I often come across differences between Goal Setting and Prescriptive Legislation when dealing with radiation protection issues. This came up again last night when discussing with a client the 'length of a tunnel' required for x-ray screening equipment (e.g. for security). This reminded my of a Blog entry I wrote some while ago. Having reviewed this I thought it was a good example to share. In the discussion I look at radiation protection related legislation and standards (I do not consider specific machine guarding legislation). The blog entry in question goes as follows:
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Our first visit was to run a day long training course for the Sapphire Alliance (a group of companies who make x-ray screening systems for the food industry and similar). The training was a challenge since the delegates were from the UK, US, France, Germany, Denmark, Spain and Australia. Working from IAEA downwards we managed to pitch the training for all delegates at the right level, but still came up with some interesting differences.
Emission Dose Rate
Take the maximum allowable dose rate from the exterior surface of the x-ray machine. In the UK there is no specific legal limit (other than avoidance of Controlled Area levels etc). Indeed, even at the EU Directive level (96/29/Euratom) there is still no specific limit. Despite this, nationally and in the EU (and mostly worldwide) the agreed limit is 1 uSv/h. There are some slight variants of this – in the UK this is ‘the instantaneous dose rate averaged over a minute’. Also, this may be specified as a surface dose rate or one taken at 5 of 10cm. Ionactive uses 1uSv/h at the surface. So where has this come from? ... it’s certainly not written in law in the UK (or I believe most similar local legislation of the EU).
One derivation comes from a slight misinterpretation (in terms of purpose) of Article 3 of the Basic Safety Standard (96/29/Euratom) which deals with reporting and says:
No reporting need be required for practices involving the following: .... ‘the operation of any electrical apparatus to which this Directive applies, other than that referred to in subparagraph (e) provided that:(i) it is of a type approved by the competent authorities of the Member State; and(ii) it does not cause, in normal operating conditions, a dose rate exceeding 1 uSv/h at a distance of 0.1 m from any accessible surface of the apparatus’.
The wording above is almost transferred directly to Schedule 1 of the Ionising Radiations Regulations 1999 (IRR99). However, this is effectively about notifications to the regulator (competent authorities). In the UK, regardless of whether the enclosure of an x-ray machine meets the 1 micro Sv/h criteria, it still has to be notified – as no such equipment has ever been type approved.
So in the UK, and at least at EU level (the Directive), there is nothing specifically which gives us a performance limit. Despite this, no manufacturer of x-ray equipment in the UK (be it for food screening or security), would dare to sell equipment not meeting the 1 uSv/h level. It is therefore treated as a legal limit (and is still seen as very much an upper level with actual dose rates near background).
Tunnel Length (to restrict exposure)
Let us move our attention to ‘tunnel’ length, or distance between an open port in an x-ray enclosure and intersection with the primary beam (see picture). This can cause great debate in the UK (and beyond) – i.e. what should that length be? In the UK it is basically down to ‘restriction of exposure’ – either so far as is reasonably practicable (ALARP), or in the case where a dose limit could be exceeded in a short period of time, an absolute requirement. However, nowhere is there an actual dimension and many would argue that this is the correct approach. Many would argue its down to ‘goal setting’ legislation – i.e. conduct a suitable and sufficient risk assessment based on actual measured dose rates, and put in place protective features to mitigate that risk.
It is worth pointing out that the UK does have the Generic Prior Authorisation (GA) for X-ray Equipment – although this does not cover x-ray screening equipment (using x-ray tubes) unless the equipment is used for research. However, whilst a typical piece of x-ray screening equipment does not need to comply, following the spirit (or actual requirements, so far as is reasonably practicable) is certainly good practice. Furthermore, compliance with Regulation 8 of the Ionising Radiations Regulations 1999 is a must – and most of the GA requirements are covered here (the degree of compliance then comes down to the risk assessment under Regulation 7). However, the GA is still designed for specific types of x-ray equipment (e.g. Industrial Radiography), and its more prescriptive nature still does not cover x-ray screening equipment.
Commonly in the UK, ICRP reference man arm (85cm) has been used to set tunnel length on x-ray equipment (or more precisely, the distance between the open end of the tunnel and the primary x-ray beam) – e.g. when you cannot avoid passing a sample in one end (through leaded curtains) and out through the other. This is of course a familiar practice at any airport when getting your hand baggage screened. However, in determining the distance one is still left with risk decisions (i.e. should the distance to the primary beam reflect average arm length, or should one consider someone contorting and reaching in far beyond the curtain?). These are questions for designers, manufactures and suppliers. In addition, it is an excellent example of when to consult a Radiation Protection Adviser.
Something more prescriptive perhaps?
So goal setting it is? Based on Risk? What is the alternative?
How about US [21 CFR 1020.40]. This is the US Performance Standard for ‘Cabinet X-ray systems’. The standard was written in 1974 and then revised April 2007. In September 2007 the FDA issued a guidance document ‘Guidance for Industry and FDA Staff - Compliance Guide for Cabinet X-Ray Systems’. The status of the guidance is broadly similar to an Approved Code of Practice (ACoP) in the UK.
Using the examples already mentioned, it is interesting to test the FDA standard against what we have in the UK. For example, let us look at emission rate.
Emission Limit [21 CFR 1020.40(c)(1)(i)]:Radiation emitted from the cabinet x-ray system shall not exceed an exposure of 0.5 milli Roentgen in one hour at any point five centimetres outside the external surface.
Putting into SI units - that is 5 micro Sv/h at 5cm from any surface. That is a prescriptive limit – like nothing we have in the UK. To be clear, the standard actually requires that the limit is 5 micro Sv total in anyone hour – there is no restriction on peak dose rates during the hour. The impact of this is similar to the UK position in terms of dose rate averaging (here we do it over one minute).
Ports and Apertures [21 CFR 1020.40(c)(3)]
i) The insertion of any part of the human body through any port into the primary beam shall not be possible. (ii) The insertion of any part of the human body through any aperture shall not be possible.The guidance to this regulation is helpful :
'This requirement is intended to prevent accidental and routine operator exposure to the primary x-ray beam. Under this provision, insertion of any body part into a port must not be a standard operating procedure. This requirement is not intended to prevent people from intentionally attempting to defeat system safety features to reach the primary beam. Contorting to reach into, crawling into, or riding through the port into the system are examples of intentional defeat of safety systems.'
This is a useful clarification which the UK lacks explicitly. For the UK the judgement is again down to risk assessment – and ultimately perhaps demonstrating to the regulator (or customer) that the tunnel length (entrance to primary beam distance) will protect a ‘reasonable / sensible’ person.
The guidance to the standard goes on to state:
'A system with a straight tunnel that is at least 36 inches {91 cm} from any port to the primary beam complies with this requirement. A system with a straight tunnel less than 36 inches from any port to the primary beam should have some means other than distance to make the primary beam difficult to reach. One example includes incorporating photoelectric sensors that turn on the beam when interrupted, and placed so they are unlikely to be triggered by someone in a normal posture reaching into the cabinet. Another example is locating system ports lower (near the floor) or higher (at head height) than is conventional. Also, integrating the system into a production line where the products being examined move fast enough to limit access to the port itself.'
The UK does not have this level of guidance – something which would be very useful in the x-ray security screening industry. We are often asked ‘how long should we make the x-ray tunnel’. If we say ‘X’ and ‘X’ is too long (for the designer), we are sometimes asked to point to where this dimension exists in UK legislation. Of course – it does not!!
So where does this leave us – what is best? I think a mixture of good guidance, prescriptive and goal-setting legislation is best. Certainly a more prescriptive standard for ‘cabinet’ type x-ray systems in the UK would be welcome by many (e.g. manufactures and supplies). However, the reality is that x-ray screening systems produced and used in the UK certainly meet (in our experience) best practice, and exposures to workers who are resident around these devices for the majority of the year are unlikely to receive measureable radiation above background.
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There you have it. You can read the blog at http://www.ionactive.co.uk/blog.html" onclick="window.open(this.href);return false;
Mark
