IRR17 - Formal investigation levels (where you don't measure personal exposure with dosimetry)

Ionactive statement: This particular blog article is Ionactive musing and does not represent formal Radiation Protection Adviser (RPA) advice. We always intend our clients to follow the legal requirements set out in the Ionising Radiations Regulations (IRR17) - and this includes direction from our regulators. However, it is appropriate and interesting to challenge IRR17 and see if the legislation always makes sense.

Prelim

Not too long ago we were approached by a client who was in a bit of a fix with an x-ray unit (food quality assurance screening). They had the pleasure of a visit from HSE (Health and Safety Executive) and an improvement notice had been issued. We were brought in to act as RPA and help them improve their radiation safety systems within the notice period given by the regulator. We did this and new documents and responses were sent back to HSE. The good news is the regulator accepted the improvements and closed the case. The slightly less good news is that the regulator pointed out something that Ionactive (RPA) had written in the risk assessment - it was incorrect (or certainly worded incorrectly). No one likes to admit they are wrong, but from time to time we all are - and admitting it can be difficult! But here we go!

In the risk assessment we had written the following with respect to investigation levels "A dose investigation level is not formally required". The reasoning for writing this was that we had taken dose rate measurements around the unit and also in positions to simulate a shielding curtain failure (etc). Examples of measurements taken were as follows:

  • Instantaneous dose rate (IDR) around the surface perimeter of the unit were substantially < 1 micro Sv/h.
  • Tunnel area (just beyond beyond curtains) - finger dose rate of 0.3 micro Sv/h. This figure was also used to risk assess partial or complete shielding curtain failure.
  • Dose rate to hand at arms length into infeed / outfeed (prohibited but possible) - 10 micro Sv/h.
  • It was not possible (due to physical restriction and tunnel length) to place hand (or body) directly into main x-ray beam.

The conclusion from the risk assessment was that radiation exposures during normal routine use and in reasonably foreseeable events (e.g. curtain failure etc) would not yield exposures that were materially above background (i.e. if dosimetry had been worn, nothing would be recorded). This was further backed up by the weekly radiation monitoring, such that IDR measurements could be used to demonstrate the findings of the risk assessment. In effect, the IDR was acting as an 'investigation level' (i.e. approaching 1 micro Sv/h and beyond would lead to asking 'why?' and 'will this get and worse?' etc).

We also stated the following in the local rules.

"...For the purposes of the Local Rules, an investigation will be carried out for any unplanned / unexpected event and this will include dose estimation / calculation if relevant. In effect, an investigation level is taken to be ‘any suspected exposure above background arising from an unplanned event... "

However, the regulator responded that " ... a dose investigation level is a legal requirement under IRR17 Regulation 9(8)...". They did say that in this particular case the investigation level was '... quite arbitrary when dosimetry is not in use ...' and further suggested it was appropriate to set it at 1 mSv / year. Interestingly, the 1 mSv relates to whole body dose whereas the area of the body that could receive the highest exposures (small as they would be) are the hands in this particular case.

The regulator is correct, there is a legal requirement (for a formal investigation level) - therefore our wording above ("A dose investigation level is not formally required") was not correct (even though the meaning we were trying to convey has merit).

In addition it is quite clear that local rules require a dose investigation level (see next section where we look at what the ACoP states) . In this specific case formal local rules (as specified under IRR17) are in place because the area inside the x-ray cabinet is declared a controlled area.

[Ionactive comment: The issue of declaring the interior of small cabinet x-ray systems, regardless of findings of a risk assessment, as controlled areas is somewhat contentious in the radiation protection community. However, our regulator has been very clear that they expect controlled areas in these circumstances - this is the subject of another blog article to be released in the future].

So the 1mSv (effective) dose investigation level now appears in the local rules. The regulator did endorse our advice regarding investigating unplanned events.

All is well that ends well?! Well not quite. Anyone can just stick a dose investigation level in local rules - it takes a single line of text. Then what?

Since there is a requirement to always specify a dose investigation level in local rules, our view is that this must always be backed up with analysis and evidence that the level is being complied with (or not). It is not good enough to simply state the investigation level and then move on - to do so is meaningless. We would have preferred an opportunity not to necessarily state an investigation level (if IRR17 allowed) and instead pre-risk assess exposure and simply demonstrate that whatever happens a maximum exposure will be "x micro Sv/year". But we are not allowed to do this - so if an investigation level must be specified you need to have a process to detect when this level is about to be exceeded or could be exceeded. This must be demonstrated even when dosimetry is not being used (and it does not need to be used in our example - and the regulator agreed with this too). Whilst our analysis shows that the client would absolutely never exceed 1 mSv/year effective dose - what is the point of using this as an investigation level, unless there is a line in the sand that indicates 'the level has been exceeded, or is about to be exceeded'.

One way of doing this is to state the formal investigation level is any exposure measured above 1 micro Sv/h. We had this in the local rules, but unfortunately had not called this a 'dose (rate) investigation level'. And even if we had, it would not have met the requirements of the regulator - since it is supposed to be an accumulated effective dose according to IRR17 - 9(8).

Let's go and look where formal (dose) investigation levels appear in IRR17. 

Where formal investigation levels appear in IRR17

The term 'dose investigation level' appears once in the entire ACoP for the IRR17 regulations [para 336 'Local rules - essential content' - Regulation 18(1) - 18(2)]. Note that this is in the guidance section of the ACoP. The phrase 'essential content' is perhaps somewhat at odds with the status of guidance which is defined as follows (taken directly from the ACoP):

Guidance
The Regulations and Approved Code of Practice (ACOP) are accompanied by guidance. Following the guidance is not compulsory and you are free to take other action. But if you do follow the guidance you will normally be doing enough to comply with the law. Health and safety inspectors seek to secure compliance with the law and may refer to this guidance as illustrating good practice.

We are not suggesting that diverging from the guidance should be taken lightly or often. In most cases the use of a 'dose investigation level' (note the precise phrase - this is important later) is absolutely necessary in local rules. However, note that in IRR17 - 9(8) 'investigation' the phrase 'dose investigation level' is absent. This regulation says, in summary:

  • Ensure an investigation is carried out when the effective dose of radiation exceeds 15 mSv for the first time in a calendar year;
  • Or when a lower effective dose is specified by the employer is exceeded;
  • The dose must be specified in local rules, or in some other way if local rules are not required.

So what if the effective dose for routine work and reasonably foreseeable events is determined to be no greater than background over a complete year? What if the principal check on exposure is by the measurement of radiation exposure during a weekly dose rate survey with a radiation monitor, where the investigation level is stated as 'anything > 1 micro Sv/h measured at the surface'. Assume you had a choice to state one or other of the following options in the local rules (but not both):

  • 'The dose investigation level is set at 1 mSv effective whole body dose per year'. Or;
  • 'The dose rate investigation level is where any exposures are recorded at > 1 micro Sv/h as measured at any point on the surface'.

For the specific case we are discussing the second option is significantly more useful than the first! Yes - we can hear you shouting 'well just use both bullet points then!'. This is true, and probably the easy (and legal?) way out. However, we don't like putting information into safety documents (e.g. local rules) which provides absolutely no tangible benefit or use for the end user, RPS or management. The second bullet point provides significant tangible benefit, and it is this measurement (together with other maintenance checks on interlocks etc) which keep exposures at the ALARP level.

The risk assessment (and the above mentioned real time dose rate measurements) can prove that the dose investigation level (say 1 mSv whole body effective dose / year) cannot be exceeded. But does its placement, as stated in the first bullet point above, provide any safety value? Would placing it in the risk assessment be more suitable (like we did in the specific case which prompted this blog article?).

The role of the regulator is not an easy one for sure. Among many things they have to ensure that the law is obeyed as written in legislation. IRR17 and most other health and safety legislation is written with a goal setting approach - driven by the risk assessment.

In our specific example, radiation safety is driven and proved by taking regular dose rate measurements around the unit, and initiating an investigation if dose rates exceed our limit (1 micro Sv/h). This is a clearly stated requirement, set out in the local rules. There appears to be negligible benefit gained from stating a 'dose investigation level' (effective whole body dose accumulation) in the local rules. To do so implies that you must be able to measure the effective dose and implement an investigation if exceeded - this is not going to work with our example. Furthermore, if a dose investigation was set at 1 mSv/year effective dose (and recorded in the risk assessment), the above mentioned dose rate measurements would ensure that it could never be exceeded. This was our intention in this case all along - our target was that no person would ever exceed 1 mSv effective dose per year. And as stated above, the dose investigation level is only mentioned once in IRR17, in the IRR17 ACoP guidance - and following this guidance is not mandatory.

[Ionactive comment: The use of < 1 micro Sv/h in the above example, is also arbitrary. It is often used by suppliers / installers as a 'limit' but it is not recognised as a legal limit in the regulations. It appears in schedule 1 of IRR17 and specified at 10 cm from a surface, but this schedule is unusable for x-ray units since none are 'type approved by the executive'. However, the clear meaning of schedule 1 is to take certain ionising radiation uses out of some of the the regulatory regime (i.e. to register), so the < 1 micro Sv/h can be taken as a level approaching background -  providing negligible radiation risk (ensuring whole body effective dose over a complete year for a specific user is < 1mSv). ]

Now let's consider some other scenarios. 

Working with H-3 (tritium) in a university laboratory
 

Radioactive tritium (H-3) is a common radionuclide used in research  - particularly in the university and R&D sector. It is a low energy beta emitter which a beta max energy of just under 19 keV and a half-life of 12.3 years. A detailed radiation safety review of H-3 can be found on our website here: H-3 (Tritium) Radiation Safety Data (opens in a new window). 

Typical activities will vary with experimental aims and objects but will be something like this:

  • Stock activity may be up to 185 MBq
  • Sample activity may be up to 1.85 MBq

There is no external radiation hazard from working with H-3, and typically real time (active) monitors and passive radiation dosimetry will not record exposure.  To 'monitor'  for tritium will require use of liquid scintillation counting (LSC) (i.e. taking wipes / smears and then counting them on a LSC). This is an indirect measure of H-3 surface contamination, and apart from the issues with sample preparation and counting, it also requires assessment of pick up factors, determining areas to be wiped, counting efficiency and so on.  This is not an immediate measurement - and even when you have converted CPM (counts per minute) into DPM (disintegrations per minute), and then on to activity (Bq/cm2 etc), there is more interpretation required - since you still have NOT measured the exposure of a person working in the area!

[Ionactive comment: It is possible to assess intake of H-3 via bio-sampling (e.g. urine sampling followed by counting in a laboratory). However, at the levels used in a typical university or research establishments, this would not be the usual undertaking].

Following monitoring, and determining an H-3 surface contamination level (or absence of - i.e. background), further assessment is needed to assess the likely ingestion (or inhalation) of H-3 which might lead to an internal dose. For illustrative purposes, the following could be determined from the literature (e.g. ICRP, 2016. Occupational Intakes of Radionuclides: Part 2. ICRP Publication 137). For this article we are assuming tritiated water, other dose coefficients could be used (e.g. for organically bound H-3). 

  • Ingestion  - activity (MBq) for whole body effective dose of  20 mSv - 480 MBq
  • Inhalation - activity (MBq) for whole body effective dose of 20 mSv - 490 MBq

Other literature such as 'Delacroix et al Radiation Protection Data Handbook 2022' goes further and suggests maximum activities which might be suitable for certain laboratories or available equipment, such as (for tritiated water):

  • Supervised areas:  Bench (3 MBq), Fume hood (30 MBq)
  • Controlled areas: Bench (10 MBq), Fume hood (100 MBq) and Glove box (10 GBq)

[Ionactive comment: Long gone are the days from IRR85 where activity limits for surface, airborne and total activity holdings for individual radionuclides were specified - now it is down to risk assessment]. 

Overall, given the above values it is likely that much H-3 work will be conducted in a controlled area, with some in supervised areas. Regardless, common practice (Ionactive experience) is that such work is conducted under Local Rules (much of this will relate to environmental compliance, but recall that local rules are made under IRR17). 

And so we return to the theme of this blog article - where there are local rules there must be an investigation level specified as discussed earlier. That is, an effective dose should be specified - and this will be committed effective dose from the intake of radioactive H-3. 

So how are you going to tell when the H-3 intake dose investigation level is exceeded? 

You are going to do what we did with our x-ray client; you are going to find a suitable surrogate to show / demonstrate / prove that the investigation level cannot be (or is it not) exceeded.  A simple way of doing this might be where the total activity in the area is < 24 MBq (i.e. a full intake by inhalation or ingestion would not yield a committed whole body effective dose which would exceed an investigation of level of 1 mSv whole body committed  effective dose ). This would work for sure, but is somewhat restricted and unrealistic (and it's not ALARP). 

Instead you are likely to work on the basis of:

  • Where reasonably practicable demonstrate ALARP (i.e. all surface contamination should be cleaned down to background levels).
  • More likely, you may work to derived limits in Bq/cm2 which will then be based on transfer factors which take a maximum allowable intake (inhalation or ingestion) and then work back to derived working levels (loose and fixed surface contamination, airborne contamination etc). 

Whatever the method - you are not measuring effective dose. At best you are measuring a surrogate for it - let's say X Bq/cm2 of H-3 removable contamination from a surface. So the constraint implemented (e.g. stop work, carry on, undertaken an investigation) is very similar in principle to our food x-ray system outlined at the beginning of this blog (i.e. no more than 1 micro Sv/h around the perimeter surface of the x-ray unit). 

It's more meaningful / useful to use a dose investigation level based on a measurement of X Bq/cm2, rather than quote committed dose equivalent.

Working with security screening x-ray equipment in freight, cargo and airport sector

The observations in this section on security x-ray equipment are similar to that given earlier during our discussion on the food industry x-ray unit. It is true that many of the security screening systems have bigger enclosures, some of which can by physically entered and walked through (if permitted when x-ray generation is disabled). Most of them will take the form of a cabinet x-ray unit with entrance and exit points and one or more layers of leaded shielding curtains. Some systems may be fitted with labyrinth entry and exit points (e.g. "S-bends") to avoid the use of curtains (useful for light postal packages). Other systems will not confirm to the typical  x-ray cabinet layout - such as mobile and fixed linear accelerators for large cargo containers and lorries (these will not be considered further here). 

As noted earlier, the interior of the cabinet will be designated as a controlled area and this is regardless of the size of the enclosure or ease of person access (deliberate or otherwise).  The systems come under the full force of IRR17 and will require local rules, radiation risk assessment and contingency arrangements (noting that 'plans' are only applicable where a radiation accident is reasonably foreseeable).  

Note that passive or active dosimetry  for the operator is not typically used (the radiation risk assessment will conclude this). Service engineers are more likely to wear dosimetry as they could be removing shielding curtains or panels, or adjusting collimators underneath the cabinet cosmetic outer covers etc. 

As for the food x-ray unit, the industry standard requirement for dose rate is that it will not exceed 1 micro Sv/h around the perimeter (whether this is at the surface or at 10cm is mostly arbitrary). It is likely that more rigorous analysis is required where there are large shielding curtain displacements (e.g. with the larger cargo / freight units).  This may show that whilst instantaneous dose rate (IDR) exceed 1 micro Sv/h for a short duration during cargo / freight movement through the curtains, time average dose rates over an hour will be less than 1-micro-Sv-in-any-one hour (this is often determined by taking accumulated dose measurements over an hour at maximum workload). 

We are therefore in exactly the same situation regarding formal dose investigation levels. These will be specified in local rules, and will be expressed as whole body effective dose (i.e. typically an investigation will take place where 1 mSv whole body effective dose is exceeded for the first time in a calendar year).  However, as is obvious by now, it is other factors that will actually determine when an investigation takes place such as:

  • Obvious signs of damage to the shielding curtains.
  • Obvious signs of damage to the bulk structure of the x-ray unit. 
  • Where IDR dose rates exceed 1 micro Sv/h (away from areas which have variable dose rates such as entry and exit curtains).
  • Where time average dose rates exceed 1-micro-Sv-in-any-one-hour, as measured at the entrance and exit points of the x-ray cabinet.

All the above are tangible and sensible action points where an investigation might take place.  Despite this, the formal dose investigation level will need to be expressed as a whole body effective dose in local rules, as IRR17 requires. 

Working with x-ray inspection equipment in the electronics industry

These are normally cabinet x-ray inspections systems with an interlocked door (i.e. no infeed and outfeed or leaded curtain etc). Usually there is no exposure measurable above background around the perimeter of the cabinet (Ionactive findings for over 90% of units monitored). Suppliers will adopted the < 1 micro Sv/h around the perimeter of the unit. 

But - the expectation of our regulator (HSE) is that even the smallest interior cabinet x-ray space will be designated as a controlled area. This is in spite of the fact that:

  • Many such units contain at least two independent door interlocks.
  • Many of the newer units provide a solenoid door interlock such that the door is physically locked shut before any x-rays can be generated. 
  • Overall, no part of the body can have access to the interior of the x-ray cabinet when x-rays are being generated.

Since a controlled area is required, the x-ray unit will require local rules under IRR17, and these will state a formal dose investigation level (perhaps set at 1 mSv whole body effective dose for the first time in any calendar year ). Following the ALARP concept, and given the low exposure potential (low = no greater than background),  the investigation level might be set lower - say 0.2 mSv. But there is little point in a lower level - what are you expecting to measure, and from what circumstances? And how you will measure this? 

No passive dosimetry will be worn for this type of x-ray system, so you are left with real time active monitoring - perhaps verifying that < 1 micro Sv/h is maintained around the perimeter when measured once a week (or at other suitable intervals). 

[Ionactive comment: For this  x-ray equipment a risk assessment can show that no radiation accident is reasonably foreseeable. It can be further shown that no exposures above background will be received by anyone using the x-ray equipment, or anyone else close by, in all reasonably foreseeable circumstances - and certainly << investigation levels. Despite this, an investigation level must be set out in local rules, and this will be whole body effective dose. So you had better ensure you have a radiation monitor to hand to at least attempt to measure something!]. 

Final thoughts

We hope you have found this blog  / analysis useful. We hope you do not think we are being pedantic, but perhaps we are?! But if we are then so is the regulator, and perhaps the legislation too! 

Feel free to email comments / thoughts to [email protected] 

Mark Ramsay

Radiation Protection Adviser

Ionactive Consulting Limited

April 2024

 

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