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15
15.1 INTRODUCTION
The objective of radiation protection is to protect people and the environment against the harmful effects of ionizing radiation. In order to achieve this objective it is necessary to estimate the chance that something could go wrong, evaluate the consequences that would result and install sufficient control and protection systems to ensure that these consequences are acceptably low. This is achieved through a process called risk assessment, which involves:
⦁ identifying the hazard;
⦁ estimating the size of the risk; and
⦁ assessing its importance in comparison with other risks.
The results of the risk assessment should be recorded appropriately and used as the basis for making decisions about how to manage the risk. Finally, each risk assessment needs to be reviewed and updated periodically, and when new equipment or work practices are introduced.
15.2 HAZARDS AND RISKS
In general conversation, the words hazard and risk are often used interchangeably to denote a chance event whose outcome is undesirable, e.g. the risk of being struck by lightning. In formal risk assessments, however, it is useful to reserve the term hazard to describe any inherent property of an activity or situation that could potentially cause harm. Making this distinction, it follows that, just because something is hazardous, it does not mean that it will always result in a dangerous outcome – golfers will readily appreciate that hazards such as bunkers and trees are only dangerous if the ball is played into them!
As hazard is an inherent, dangerous characteristic of something, it is therefore a certainty. Risk, on the other hand, is a chance or, in mathematical language, a probability.
Note that the value of any probability always lies between 1 (event certain to occur) and 0 (event certain not to occur). Risk can be defined as ‘the probability that some harm will happen due to the realization of a hazard’. Sometimes risk is a single numerical probability but in most situations it is the product of a number of probabilities. It is very often expressed over a specific time period, e.g. a year, and then it becomes a frequency. Usually there is also an indication of how serious the harm could be, such as ‘risk of death’. Table 15.1 compares the approximate values of a number of familiar risks.
Example 15.1
The risk of death from rock climbing is estimated to be about 4000 per 108 hours for the people involved. Calculate the annual risk of death for a climber who participates in the sport for 50 hours each year.
A risk rate of 4000 per 108 hours equates to 4000 × 10–8 or 4 × 10–5 per hour. The rock climber’s annual risk of death is therefore
50 (hours per year) × 4 × 10–5 (risk per hour) = 2 × 10–3 or 1 in 500
15.3 THE BASIC STEPS IN RISK ASSESSMENT
The basis of all risk assessments is to ask a series of ‘what if?’ questions (e.g. ‘What if the containment should fail?’) and ensure that the answers are as comprehensive and accurate as possible. The answers are obtained by reference to practical experience of similar situations, experts’ judgement, manufacturing standards and test results, and so on. The risk analyst then estimates the probabilities and consequences of all the foreseeable operating conditions that could arise, including non-standard operations, and makes an assessment of the overall risk. He then has to decide how to present the information so that it will be of maximum value in reaching decisions about the most appropriate control measures to be adopted.
Any risk assessment consists of a limited number of individual steps, as shown in Figure 15.1. These steps need to be carried out for any risk assessment, no matter how simple or complicated it is. However, it is important to ensure that each risk assessment is suitable and sufficient for the operation in question. Completeness is essential but the aim should be to achieve this without overcomplicating the risk assessment process unnecessarily.
Table 15.1 A comparison of some familiar fatal risks
Familiar risks Risk of death
(1 in N )
Smoking 10 cigarettes per day for a year 200
Being a deep-sea fisherman for a year 500
An effective (whole-body) radiation dose of 20 mSv (UK annual dose limit)* 1000 Radiation-induced fatal cancer from having one CT scan of the abdomen 2500
Being a coal miner for a year 6500
An accident at home over a 12-month period 10 000
Being murdered in a 12-month period 100 000
Being hit in your home by a crashing aeroplane in a 12-month period 250 000
Drowning in the bath in a 12-month period 685 000
Radiation-induced fatal cancer from having one chest X-ray examination 1 000 000
Being struck by lightning in a 12-month period 6 200 000
Death from a nuclear power station accident in a 12-month period 10 000 000
*It is common to express radiation risks as the probability of some adverse effect per unit dose. For example, the risk of fatal cancer is estimated to be about 5 ¥ 10–2 per sievert effective dose, or 5 per cent per sievert. This means that if 1000 people each received an effective (whole-body) dose of 100 mSv then, on average, five of them would die of a radiation-induced cancer.
The basic steps in risk assessment 185
Step 1: Identify the hazard(s)
There are various ways in which the hazards in a workplace can be identified. These include:
⦁ physical inspection of the workplace to see what could reasonably be expected to cause harm;
⦁ checking the manufacturer’s instructions and data sheets to obtain informa-tion about the hazards associated with any new equipment or sources;
⦁ carrying out a review of experience gained from similar operations;
⦁ asking the operators – they may have practical insights not obvious to the risk assessor;
⦁ reviewing the previous accident and ill-health records to see if there are any less obvious hazards.
All types of possible hazards should be considered, and one type, for example radiation, should not be solely concentrated on, to the exclusion of conventional industrial hazards.
Step 2: Decide who (or what) might be harmed and how
For each hazard, it is necessary to identify who might be harmed by it. For some hazards the potential damage may be to the natural environment rather than to people and this needs to be factored into the risk assessment. Having identified who or what might be harmed by the hazard it is then necessary to decide how they might be harmed, i.e. what type of injury, damage or ill health might occur. The following points are important:
⦁ Different groups of workers, for example operators, contractors, maintenance staff and cleaners, are likely to be in the workplace for different periods of time and will therefore be exposed to the hazard in different ways.
⦁ If members of the public could be exposed to the hazard this introduces ad-ditional complications in terms of the control and monitoring that can be applied to them. For example, the radiation risk factors for children are higher than for an adult population because (a) children are more likely to be alive for the whole latency period of the cancer, (b) children are more likely to have children of their own and (c) children’s tissues are more radiosensitive than adults.
⦁ There are particular requirements for the monitoring and protection of some workers, such as new workers, and new or expectant mothers, that need to be taken into account.
⦁ If the hazard is likely to damage the natural environment it is necessary to determine the route(s) by which this might occur.
Feedback loop
Figure 15.1 The basic steps in risk assessment.
Step 3: Evaluate the risks
Having identified the hazard and who or what might be harmed by it, the next step is to evaluate the consequential risk, i.e. ‘the probability that some harm will happen due to the realization of the hazard’. This is the core of the risk assessment process and relies on obtaining or generating a realistic estimation of the probability of harm. In some situations the probability of harm is quite obvious from a comparison with established good practice or industrial experience. However, in other situations, especially where the hazard is not immediately obvious to the normal human senses (e.g. ionizing radiation), the estimation of the probability of harm may be considerably more complicated. It usually involves estimating a number of probabilities and then multiplying them together, as shown in the following generic equation:
R = P(haz) × P(esc) × P(int) × P(dam)
where R is the risk; P(haz) is the probability that the hazard exists and is capable of causing harm to the subject of the risk assessment; P(esc) is the probability that the hazard
‘escapes’ from the devices incorporated to control it; P(int) is the probability that the hazard interacts in a damaging way with the subject of the risk assessment; and P(dam) is the probability that the interaction causes a specific type of damage in the subject of the risk assessment.
Every risk assessment should begin with an estimation of the harm that would result if the hazard were allowed to interact with the target (i.e. the person(s) or objects that might be harmed) without any specific precautions in place. This is sometimes known as the ‘bare’ risk assessment. In the ‘bare’ assessment, P(haz) and P(esc) are normally 1.
P(int) is usually also 1, although there may be circumstances where it is less than 1. For example, members of the public would not normally suffer a damaging interaction with a sealed source housed in a controlled area unless there was an accident, such as a fire, that damaged the container of the sealed source and vaporized the radioactive material. In this situation, P(int) would be very small.
The final probability in the risk equation, P(dam), is relatively straightforward to assess when the potential harm is to the safety of the subject of the risk assessment, e.g.
the probability that a dropped load which falls on a worker will result in a broken limb can be estimated from the weight of the load, the height of the fall and the likelihood of the worker being in the path of the falling load. However, in the case of ionizing radiation and toxic chemicals, where the potential harm is to the health of the subject, a whole range of possible health effects may need to be taken into account, both to the workers exposed in the workplace and to any members of the public exposed to releases from the workplace.
For ionizing radiation, the probability and extent of such health effects will depend on the type and intensity of the radiation emitted, the age of the workers, whether any of them is pregnant, the composition and dietary habits of any groups of the public who may be exposed, the chemical form of any radioactive material released to the environment, and so on. Much of the information needed to form a reliable estimate of P(dam) is available in the recommendations and guides of the International Commission on Radiological Protection (ICRP), as well as in other national standards and guides, though additional information and tests may be needed in some situations, such as clean-up and decommissioning operations, in order to form a reliable estimate of P(dam). The normal approach for sealed sources is to estimate the unshielded dose rates (to the whole body and extremities) for the
The basic steps in risk assessment 187
proposed activity, multiply these by the time over which the activity is going to take place and compare the results with the relevant regulatory limits and local dose constraints. If the predicted dose exceed the limits or constraints then additional precautions, such as shielding and exposure time control, should be incorporated.
Having estimated the four probabilities it is then quite straightforward to arrive at the corresponding risk by multiplying them together. Note that it is often neither practical nor necessary to attempt to ascribe actual numerical values to the four probabilities in the risk equation. In many cases it is sufficient to decide that each of the probabilities is either high, medium or low. Table 15.2 illustrates, in broad terms, how the qualitative value of the risk might vary according to the qualitative values of the four probabilities.
Table 15.2 Illustration of qualitative risk assessment
P(haz) P(esc) P(int) P(dam) R
High High High High High or very high
Medium High High High High
Medium High Medium High High
Medium Medium Medium High Medium
Medium Medium Medium Medium Medium
Low Medium Medium Medium Medium
Low Medium Medium Low Medium
Low Low Medium Low Low
Low Low Low Low Low or very low
When carrying out an actual risk assessment, however, it is always necessary to look carefully at the importance of each of the probabilities in the context of the hazard being controlled rather than following this table ‘blindly’.
Example 15.2
Calculate the risk that a cyclist will be seriously injured by a car at a main road junction based on the following information:
Probability that a car is approaching the main road as cyclist passes the junction: 0.01 Probability that the car fails to stop (due to driver error or inattention, road conditions, equipment failure, etc.): 0.05
Probability that the car hits the cyclist (either the driver or the cyclist may take successful evasive action): 0.5
Probability that the impact causes serious injury to the cyclist (likely to be less than 1 if cyclist is wearing a protective helmet): 0.8
Using the equation R = P(haz) × P(esc) × P(int) × P(dam) The risk in this case is R = 0.01 × 0.05 × 0.5 × 0.8 = 2 × 10–4
This means that, under these assumptions, if 10 000 cyclists use the main road every year, it is expected that about two of them will be seriously injured in a crash at this junction.
Step 4: Decide on the precautions
Having estimated the ‘bare’ risks associated with a particular work activity it is then necessary to decide on the precautions required. In some circumstances, there will be national guides or regulatory requirements that define the risk level at which specific precautions must be introduced. These may also define the type of precautions that should be taken. In other situations, there may simply be the general legal requirement to do everything ‘reasonably practicable’ to protect people and the environment from harm or to ensure that doses are as low as reasonably achievable (ALARA). For some work activities, what is ‘reasonably practicable’ can be deduced from a comparison with accepted good practice. Where this is not possible, the responsible employer may have to work out what is
‘reasonably practicable’ for themselves, following the rule that precautions to eliminate or reduce the risk should be incorporated until their cost becomes quite disproportionate to the value of the risk being averted.
When controlling risks, the following principles should be applied, in the order shown where possible:
⦁ consider a less risky option (e.g. use a smaller radioactive source or a less hazardous chemical);
⦁ prevent access to the hazard (e.g. by using a ventilated glove box);
⦁ organize the work to limit exposure to the hazard (e.g. establish contamina-tion control areas and barrier controls);
⦁ define and issue appropriate protective equipment (e.g. clothing, footwear, masks, goggles);
⦁ incorporate the above in appropriate operating procedures and provide train-ing as necessary;
⦁ provide welfare facilities (e.g. first aid and washing facilities for the removal of contamination); and
⦁ if indicated by the risk assessment, provide facilities and resources to deal with potential accident situations.
Having decided on the precautions it is then necessary to follow the feedback loop shown in Figure 15.1 and redo the risk estimation with the precautions included. This will alter some or all of the probabilities in the risk equation and result in a new estimate of the risk. This is again compared against the target for reasonable practicability to decide if it is acceptable. If it is still unacceptable, further precautions are included and the process repeated until the situation is judged to be satisfactory. Note that the introduction of a precaution or control can increase the risk of injury from some other hazard. For example, medical X-ray staff wear lead-impregnated aprons that are 0.25 mm lead-equivalent to protect them from scattered radiation (attenuation factor around 90 per cent). Higher lead equivalences (e.g. 1 mm) would obviously give greater attenuation but the risk of back injury from the weight of the apron begins to outweigh the increased radiation shielding benefit.
Step 5: Record the findings and implement them
It is always good practice to record the findings of each risk assessment. The written record provides the basis for improving the work activity and allows the results to be shared with the staff who will be involved, as well as with other people who might be affected by the
Hazard and risk in radiation protection 189
work activity. It allows appropriate operating rules to be developed and assists in defining training requirements.
Step 6: Ongoing review
It is essential to ensure that the risk assessment for any work activity is up to date. When any changes are made, such as bringing in new equipment, substances or procedures, the risk assessment should be reviewed. It should also be reviewed on a regular basis to see if there are any improvements outstanding from previous reviews, if the operators have identified any problems ‘on the ground’, or if there are lessons to be learned from accidents or near misses.
15.4 HAZARD AND RISK IN RADIATION PROTECTION
Radiation protection is essentially concerned with controlling the risks that arise from the use of radioactive materials or machines that produce ionizing radiations. The nature of ionizing radiation means that it is normally impossible to eliminate all risks from activities involving these sources of hazard. In a facility that has been designed, maintained and operated properly, the risk should be very small, but it is not zero. In any situation, the risk is estimated by identifying all the possible chance events (such as material leaking from a radioactive source or a radiation machine malfunctioning;
a worker or member of the public being in the ‘path’ of the escaping radiation, etc.) that could lead to the ionizing radiation reaching and interacting with its ‘targets’.
Then a probability is ascribed to each of these chance events happening, either from experience or from experiments, measurements or modelling, and the risk is calculated by multiplying all these probabilities together, as shown by the previous generic risk equation. This is a simplified description of the process of risk assessment, which can often turn out to be quite complicated in practice. The following two examples illustrate how the risk is assessed in two quite different situations, following the six steps in Figure 15.1.
Example 15.3: risk assessment prior to commencing work with a sealed source in a radiochemistry laboratory
Regulation 7 of the UK Ionizing Radiations Regulations (1999) requires that:
‘before a radiation employer commences a new activity involving work with ionizing radiation in respect of which no risk assessment has been made by him, he shall make a suitable and sufficient assessment of the risk to any employee and other person for the purpose of identifying the measures he needs to take to restrict the exposure of that employee or other person to ionizing radiation.’
Step 1: Identify the hazard
Although the general hazard is clearly ionizing radiations from the sealed source, the risk assessor needs to know the specific radioisotopes present, their activity levels and principal emissions, and the energies of the principal emissions. This information will be required later to allow the doses to various subjects to be estimated.