this is tolerable from a social point-of-view.
An additional difference has to be made between a tolerable and an acceptable risk. The tolerable risk becomes acceptable when the protection is being optimized.
In its latest recommendations, the ICRP also emphasized the fact that sometimes there is an unequality in the individual dose distribution. Situations where the personnel is exposed, during work of even due to an action to reduce the risk, can lead to important dose differences. Differences being that important as to reduce them.

This means that the optimization may not only be focused on the reduction of the collective dose, but also it has to be careful for the reduction of the highest individual doses.

        Fig.1. : Relation between the limits, the tolerable and the acceptable risk, and the optimization
 

2. The optimization procedure
The application of the optimization principle means essentially putting into use a methodology that allows to identify, evaluate and select actions in the field of radiation protection with the aim of keeping the dose for the workers, public and patients as low as reasonably possible.
This means concretely that one takes into account the extent of the means, the protection level that can be reached, the marginal conditions and other factors so that the best possible protection can be given taking into account the economic and social circumstances.

The search for a balance between the expenses for protection and the advantages of this protection can become, in some cases, very complex. In some cases the scientific analysis has to be coupled to judgments on the relative importance of certain risks or other judgments of the decision takers or exposed persons. In this context, decision techniques can be used to define the level of protection taking into account the different aspects and value judgments.

These techniques can be applied after a clear identification of the alternatives and the marginal conditions that are important in the process. The various factors characterizing the options have to be identified and qualified. The different steps in the optimization process are given on the next figure. The place where decision techniques can be applied is indicated.

        Fig.2. : Components in the optimization procedure
 

The most important part of the job is the structure of the problem and the definition of the options, factors and marginal conditions. Factors that characterize certain options are mainly : 
 
  • the collective dose  
  • the individual dose distribution  
  • investment costs for protection  
  • the variation of the exploitation costs.

    The analysis of this information is the most difficult and time-consuming part of the work. In some cases it is also necessary to take into account the dose transfers between different groups (workers and public, internal and external workers) as well as geographic or time-connected exposure and the economical, social or ecological impact.

  • 3. Simulation techniques in ALARA problems
    The application of the ALARA principle in operational nuclear installations is not always obvious. The dose uptake for a task is defined by factors as geometry, materials, source distribution and the organization of the work. This even becomes more complex for installations that are being converted or dismantled and where geometry, material, source distribution and work organization can change in a very short time.

    We can ask ourselves which means we need to carry out a good ALARA study. The first part of ALARA, the "As Low As" part, refers to the dose reduction.
    In order to make a study about this, we have to be able to make a dose prognosis of the planned actions, but also to evaluate the effect of the dose reducing measures. The dose prognosis and the effectiveness of the protection measures depends on the kind of work, the duration of the work, the number of workers, shieldings and actions having a direct impact on the sources (i.e. chemical decontamination). We are confronted with a problem where a lot of parameters interfere.

    From this problems the development of simulation programmes as VISIPLAN 3D ALARA planning tool was started. These programmes allow to simulate a complete work scenario in a 3D environment including the dose estimation.

    The use of these simulation techniques to search and judge the means or actions to reduce the dose happens in several steps (see figure 8).

    The first phase is the establishment of an adequate model of the workplace. This includes the geometry, the materials and the sources.

    In the second phase, based on the model, an estimation is made of the exposure risks on the workfloor by calculating the dose rate cards on which the locations having a high dose rate are visible. Based on these cards containing information on the different sources, the dose and the work locations, some propositions are made to reduce the dose. This can be done by placing some shielding, filling with water, decontaminating or removing sources if possible. With the simulation programme these options are analysed on their efficiency by comparing the dose rate cards of the new situation to the original one. The analysis allows to find the best solution for the dose reducing actions defined by the geometry of the source distribution.

    In the third phase, trajectories and operator work positions are analysed in detail. Tables are set up containing information on the work position, the work duration, the work description and the uncertainty on the work duration. The latter is an important factor when one wants to do a sensitivity analysis. In many cases only a rough estimation of the work duration can be done because some circumstances, as for instance a stuck bolt, can increase the duration. Based on the tables the programme calculates the individual dose for the operators. This information can be gathered in tables grouping several trajectories into scenario's, from which the maximum collective and individual dose can be deduced. The trajectories and scenario's are calculated for the basic situation (without supplementary dose reducing actions) and also for the different situations where actions are taken in order to evaluate the efficiency of the actions. The propositions found are then confronted with other factors playing a rol in the ALARA process, the "Reasonably Achievable" aspects. Before this, the results of the different options and their effectiveness will have been communicated to the people who plan the work. During this analysis, an option is chosen and dose objectives will be predefined.

    In the last phase, the real dose uptake (measured with operational dose meters) will be compared to the dose estimated for the different tasks. In case there is a large discrepancy between both of them, this can be due to different causes: whether the estimated dose is incorrect, i.e. due to a bad characterization of the work location or when a new source is found during the works. It can also be possible that the work has not been carried out following the procedures. In both cases, an analysis will be done in order to take corrective actions.

    The scheme below gives the different phases when using a simulation programme.

            Fig.3. : Methodology for the use of a simulation programme in the optimization process
     

    Example of an analysis by simulation technique

    Below, an example is given of an "As low As" analysis by means of the simulation technique.

    Description of the workplace

    In a location given on figure 4, a switchboard and a valve have to be installed. We presume that this action is justified, that we know the strength of the source and that, in the simulation programme, an adequate 3D model for the site was built.

    The valve will be placed by 1 person and will take 1 hour. Placing the switchboard will be done by 1 person and will take maximum 2 times 15 minutes: 15 minutes for the drilling and 15 minutes for putting it in place.

    During a preliminary analysis of the workplace it was stated that there were sources in the waste drum, the pump and in the elbow of the pipe. The source in the central drum is concentrated in the deposits on the bottom of the drum. From our experience we know that this type of deposit does not dissolve in water.

    Further more we know that on site we have the disposal of lead shieldings (0.5 cm equivalent thickness), of a bridge for transporting weights, and that the central drum is empty for the moment but that it can be filled with water.
    There is an additional marginal condition for the waste drum: it has to stay on site.

            Fig.4. : Workplace before and after installation of the switchboard and the valve
     
    Analysis

    As in this example the model is already available in the simulation programme, we can proceed immediately with the general analysis phase. Therefore, we calculate the dose rate cards on the locations where the work will be carried out. The results are given hereafter.

            Fig.5. : Dose rate card without dose reducing means
     
    At the location where the valve has to be installed, the average dose amounts to 1.5 mSv/h; at the location of the switchboard, the dose is approximately 0.4 mSv/h.

    The most important sources contributing to the dose are the waste and the deposition at the bottom of the tank. The source located in the elbow is of less importance and the one in the pump can be neglected.

    The options we have to reduce the dose are the following ones: to shield the waste drum, to move the waste drum, to fill the tank with water. When we investigate the effect on the dose, we find the results by examining the dose rate cards.

    It appears that shielding or moving the waste drum does not drastically decrease the dose in the workplace. When the central waste drum is filled with water, the dose at the location of the switchboard decreases to rates of 5 µSv/h. We can also strongly reduce the dose at the location of the valve by moving the waste drum to the other side. The dose rates are then in the order of magnitude of a few µSv. The most advantageously dose reducing strategy is clearly the filling of the drum with water.

    Fig.6. : 
    Dose rate cards after shielding of the waste drum with 0.5 cm lead (above at the left) and after moving the waste drum to the lower floor without shielding (right). The option of filling the drum with water is examined in the lowest two cards (on the left with the waste drum in is original position, on the right with the waste drum on the other side of the central drum). The scale of the dose rate cards is the same one as on figure 5.
     
    Analysis of the trajectory

    The strategy for the dose reduction being chosen, we can proceed to the dose analysis on the trajectories. The basic work scenario, thus without dose reducing measures, is composed of three trajectories. The first one simulates the drilling of the holes for the switchboard, then installation of the switchboard and finally, putting into place of the valve (figure 7).

            Fig.7. : Trajectories for placing switchboard and valve without dose reducing actions on the workfloor
     
    The trajectories are put together to result in the reference or basic scenario.

    Description of the trajectory Time (h) Average dose rate (mSv/h) Collective dose (man*mSv)
    Installation of the valve 1 1.5 1.5
    Drilling of holes for the switchboard 0.25 0.4 0.1
    Putting into place of the switchboard 0.25 0.4 0.1
    Total 1.30 - 1.7

    The scenario in which the central drum is filled with water consists of four trajectories as, for the work at the valve, the waste drum has to be replaced to the other side of the tank. This is done with the bridge. The supplementary work is to couple the drum to the bridge, which is supposed to take 1 minute. After the installation of the valve, the drum is put back on its place.

    Note that after filling the tank with water, the work on the switchboard is started. The trajectories of the new situation are given on figure 8.
    Fig.8. : 
    Dose rates on the trajectories to put the switchboard and the valve in place. For the installation of the valve, the waste drum was placed on the other side of the central drum.
     

    Description of the trajectory Duration (h) Average dose rate (mSv/h) Collective dose (man*mSv)
    Drilling of the holes for the switchboard 0.25 0.006 0.0015
    Installation of the switchboard 0.25 0.006 0.0015
    Transfer of the waste drum 0.0167 1.5 0.025
    Installation of the valve 1 0.020 0.020
    Putting the waste drum back on its place 0.0167 1.5 0.025
    Total 1.53 - 0.073

    The filling with water and the reorganization of the work instruction present a large dose gain. The collective dose decreases from 1.7 man*mSv to 0.073 man*mSv, i.e. a gain of a factor 23.
    Remark: This analysis is correct when the source deposit in the central drum is not dissolved in the water. When this would be the case the gain would be less important.

    Characterization of the workplace
    The characterization of a workplace comprises a technical, geometrical and radiological analysis.

    The technical analysis is focused on the function of the workplace, the materials, the components present and the logistic means present or available to use.

    The geometrical analysis determines the dimensions of the workplace. It gives information about the accessibility and the degrees of freedom to use expedients in order to reduce the dose rate.

    The radiological analysis is used to identify the exposure risks. This can be done based on a simulation model or by carrying out measurements. It is clear that, in some cases, a simutation model is also set up by the measurements done. The radiological characterization is done by dose measurements, spectral analysis or smear tests, and by the analysis of historical information on the workplace.

    In some cases, the radiological characterization is attended by an important dose. Therefore it is important that, in the field of radiation protection, the most efficient equipment is used. Thus, for low dose rates, it is reasonable to have the measurements done by a radiation officer. For higher dose rates and hot spots, it is better to use telescopic dose meters. In the case of even higher dose rates, the use of gamma camera's and gamma scanners is justified.

            Fig.9. : Important aspects for the characterization of a workplace
     

    Communication during the ALARA process
    As mentioned before, optimization is a multidisciplinary method. This means that the different partners have to communicate: the planners, the radiation officer, the technicians and the workers. Everyone in this group has his responsibility. On the figure below, an example of the (or a possible) communication structure is given, indicating also the information each person disposes of. During the evaluation and identification of the radiation protection means, the radiation officer has to co-operate with all partners in order to find the best solution.

            Fig.10. : Communication between the different partners in the ALARA procedure
     

    Once the decision taken about how to carry out the work, the workers have to be prepared, more specific by explaining them the work procedures and the exposure risks. The latter one is important in order to stimulate them to reduce the individual and collective risks. Dose rate cards or virtual presentations of the workplace with indication of the dose can elucidate the communication. This way, the workers become familiar with the work environment before the start of the operations. An example is given on the following figure(s).

            Fig.11. : Virtual reality used in the communication of risks to the workers on the workfloor
     

    Conclusions
    The optimization principle is set up in order to manage the residual risk of exposure to ionizing radiation. One has to find a compromise between the costs of the dose reducing actions and the obtained advantages to keep the exposure as low as reasonably possible, taking into account the economic and social circumstances, all of this in order to have an acceptable residual risk.

    An overview of structures and means that can be used in this analysis is given in the previous chapters. Besides these means and structures, we have to emphasize that ALARA also demands an attitude based on precaution, responsibility and transparency.

    Precaution 
    Because the actual scientific knowledge does not allow to accept a treshold model for low doses.

    Responsibility 
    Because there are no unlimited means available and the protection of one individual may not be at the cost of the other one. The means have to be divided fairly to keep the residual risk acceptable.

    Transparency 
    Because looking for a balance between individual protection and social equality can only be achieved on basis of a compromise between all actors. It is necessary that everyone knows the exposure risks as well as the means used to control them.