 Table of Contents ECNDT '98 Session: Radiation Safety | PREDICT, an alternative approach to radiation protection in industrial radiographyPaul van RooijenAIB-Vinçotte Nederland B.V. P.O.Box 6869, NL 4802 HW BREDA, The Netherlands Corresponding Author Contact: Paul van Rooijen AIB-Vinçotte Nederland B.V., P.O.Box 6869, NL 4802 HW BREDA, The Netherlands Email: p.van.rooijen@vincotte.nl, Internet: www.vincotte.nl |
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"We now have many other inspection methods, but radiography is still the best method for a large number of tasks. ...The risks of radiography may, generally speaking, be considered very low compared to the risks normally accepted in industry and society in general in Europe. ...There must be a reasonable balance between the advantages with regard to safety by using radiography and the risks connected to the performance of the examinations...It is my hope that these thoughts will be used...in discussions on radiation safety in the near future." ECNDT President Bjarne Larsen, [7]
When one reads articles written by representatives from NDT-societies it looks like times of change have begun and the quest for trend-setting safety-programmes have started. The contents of this paper should be placed against this background to understand its value and its practical impact. To illustrate this background the paper starts with one these comments. Similar thoughts can be found in [8] and [9]. Typical problems encountered in practical radiation protection where industrial radiography is concerned are mentioned in [10] e.g.: "control point to close to source; no local shielding; poor local communication; failure of radiographer to retreat an adequate distance;... inadequate use of radiation monitors.". Similar problems will be discovered in each country all over the world.
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In this paper a pragmatic, dynamic and proactive approach to protect workers which are involved in industrial radiography against the hazards related to the use of ionising radiation, the so called PREDICT approach, is presented.
The basis of this approach lies in the optimisation- principle, which is generally accepted by radiation protection practitioners. To be able to successfully apply the optimisation-principle in practice, and use it as an effective management-tool, it will be presented in combination with two other basic concepts which are used in occupational health and safety, namely risk-assessment and continuous-improvement-programmes.
Exposure to ionising radiation, in general, is harmful for human beings. Although the health-effects after exposure to high doses are well known, there is still a lot of uncertainty about the effects resulting from exposures to low doses. In the normal day to day practice an radiographic operator will only be exposed to low dose-levels. There is one certainty, namely that the probability for cancer-induction increases with the level of exposure.
Because of these uncertainties the international community has accepted a very careful approach to radiation protection [2]. The assumptions for this approach are based on the probability of cancer-induction:
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On the basis of the above mentioned assumptions, even a very low dose could lead to harmful health-effects.
The International Labour Organisation (ILO) adopted the 'Radiation Protection Convention 1960', in which article 5 reads [6]: "Every effort shall be made to restrict the exposure to workers to ionising radiation, to the lowest practicable level, and any unnecessary exposure shall be avoided by all parties concerned"
The International Commission on Radiological Protection (ICRP) provides recommendations on all aspects of radiation protection. In its Publications 26 and 60 the ICRP issued its basic recommendations for the principles of radiation protection [2, 3, 4]. The objectives are to provide suitable protection such that 'useful' practices of radiation can be performed in a safe manner. The principles for normal operation of a radiation source constitute a 'system of dose limitation' that has three components namely:
a) justification of a practice: "no practice shall be adopted unless its introduction produces a net benefit";
b) optimisation of radiation protection(ALARA): "all exposures shall be kept as low as reasonably achievable, economic and social factors taken into account";
c) individual dose and risk limitation: "the dose equivalent to individuals shall not exceed the limits recommended for the appropriate circumstances by the Commission";
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The system of dose limitation is generally accepted all over the world and is the basis for most national radiation protection regulations. In ICRP publication 60 'dose constraints' were added to the system, each at a fraction of the dose limit. These constraints are to avoid an individual exceeding the dose limit, when exposure to several sources takes place. It is expected that these will be part of national legislation in the very near future.
The aim of the optimisation-principle is to achieve the best levels of protection in the circumstances. But what is considered to be an acceptable ALARA-level?.
The ICRP publications [2,3,11] show several ways to deal the aspect optimisation based on complex cost-benefit analysis. These analysis are for instance already used in decision-making-processes in the Nuclear Energy Industry [1]. They even got a ALARA-centre, which can be visited on the world wide web. Maybe such a complex analysis will be useful for industrial radiography, but these analysis are time-spending and can only be performed on a broad basis.
The PREDICT approach uses a simplified method of cost-benefit-analysis.
This paper will not discuss the aspect 'acceptable ALARA-level' in detail. Of course this question remains crucial for future decisions, e.g. when 'dose constraints' for industrial radiography will be introduced into the national legislation.
The European Union issues general directives for occupational health and safety which contain the minimum requirements and should be implemented by all member-states into their national legislation. Within the context of this paper two directives should be considered:
Nowadays risk-assessment (figure 1) is generally accepted as a management tool. In this paper 'hazard' and 'risk' are considered to be equal terminology. Assessment-methodologies are applied in those conditions where there is a need for reduction and preferably elimination of exposures to hazardous substances or circumstances. Risk is generally expressed in terms of 'probability of occurrence' of a certain 'scenario' and the 'seriousness of the undesired harmful effects'. The probability of occurrence is calculated using exposure-frequency, average exposure time and number of exposed workers. For exposure to ionising radiation, the probability of occurrence, expressed in terms of individual or collective dose, is the main point of attention when designing suitable protection programmes.
The risk-assessment starts with the global 'identification' of all activities of the work process in which hazards are encountered. The protection measures which are already implemented, and their effectiveness are part of this identification. All hazardous activities regardless the seriousness, are included; e.g. hazards which do not result in acute high doses but where exposure-frequency is high should also be included. If necessary a more detailed identification for specific risks will be conducted.
The identification-stage is followed by a more detailed 'evaluation'-stage during which the aspects residual risk and possible need for additional protection measures are considered. The 'residual risk' is the remaining component of risk after implementing protection measures. When its value falls below a level which is considered to be acceptable, no further action is needed. Risks are then categorised using quantitative or qualitative techniques, e.g. relative ranking. The results of the risk assessment can be used to set priorities and targets for future protection-programmes.
Fig1: Risk assessment
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This process is repeated after a certain time period, or whenever circumstances are changed, to determine the effectiveness of the implemented additional protection measures. The concept 'continuous improvement' is a way of thinking about the control and management of organisational processes. It originates from recent ideas of quality control and the same concept has nowadays broadened into the fields of health-, safety- and environmental-protection. The Institution of Occupational Safety & Health (IOSH), Europe's leading body for OSH-professionals, recently published a draft policy statement on integration of management systems[5], from which the following text is recited:
"In the past, many organisations managed OSH, environmental protection and quality reactively; they took few preventive measures until something went wrong. Subsequent action was limited to preventing a recurrence of that undesired event. The contemporary view is that organisations should take a proactive approach; risks should be identified and controlled before the first adverse event. Such an approach is in principle more effective, but also more challenging. Success demands the design and implementation of robust management systems that incorporate, among other things, clear policies, procedures for planning and implementing risk assessment and control, and suitable arrangements for monitoring and reviewing performance leading to continuous improvement.".
This way of thinking is used in the PREDICT-approach.
In the following paragraphs the main elements from the PREDICT-approach shall be explained in more detail. Because specific monitoring-information is still confidential only the highlights can be discussed. Note: PREDICT does not cover exposure to radiation from X-ray-tubes; in the future the calculus-model will be extended for this purpose.
Radiation Protection can be practised in different ways. Until now dose-limits where the point of attention to determine maximum secondary dose-rate levels. Stay below these levels and it is safe. There is nothing wrong with this approach, but it is incomplete and not covering all aspects of dose-reduction, e.g. optimisation is difficult to perform. The PREDICT approach provides some additional aspects, e.g.:
To predict a dose for a specific circumstance where exposure time and dose-rate are known is quite simple. It gets more complex when dose-rates and exposure-times are changing. To calculate a dose for a whole working day all the activities should be analysed. This was done by performing a task-analysis the result of which is presented in a flow-diagram lay-out. (Figure 2). The tasks were then grouped, based on similarity in dose-calculus. Then for each group the dose-model was designed (figure 3), and field-experiments were performed to obtain valid constants and to check rules of thumb.
Input variables for the dose model are: number of exposures and objects, average film-exposure-factor, time-intervals for all phases, source strength and type, length of front end, length of wind-out, transmission of collimator, position of operator during film-exposure, and some other transmission-constants.
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Fig 3: dose rate-curve: basic calculus of dose, by integration of (coloured) area beneath the curve; the numbers at the top refer to the flow-chart form figure 2
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| Fig 2: task analysis - flow-chart of one working job; numbers refer to specific task; letters on the right side refer to the main groups of tasks, used in the dose-model, and for which dose-calculus is similar. | |
For several different combinations of input-variables the output in terms of predicted dose could now be calculated. The results of six different combinations are shown in table 1. Situations can now be compared in terms of dose-reduction. In combination with the statistical monitoring reduction-strategies are defined on basis of the impact of protection measures. In the decision-making process weighing factors for technical, organisational and economic feasibility are added.
| INPUT | No 0 | No 1 | No 2 | No 3 | No 4 | No 5 | No 6 | unit |
| length wind-out | 10 | 10 | 15 | 10 | 15 | 15 | 15 | m |
| length front-end | 1,4 | 1.4 | 1,4 | 1,4 | 1,4 | 1,4 | 0,7 | m |
| material-collimator | Pb | Pb | Pb | Pb | Pb | W | W | |
| thickness collimator | 20 | 20 | 20 | 20 | 20 | 27,50 | 27,5 | mm |
| source-activity | 30 | 30 | 30 | 15 | 15 | 15 | 15 | Ci |
| max. distance during film-exposure | 10 | 30 | 30 | 30 | 30 | 30 | 30 | m |
| OUTPUT | ||||||||
| dose-exposure-phase | 139,0 | 19,6 | 17,0 | 17,5 | 16,2 | 3,9 | 3,9 | µSv |
| dose front-end-phase | 51,4 | 51,4 | 22,9 | 25,7 | 11,4 | 11,4 | 5,7 | µSv |
| dose-other phases | 8,1 | 8,1 | 8,1 | 4,0 | 4,1 | 4,1 | 4,1 | µSv |
| total dose | 198,5 | 79,1 | 48,0 | 47,2 | 31,7 | 19,4 | 13,7 | µSv |
In the final part of this paper the most important part of optimisation, namely promoting the philosophy to all members of the organisation, is discussed. As stated in [1]:"In most cases, implementing ALARA is just common sense, and one must therefor rely on each person's awareness...In theory the ALARA principle corresponds mainly to a 'state of mind'..."
I will add my own personal vision to this awareness-idea:
Changes should be anchored into the organisational system. The complete organisation should be made aware of the general policies. In the process of the detailed filling in of these policies communication with all levels is advisable. Some resistance is always to be expected because where people work together the existence of different opinions is a given fact. New ideas are always approached with some caution, because change automatically implies that time has come to say goodbye to old habits. Management can steer this process and use the difference in opinions, and the associated resistance against change, in a creative way. An individual's own ideas will be accepted more easily. If management is prepared to meet the resistance on forehand, it can use these ideas to increase the acceptance for changes. Compromises need to be accepted by all parties involved in the decision-making process.
When a certain target-point is chosen, the success will still depend on the given directions for the ways to get there. Another crucial point is that in an organisation which has chosen for transformation and continuous improvement the target-points, ways and directions will also change continuously. 'Reflect and react' is part of this process. Therefor some flexibility is required especially in the written procedures and instructions.
Another pitfall lies in the control-systems itself. Never should 'the being of the control-system' be the main point of attention: the systematic approach is a tool and not a result by itself.
Safety is not achieved with the design of systems alone, safety is made by the human-beings involved. Never try to control the people, but 'give them the facts and they will act'.
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