X-ray cabinet sterilisation (irradiation) – UK registration or consent (IRR17)?
Published: Feb 23, 2025
Prelim
We have been meaning to write this blog article for some time. Some of the themes have been explored previously e.g. 'When is an industrial radiography walk-in enclosure not an industrial radiography enclosure (and therefore does not require an HSE consent)?' This looked at the use an x-ray facility for x-ray screening of objects, as compared to industrial radiography (taking place using the same room and equipment), and for each explored how IRR17 applies. This latest article explores industrial irradiation defined in IRR17(2) as 'the use of ionising radiation to sterilise, process or alter the structure of products or materials'.
We think this article is particularly timely as many users of the smaller irradiators containing HASS (high activity sealed sources), typically using Cs-137 in the 10’s TBq range, having been encouraged to dispose of them on security grounds – at significant cost (typically many £10’000s). Some may then find that the regulatory burden is less simplified than they may have first assumed.
[Ionactive Comment. This is the blog of Ionactive where we will from time to time vent our ideas, frustrations and discussions on radiation safety matters, principally in the UK, but often the articles apply far beyond. Our day job is providing Radiation Protection Adviser (RPA) advice and radiation training services. So whilst we will explore this subject matter and ask questions of ourselves, the operators of irradiators, and our regulators; we will professionally toe the line when providing official advice to our clients. At the time of releasing this article, if you come to Ionactive (RPA) and ask what you need in order to operate a desktop based x-ray sterilization system, we will state you need a consent (and so will the HSE). In our opinion this does not meet the graded risk approach that IRR17 introduced (i.e. notification < registration < consent). For the time being we appear to be stuck with this situation, so the idea of this article is to layout the radiological safety facts which might help simplify a consent application (and one day may result in the requirement being downgraded to a registration for radiation generators)]
If you need a reminder of the general requirements for consents, then please visit the following blog article 'New UK Consent process for users of Ionising Radiation'.
A word about terminology
The situations described in this article are more about ‘what you are doing’ / ‘what your equipment says it is doing’, rather than radiation physics, radiological risk or exposure potential.
The principle definition of 'industrial irradiation' is important. As noted above this is defined in IRR17 (2) and states specifically 'the use of ionising radiation to sterilise, process or alter the structure of products or materials'. It's also mentioned in IRR17 (7) which details consents and further mentioned in the consent information available from HSE and ONR, such as here: Work with ionising radiation that requires consent (HSE). Note that HSE deals with non-nuclear uses of ionising radiation, whereas the ONR deals with nuclear uses (i.e. on nuclear licensed sites).
With this key definition in mind, let’s start by considering a device (an ‘irradiator’) which contains a radioactive HASS source. Something like the following research irradiator illustrated below, which will contain perhaps 10-20 TBq of Cs-137.
Typical research irradiator using a radioactive Cs-137 HASS source
If the above device is used for the process of sterilisation (e.g. of cells in a scientific process) then it meets the definition of industrial irradiation and requires a consent. [Ionactive Comment: the term ‘industrial’ is a potential problem and causes some confusion for end users, who reasonably argue that their process is not ‘industrial’. Should IRR17 be amended in the future, it must carefully review definitions, and particularly specified practices].
If the device is used for calibration of dosimeters - the practice is not industrial sterilisation. Instead it is use of HASS sources. This is despite the fact that if you deliver 10’s Gy (for any reason) to a dose dosimeter, this may cause ‘sterilization’ or 'alter the structure' of the materials being irradiated. So the physics of the process is not so important, whereas what you say you are doing is.
Despite these definitions, using the (radioactive source) based irradiator for industrial irradiation (sterilization / cross linking polymers etc), or for calibration, requires a consent (either for industrial irradiation or for use of HASS sources). This discussion can be extended to users of e-beam systems (particle accelerators) which can be used for similar purposes (either requiring an industrial irradiation consent, or an accelerator consent).
As we shall now discuss, all the above definitions can be applied to ‘specified practices’ using x-ray tube technology (i.e. radiation generators). Most radiation generators (typically x-ray tube based) equipment (such as x-ray security screening, medical diagnostic units, x-ray inspection cabinets for quality control) normally require a registration (this being the medium radiation risk category). This is not that surprising since most (but not all) x-ray generator uses are within shielded cabinets, or specialised areas (hospital x-ray department). Overall exposure potential is low (excluding patient exposures which are an important sub-category, always optimised but will not be considered further here since they are not occupational exposure).
An example of a higher exposure potential using x-ray generators, is their use in open site industrial radiography (NDT), or in NDT enclosures that can be readily entered (i.e. walked into). Not surprisingly, this use is a specified practice and requires a consent for industrial radiography.
[Ionactive comment – even here things may be not be quite so clear. Revisit this the blog article 'When is an industrial radiography walk-in enclosure not an industrial radiography enclosure (and therefore does not require an HSE consent)? (noted earlier), since if you are screening a painting or geological specimen in an enclosure using ‘NDT type equipment’, you are not performing industrial radiography and you only need a registration – despite the radiological risks being more or less identical].
We will now turn our discussion to x-ray cabinets for sterilisation. To do this we will compare and contrast two units: one for sterilisation (industrial irradiation as defined), and one for imaging.
X-ray cabinets - comparison
After a chat with RPS Service LTD, we are considering examples of equipment they supply – the subject matter of this blog will obviously also apply to similar equipment and applications from other suppliers / manufacturers.
First up is the CELLRAD+ X-ray irradiator by Precision X-Ray which is shown below.
CELLRAD+ X-ray irradiator by Precision
CELLRAD+ summary technical details
Relevant statistics and features of this cabinet x-ray irradiator system are as follows:
- 10 - 150 kV (peak)
- 0.1 - 6.25 mA
- 940 W (max)
- Internal filtration: 1.5mm glass + 0.8 Be
Much of the available product literature does not specify the dose rate in Gy/min. However, research literature does provide limited useful data, such as 'Dosimetric characterization of an X-ray irradiator for use with cells'. In this paper we can find the following:
- X-ray generator set at 130 kV and 5 mA.
- Source to shelf distance (i.e. source to dose rate measurement point) = 33cm.
- 1.34 - 6.72 Gy/min (with and without 0.5 Al of filtration, respectively).
The dose rates were measured using dosimetry and you will need full access to the above linked article if you want to read the details of the measurements made.
Let's use the Ionactive x-ray widget to explore potential dose rates. To do this we will normalise the above dose rates to 1 m (assuming inverse square) for easier comparison. Identical results are not expected since the widget uses a minimum of 1 mm aluminium filtration, and it's modelling is a necessary simplification of the full physics. We then have the following:
- 0.136 Gy/min at 1 m with 0.5 Al filtration (130 kV, 5mA).
- 0.732 Gy/min at 1 m without Al filtration (130kV, 5mA).
Have a play below, this is a live interactive X-ray tube dose rate widget!
Let's input 130 kV at 5 mA with 1 mm Al (the nearest we can get with the widget). Try this if you like - the output will look like the image below.
Ionactive X-ray Widget (130 kV, 5mA with 1mm Al)
Taking results from the above image we have:
- 0.12 Gy/min (at 1m) for soft tissue.
- 0.6 Gy/min (at 1 m) for bone.
A hybrid dose rate ("tissue equivalent") can be obtained by weighting the above results for say 10% bone and 90% tissue (on the assumption that the dosimetry used in the above referenced paper may have been of HP10 specification). The result of this is then 0.168 Gy / min at 1m (pretty close to the reported dose rates in the research paper - considering the unknowns and allowances made). In fact, if the research data was reporting dose rate in air, then the measured dose rate is even nearer that reported by the widget.
However, whilst accuracy in actual dose delivered is necessary in experimental terms, it matters much less in terms of radiation risk - which is what we are considering here.
So let's go with 0.168 Gy/ min at 1 m (1.54 Gy min at 33cm inside the irradiator) at 130 kV and 5 mA as a reference.
Next up, we compare the x-ray irradiator with a Faxitron Pro+ (digital radiography system by Hologic) often used to image scientific or medical samples (such as breast tissue). This is shown below.
Faxitron Pro+ (digital radiography system by Hologic)
Faxitron Pro+ summary technical details
Relevant statistics and features of this digital x-ray radiography system are as follows:
- 20 - 100 kV (peak)
- 1 mA
- 12 W (max)
- Internal filtration: 0.25 Be
This system is far less powerful than the x-ray irradiator featured above - completely understandable given it's intended use. In order to make a comparison we have to manipulate some of the unit data, make some reasonable assumptions, and accept this is still very much a 'ballpark' figure.
The maximum power is 12 W. Therefore, to run the unit at 100 kV we can only run the x-ray tube at 0.1 mA. Alternatively we could run the unit at 50 kV and raise the current to 0.24 mA. Try the above figures in the x-ray widget (you will need to account for current by adjusting the output dose rates manually, since the minimum current used by the widget is 1mA). The results are as follows (adjusted for tissue type and calculated at 33 cm as described above for the irradiator).
- 0.0331 Gy/min at 33 cm (in air) [50 kV and 0.24mA].
- 0.0542 Gy/min at 33 cm ("tissue equivalent") [50 kV and 0.24 mA].
- 0.017 Gy min at 33 cm (in air) [100 kV and 0.1mA].
- 0.0265 Gy/min at 33 cm ("tissue equivalent") [100 kV and 0.1 mA].
Using the tissue equivalent data for comparison as we did for the irradiator, and assuming an average use between 50 and 100 kV we end up with 0.04 Gy/min at 33cm. The above analysis is probably too complicated given the unknowns, but we now end up with two figures we can use to compare the irradiator with the digital radiography system:
- Irradiation x-ray system: 1.54 Gy / min at 33 cm (i.e. inside the unit).
- Digital radiography x-ray system: 0.04 Gy/min at 33 cm (i.e. inside the unit).
It looks like the irradiator is capable of producing about 40 times the dose rate of the digital x-ray system . Let's look at this per hour (noting that total dose delivered inside each unit is dependent on the time of exposure).
- Industrial irradiation x-ray system: 92.4 Gy/h (inside the unit)
- Digital radiography x-ray system: 2.4 Gy/h (i.e. inside the unit).
And here is the thing: The accuracy of these figures is not that important, their relative size to each other is. In actual fact - both these dose rates are huge - and as far as occupational radiation protection is concerned - it makes little difference which one you consider.
But of course - you might be asked "What does it matter Mark, they are both in shielded enclosures where you cannot foreseeably receive an exposure!". You might very well think that; I couldn't possibly comment (but I will).
Before we finish considering dose rates - what are other typical dose rates from x-ray units, where actual exposure potential is greater (at least greater than from the two shielded x-ray enclosures considered above)?
Airport security X-ray screening systems
CT technology is now common place in airport security screening (for carry on baggage, hold baggage and cargo). Typical machine specifications will be:
- 160 kV (typical).
- 20mA (typical).
The above widget will report dose rates in the region of 32-36 Gy/h at 1 m (294 - 331 Gy/h at 33cm to normalise with the above discussion). This is from the raw uncollimated output of the x-ray tube, dose rates inside the security cabinet would be much less. How much less? Consider the following back of envelope assessment.
- Ionactive has taken numerous accumulated dose measurements from passing various radiation monitoring instruments and dosimeters through CT security systems. Typically dose per pass varies between 0.5-2 mSv depending on unit type, scatter, orientation and similar. Let's use 1 mSv / pass as a nice round figure.
- Typical conveyor speeds are of the order of 0.5 m/s.
- Assume the residence time in the beam of an average 'person like object' is 2 second.
- Dose rate is then 0.5 mSv / s or 30 mSv/minute.
- Or - 1.8 Gy/h. [Highly collimated]. [Ionactive comment: for the purposes of this blog article, we can interchange Gy and Sv]
Notwithstanding the various safeguards on the security x-ray units, this is a hefty dose rate. Not as high as the x-ray irradiator, or the digital radiography unit, but accessible. The interior or the x-ray irradiator and the digital radiography unit are not accessible (whole body or extremity) - whereas with the security x-ray unit, taking a ride through the screening tunnel is entirely possible (absolutely not recommended).
One of the safeguards on x-ray CT security systems is that if the conveyor should stop, so will the x-ray generator (such that the dose per pass is not exceeded). However, most units will have an engineering mode that allows the x-ray generator to run continuously regardless of conveyer status. So whilst unlikely, exposures to objects (or unauthorised persons) could soon become significant.
Operators of security x-ray CT systems required a registration for radiation generators, they do not require a consent.
X-ray cabinets - common radiation protection features
Despite the two desktop based x-ray units having different uses, they both share many common features for radiation protection - the most significant are summarised below.
- Radiation shielding (to bring radiation dose rates down to the 'industry standard' of <1 micro Sv/h at 10 cm from the surface). [Readers may find the following Ionactive blog article interesting: 'A dose rate of 1 micro Sv/h. A magic line or a bit of a nonsense?'].
- An interlocked cabinet door. Units like these are fitted with 2-3 independent failsafe door interlocks using diverse methods. Any attempt to open the door will terminate x-ray generation. Whilst not fitted to either of the units featured in this article, many higher powered cabinet x-ray systems use door locks in addition to interlocks - if these are fitted the door is physically locked shut and cannot be opened whilst x-rays are being generated.
- At least one active x-ray status light (normally fail safe).
- At least one alternative x-ray status (often software driven).
- An x-ray inhibit (and / or) key operated system switch. In addition, or as an alternative, a software driven password known only to those authorised to use the equipment.
- A way achieving system isolation (e.g. e-stop, system off button, or some equivalent means of isolation).
Despite the dose rate potential of the irradiator unit being some 40 times higher than the digital radiography x-ray system, overall they are very similar in concept - doing different jobs - but fundamentally achieving radiation protection by the same methods.
Which unit poses the higher radiation exposure risk?
[Ionactive comment: This is a deliberately provocative question - the short answer is that neither presents a significant radiation exposure risk. They are both well engineered units, with similar radiation safety features expected of quality x-ray equipment.]
But - you HAVE to make a choice - which!?
Well.. for the purposes of this blog article, the lower dose rate digital radiography x-ray system presents the higher risk as it has a leaded glass window in the door. The glass is more vulnerable than a steel / lead door and could crack / break / be partially removed. The unit would (could) still function - creating significant dose rates and exposure potential beyond the confines of the x-ray cabinet.
So .. the digital radiography x-ray system requires the higher risk consent from HSE? Surely this must be the case - the hub of the Ionising Radiation Regulations 2017 (IRR17) is reg 8 (risk assessment). We have just completed a risk assessment (a crude assessment for the purposes of this blog) - the exposure potential of the digital radiography x-ray system is higher. Yes?
Well no - according to the IRR17 notification, registration & consent regime.
Risk assessment under IRR17 does not drive notification, registration or consent. In the case of consent, a "specified practice" will drive the categorisation. Let's return to the definition of industrial irradiation : "the use of ionising radiation to sterilise, process or alter the structure of products or materials". Arguably, either of the featured x-ray units can create the conditions to [sterilise, process or alter structure], the dose rates are different, but we have not considered exposure time. However, consider the intention here:
- Digital radiography x-ray system - there is no intention to sterilise, process or alter structure - requires a registration.
- X-ray sterilisation system - the intention is to sterilise, process or alter structure (i.e. meets the definition of industrial irradiation) - requires a consent.
Frustrating? The desktop x-ray sterilisation system is placed into the same category as an industrial irradiator containing 185 PBq (5 MCi) of Co-60, or a 10 MV e-beam system, either of which are capable of delivering 25,000 Gy (or more) to a product. Each of these large industrial systems provide person entry (for maintenance and setup), and as you would expect, there are rigours search and lockup systems to ensure no person is left inside.
Is this deliberate? No. The HSE, the supplier and the user know that this is not the intention of the graded approach to radiation protection, which IRR17 implements. Quite simply, the desktop x-ray sterilisation unit has got caught up in terminology - this has nothing to do with radiation risk.
Further thoughts
Nothing discussed so far is likely to change any time soon. A change to IRR17 (a statutory instrument) requires parliamentary and regulator time and money - neither is available.
If a UK company wishes to purchase and use an x-ray unit which is used to sterilise, process or alter structure - then a consent will be required. A registration takes minutes to complete online (via the RADAD system), and costs £26 to process (assuming your risk assessment, RPA, RPS, local rules, contingency and other matters are in place). The consent is a very different prospect.
Note that a prospective user / purchaser cannot trial an x-ray irradiator (on loan) until they have consent. The only exception to this would be if a supplier installs the unit into a prospective purchasers site, which is demonstrated exclusively under the supervision of the supplier (this assumes the supplier has a consent).
[Ionactive note: Don't get caught out - if a supplier / installer is operating an x-ray irradiator under the following conditions..
- Operating on their own premises, or under their own supervision, (e.g. for demo, setting up, or similar).
- Operating on a customers site, prior to handover (e.g. for setting up, completing a critical examination).
- Operating on a customer / operators site (e.g. for service and repair).
.. then the supplier / installer / service contractor will need their own consent]
The full consent process is beyond the scope of this blog article, if you need help please seek advice from the supplier, or a Radiation Protection Adviser (RPA). A summary of the needs is as follows:
- Apply on the RADAN system for a consent.
- Supply HSE with a Safety Assessment, Local Rules and Contingency plan for industrial irradiation.
- Be prepared for HSE to inspect the workplace / x-ray irradiator.
- Be prepared to pay consent fees (> £1000's).
- Be prepared to wait a few months for a decision and hopefully issue of the consent.
[Ionactive comment: For all the reasons given in this blog article, a consent for a small desk top based x-ray irradiator should be processed and issued in weeks, not months, and for a fee proportional to actual HSE resource required]
All users of cabinet x-ray systems will need at least the following (regardless of registration or consent):
- Appointed RPA.
- Appointed RPS.
- Radiation risk assessment.
- Local rules (since the interior of the cabinet will be treated as a controlled area).
- Contingency plan (if a radiation accident is reasonably foreseeable, otherwise contingency arrangements for all other reasonably foreseeable radiation incidents).
- Appropriate information, instruction and training for operators.
Consent SA for desktop X-ray sterilisation unit
A consent application will require submission of a Safety Assessment (SA) for industrial irradiators. For large industrial irradiation plants the SA is complementary to a radiation risk assessment. For the small desk top x-ray sterilisers, if you have already written your risk assessment (using the risk terms in para 70 & 71 of the ACoP to IRR17), then you should have no issue completing the SA.
The SA will require information on the following (seek Radiation Protection Adviser advice if you are not sure - you will need to appointment one anyway). This is not formal RPA advice.
| SA Section | Ionactive Comment |
|---|---|
| (1) A summary of the type of industrial irradiation performed and where this takes place. Details of the radiation generator must be given. | Include a general description - make clear it is a desktop unit using an x-ray generator. Provide the maximum kV and mA. If known, provide the maximum dose rate that can be delivered (within the unit). |
| (2) A summary of the arrangements for managing radiation protection during industrial irradiation. | The operator (user) could be a small research company or a massive pharmaceutical organisation - so make this answer proportional. Detail the organisation (perhaps use a diagram). |
(3) Details of the likely frequency of use of the industrial irradiation sources of radiation to be used, or likely to be used. Projected maximum number of occasions industrial irradiation will be carried out per year at each location. Include accumulation of radon in the working environment and any other sources of radiation. | Frequency of use is likely related to the larger irradiators (using high activity sealed sources, or accelerators) - but it's an easy one to answer for the x-ray steriliser. Estimate use. Inclusion of the radon question occurs in all consent (and registration) applications. Whilst nothing to to with your intend use of x-rays, it is recognised as a significant contributor to occupational exposure in some parts of the UK - hence HSE will take the opportunity to make you think about radon. |
| (4) An estimate of the radiation dose rates to which anyone can be exposed (employees, members of the public etc). Confirm dose rates will be < 7.5 micro Sv/h outside the shielding of enclosures. Consider where the exposure may occur (eyes, extremities, whole body). Provide evidence that all exposures are ALARP. | Recall that the consent application is a 'one size fits all' for industrial irradiation. For the x-ray sterilisation unit, the short answer is - there will be no exposures to any class of person, measurable above background during routine use, and following any reasonably foreseeable unplanned event. You may wish to state that exposures will always be < 1 micro Sv/h at 10cm from the surface (read the product literature). |
| (5) A summary of the engineering control measures and design features already in place or planned (include all these features on a plan). | The wording and inference for this part of SA application leans towards a complex industrial irradiator. You have a desktop x-ray cabinet system - read the brochure, and talk to the supplier (and don't overcomplicate for the sake of it). Things to include could be the following:
If the supplier does not have suitable product literature, you may wish to provide an annotated diagram of the x-ray steriliser, with the above features highlighted. It is probably worth denoting the interior of the cabinet x-ray system as a controlled area (HSE will expect this). |
| (6) A summary of the maintenance and testing schedules (for all features specified in section 5). | The supplier of the x-ray steriliser should provide information on the routine maintenance required (e.g. annual maintenance visit plus operator daily checks if relevant). Use this information to write an answer for this section. |
| (7) An summary of the results of any critical examination, or planned critical examination including pass / failure criteria). | A reasonable assumption is that you will not already have an x-ray steriliser (unless you have a consent already under the former system pre-RADAN, and have been asked to upgrade). You will not be able to operate the x-ray steriliser until you have the consent. It is possible it's been installed prior to this, and therefore has a valid critical examination (by the supplier who should have their own consent). Either include the full critical examination report (if available), or ask the supplier to detail what the critical examination involves (they might provide a blank pro forma report detailing the checks and pass / failure criteria). |
| (8) A summary of the planned radiation dose rate monitoring regime for irradiation areas and their surroundings, including any areas to which the public may have access. | Whilst not all users of cabinet x-ray systems will conduct their own routine radiation monitoring (i.e. leaving it to the RPA / service engineer etc), since this is an industrial irradiation consent, there will be an HSE expectation that the operator will do their own routine monitoring. So be prepared to purchase a radiation dose rate monitor - the actual monitoring process will be simple, verifying, for example, that < 1 micro Sv/h at 10cm from the surface remains the minimum standard to achieve. You will need to detail the monitoring schedule, how records will be maintained, and the actions to be taken if the dose rate criteria is exceeded (switching off and unplugging is a good start!). |
| (9) A summary of the planned personal dosimetry to be provided to employees and others. | There will be no personal dosimetry required for operation of the x-ray sterilisation unit. This should be clearly demonstrated in the IRR17 risk assessment. Note that routine personal dosimetry is not required for nearly all x-ray cabinet systems, including those with open ports (e.g. x-ray security screening units). |
| (10) The rationale for designating employees as classified persons. | There will be no requirement for any operator of the x-ray sterilisation unit to be designated as a classified person. This should be clearly demonstrated in the IRR17 risk assessment. |
| (11) A summary of the radiological protection training that will be, or has been provided, to employees and other persons, including planned frequency and refresher training. | Radiation protection related training will be required as follows:
Such training should be refreshed over a period of 3-5 years. |
| (12) A summary of the information that will be supplied to employees concerning their work with ionising radiations in connection with pregnancy and breast feeding and how you will communicate this information to them. | Ionactive always advises that reassurance information / training (some of which will be presented in the local rules), is always available with respect to pregnancy. A radiation risk assessment for this unit, will show that there would be no restriction of work by an operator of the x-ray equipment who declares they are pregnant. Information (and restrictions) regarding breast feeding is irrelevant with regards use of the x-ray sterilisation unit (no radioactive material is involved). Whilst the above advice is obvious, still show this is true by considering this specifically in a radiation risk assessment. |
| (13) A summary of possible radiation accident situations as identified in the radiation risk assessment, their likelihood and potential severity. | [Ionactive note - the definition of radiation accident is as follows '...where immediate action would be required to prevent or reduce the exposure to ionising radiation of employees or any other persons...'] In our view there are no reasonably foreseeable radiation accidents - and therefore a contingency plan is not likely to be required. This should be clearly justified in the radiation risk assessment. Disconnection from the power supply will terminate x-rays whatever the circumstances. There may be reasonably foreseeable radiation related events (following evaluation by risk assessment). Examples could include:
If any of the above are deemed reasonably foreseeable they should be mitigated via contingency arrangements (don't call it a plan as that infers you have a radiation accident). |
None of the above elements of the SA need completing for the digital x-ray radiography system. It only needs a registration (just don't go breaking that glass shielding window ...).