Potential occupational, non-occupational and accidental radiation exposures in industrial radiography using radioactive sources

General radiation safety aspects of using radioactive sources in industrial radiography

We are not experts in the practice of NDT (industrial radiography) so will not discuss contrast sensitivity, focus-to-film distance (ffd), object-to-film distance or similar. In this section we will discuss some general aspects of radioactive source use in NDT by first considering the projector.

Use of a projector

Generally a projector is used to store the radioactive source for transport and as a means of remotely deploying the source to make an exposure and then retracted back safely for the next exposure (also known as a "shot"). Whilst some projectors are designed for transport within their own right [e.g. Type B (U) Certification / Type A Approval (IAEA TS-R-1) ], others are designed so they have an outer shielding overpack which must be used to achieve designed transport compliance.

The projector will contain a tungsten (or depleted uranium) "S-bend" source shield. When the source is safely stored it will be in the centre of the bend (so no 'line of sight' out of the projector). Upon attaching a source guide tube to the front of the projector, and a source drive cable to the back, it is possible using a handle (or motor) to drive (push) the source out of the projector, and then retract (pull) the source at the end of the exposure.

Mechanical safety locks ensure that the drive cable has to be securely attached to the projector before the source can be exposed.

Whilst details will vary between projector models (and their age), the back end of the unit will generally have the following features:

• An unlock / lock key or equivalent.
• Turning to 'unlock' allows the source drive cable to interface with the projector.
• The drive cable can then be securely connected to the back end of the source cable. The radioactive source is permanently attached to a small length of drive cable (known as a pigtail) which rests within the S-bend, with its connector (pigtail connector) visible at the back of the projector when the security cover is removed. The longer drive cable connects up to the source when attached to the projector.
• Mechanical safety features ensure that the connection between the shorter pigtail and longer drive cable is secure before the source can leave the projector.

For some industrial radiography shots, such as panoramic radiography, the source guide cable at the front of the projector is replaced with a panoramic collimator. In this configuration the whole projector is placed into position (e.g. inside a large steel pipe), and the source is then remotely driven out into the collimator.

The following graphic presents some of the detail just discussed.

In the graphic above we are showing a panoramic collimator fitted onto a projector which has been placed inside a wellhead pipe. You can just see the drive cable leaving the back of the projector, this has been run through the wellhead, and you see it again disappearing into an exit hole in the wall. This graphic therefore depicts an example of enclosure radiography. We will show further features of this enclosure later in the blog.

Below we see a representation of the source moving in the projector from storage position, around the S-bend and out into the collimator.

In the next graphic we see a representation of the radiation streaming from the source through the collimator and onwards through the pipe. We have already mentioned that this is in an enclosure, so have a think about where the diagonal gamma ray path might be heading.

Industrial Radiography Enclosure

Let's now reveal the enclosure we were talking about earlier. The first view is from the side.

And below is a top view of the enclosure. At this point you may already be able to note three possible radiation exposures from this setup.

• Occupational exposure - potential sky-shine dose rates at ground level caused by scattered radiation above the enclosure --> impact on the radiographer.
• Non-Occupational exposure - potential sky-shine dose rates at ground level caused by scattered radiation above the enclosure --> impact on other employees working in the area.
• Accidental exposure - an unplanned exposure to a person working outside the enclosure.
  

We will also consider direct occupational exposure of radiographers working inside the enclosure.

Sky-shine is something we will also consider later, but follow the link if you want to review this phenomenon right now.

Ideally this enclosure would have a shielded roof. However, there are some circumstances where this is not possible when moving massive objects in and out. With calculation, local shielding, and suitable workflow, dose rates outside the enclosure can be shown to be ALARP, and in any case below the level of a controlled area. Despite not having a shielded roof, this set up is much better then relying on open site radiography.

Open Site Industrial Radiography

Open site radiography could be represented as shown in the following graphic.

The projector equipment represented above is little different to that already discussed. The main difference is that with site radiography the source tends to be deployed by manual means using a handle (hand crank) as shown, whereas with the enclosure radiography the source deployment is driven by an electric motor which is interlocked into the enclosure safety systems (more of that in a moment).

Site radiography features include:

• Demarcation of the area by a barrier.
• Passive warning signs placed around the perimeter (Controlled Area - Gamma sources / Keep Out etc).
• Radiation protection mostly provided by maximising distance from the source.
• Radiation protection further enhanced by collimation of the source and local shielding (and possibly also the intrinsic shielding provided by the radiographic object).
• Visible active warning signs (e.g. strobe lights).
• Audible indicators (e.g. audible warning before source is about to be deployed).
• Real time radiation monitoring of the perimeter by radiography technicians (as a minimum looking for < 7.5 micro Sv/h instantaneous dose rate).

All the above protective features rely on human factors - suitably qualified and experienced persons doing the right thing. This is why enclosure industrial radiography is preferred (as safety is enhanced with engineered safety systems). For this reason we will now consider some of the engineered safety features of enclosure radiography which make it so much more preferable.

Radioactive sources used in industrial radiography

There are three main radionuclides used in industrial radiography - in descending order of energy they are:

• Cobolt-60 (Co-60)
• Iridium-192 (Ir-192)
• Selenium-75 (Se-75)

Co-60 and Ir-192 are beta / gamma emitters, whereas Se-75 decays by electron capture, yielding gamma rays. In all cases it is the gamma rays which are important when performing industrial radiography.

The choice of what to use is outside the scope of this blog and into the realms of NDT, but we can provide a few considerations here. Generally, the thicker & denser the material and / or the shorter exposure time required, you would choose a higher energy emitter. This is not quite true in all cases since a relatively thin lower density material, with a high energy gamma emitter, might not produce the contrast required. However, what is true is that for a given activity (say 1 GBq) and distance (say 1 m), the dose rate will be higher as the gamma energy increases. It is also true that as the energy increases the 10th value thickness (TVT) for a given material (e.g. steel) will also increase.

To be compatible with the principles of radiation protection, the minimum activity and gamma energy emitter should be chosen which still meets the radiographic (NDT) requirements of the work. However, this is not always achieved as highlighted in the following examples:

• The time of each 'shot' will be influenced by activity, so putting NDT work (especially open site radiography) between other work may lead to higher activities being used.
• Particularly for Ir-192 with a relatively short half life (74 days), there is a temptation to purchase sources of higher activity (and therefore dose rate output), to make best use of the source over the following couple of months.

All three radioactive sources, at the activities used in industrial radiography, will be classed as HASS (High Activity Sealed Sources). In the UK this will mean that users will need a HASS permit from the relevant environment agency of the devolved administrations. In turn this will require a significant security regime which will be inspected by the relevant agency and the police via the CTSA (Counter Terrorism Security Adviser). In terms of health and safety under the Ionising Radiations Regulations 2017 (IRR17), use of the sources will require a consent from HSE. Since the specified practice is 'Industrial Radiography' it is for this practice where the consent is held, and not for HASS sources (use of HASS sources which does not include industrial radiography or industrial irradiation has their own consent category).

All this leads us to state that industrial radiography radioactive sources can be considered highly dangerous and must be careful controlled by suitably qualified persons under the permitting / consent regime. X-ray NDT is not the subject of this blog as stated at the start, but it is clear there is a preference for x-ray where reasonably practicable - since this removes the security challenges of HASS sources since the radiation hazard will cease the moment power is removed. The dose rate output of high kV / mA x-ray tubes can also be highly dangerous too, but radiation protection is still much simplified. Reasons where x-ray tubes may not be suitable include:

• The energy (kV) or rate of emission (mA) [both of which combine to give dose rate] may not be high enough for the NDT image required.
• X-ray tubes tend to be heavy and bulky, and when combined with the heavy duty HT cable (which may also contain cooling air / water), may not fit into the space required to make the shot.
• The location may not have a suitable power supply for the x-ray generator (more likely for open site radiography).
• Required collimation and minimum / maximum focal size may be more difficult to achieve with x-ray (noting the physical size of the radioactive source is tiny compared to an x-ray head). Typical dimensions of an Ir-192 source used in NDT will be about 6mm diameter and up to 24mm in length (this is for the outer capsule).
  

Occupational exposure scenarios

In this section we will describe several occupational exposure scenarios. Some are based on generic assumptions and general data, others are more related to the radiographic equipment and sources we have highlighted thus far.

Occupational exposures from handling projectors

Anyone who has worked with a projector will know that handing duration is minimised by the fact they are heavy and have manual handling risks. Once the projector has been moved into position (e.g. from storage location / transport van), the ALARP principle requires that exposures are minimised by maximising the distance between the operator and projector.

The maximum dose rate from the surface of a typical radiography projector will be < 2 mSv/h (depending on package category). A transport index (TI) of 0.1 will infer a dose rate at 1m from the projector of 1 micro Sv/h, and an index of 1.0 will infer a dose rate at 1m of 10 micro Sv/h. Both these TI values would mean the projector was a category II-yellow and that the surface dose rate was between 5 micro Sv/h and less than 500 micro Sv/h.

A typical example of a projector might be 10 Ci (370 GBq) of Ir-192 and in our example below has a transport index of 0.1 (1 micro Sv/h at 1m).

For most of the time it is reasonable to assume that the NDT technician is at least 1m from the projector, and at 1 micro Sv/h implies occupational exposure around the projector will not be significantly above background.

There will be times where the projector is carried from a vehicle to the the work area. Given the mass of the protector, this will not be a long task - let as assume 1 minute of manual handling per task, 5 days per week, 50 weeks per year (by the same technician - assumed to be a critical person). Whilst the inverse square law will not work perfectly with such a 'large' source (projector), we can use it to infer a whole body dose rate which might be located some 10cm from the projector. This is simply 100² / 10² x 1 micro Sv/h which yields 100 micro Sv/h.

This could present an occupational whole body exposure of 417 micro Sv/year. There is also the work required around the projector when connecting up the source drive cable etc, however since this is now much less of a manual handling issue, using ALARP to maximise distance (i.e. working at arms length) is reasonably practicable (so dose rate probably tends nearer to the 1 micro Sv/h).

Whilst carrying the projector there is also a potential extremity dose to the fingers. In our example we might assume the surface dose rate is up to 500 micro Sv/h. Whilst the handle is a few cm's away from the surface, 500 micro Sv/h seems a usable upper level for our example.

[Ionactive comment. We note that for a II-yellow projector, the TI can vary from 0.1-1 and the surface dose rate will vary from 5 micro Sv/h up to just less than 500 micro Sv/h. This will imply that with a TI of 0.1 the surface dose rate will be nearer 5 micro Sv/h because each increase in TI (i.e. 0.1-0.2, 0.2-0.3) would linearly increase the dose rate by just less than 50 micro Sv/h each time. However, our analysis is not based on real measurements so it is easier to use the maximums in each category (therefore we choose a surface dose rate of 500 micro Sv/h regardless of TI between 0.1-1].

Using the same analysis above we find a potential dose uptake of about 2mSv/year to the extremities. Not huge compared to the legal limit of 500 mSv/year equivalent dose to the extremities (and noting the classified person threshold of 150 mSv/year).

[Ionactive comment: We do wonder what percentage of industrial radiography technicians would routinely use extremity dosimetry whilst carrying the projector?].

This example is based on a relatively modest 370 GBq (10 Ci) of Ir-192 so the dose rates near the projector are likely to be less than specified above. Recall that the projector might be rated up to say 5.55 TBq (150 Ci) of Ir-192 and the higher activities could result in a TI of between 1 and 10 (suggesting up to100 micro Sv/h at 1m and up to just less than 2 mSv/h on the surface). Ionactive experience shows these higher activities are generally less common and / or take place in an enclosure where there may be less manual handling of the projector. It would therefore not be appropriate to simply factor up our estimates based on increased activity.

We will now consider some site industrial radiography based on our 370 GBq Ir-192 projector initially, and then explore different activities and radionuclides.

Occupational exposures from enclosure industrial radiography

[Ionactive comment. A modern NDT gamma source enclosure with shielded roof and HASS source security provision (i.e. projector does not need to be taken to and from the enclosure each day), should not need to yield occupational exposures above background. They can be engineered such that exterior dose rates are < 1 micro Sv/h which tends towards background (nominally 0.05 - 0.1 micro Sv/h) at 10 cm from the surface. For this blog we will consider our open topped NDT enclosure which might not be ideal in all cases, but was reasonably practical for the purpose it was designed for at the time].

A reminder of the layout of the enclosure.

Enclosure shielding materials

Where the enclosure is physically small and / or where space is restricted, it is likely that lead will be the preferred shielding material.

For this blog we will assume that the TVT for lead with Ir-192 is 20mm (note: this does vary and some references report as low as 10mm).

[Ionactive comment: For those that want to delve in deeper and consider TVT and its discrepancies, you may want to read our resource article here: 'How reliable is TVT (10th Value Thickness) in radiation shielding calculations?'. The problem with references is that they propagate around the internet and often don't show where they are derived from - this is why we resist putting references into these blog articles unless we are certain they are reasonable. In the case of Ir-192 with lead, the TVT is reported as 20mm in table 6.4, page 6-15 (Exposure & shielding from external radiation) from the publication - 'Handbook of Health Physics and Radiological Health' (3rd edition). Our link noted above goes into TVT in some detail, but basically considers matters such as buildup, the '1st TVT' (TVT₁) and the 'Equilibrium TVT' (TVT_e). In the publication 'Design and Shielding of Radiotherapy Treatment Facilities' IPEM Report 75, 2nd Edition TVT₁ is reported as 11mm and TVT_e is reported as 19mm (table 10.4, page 10-11). It is no wonder that references (or those quoting them) may pick and choose. What is important is real world data, and NDT technicians will know very well what actually works! The same discussion can be made for other materials such as concrete - but we will park this matter for another day].

We know that 3 TVT (60mm lead for Ir-192) will provide a 1/1000 fold reduction, so considering the dose rate at 1 m from the projector (see earlier), this would reduce 41.82 mSv per hour (at 1m) to 41.62 micro Sv/h. If the average wall distance was at least 3 m from the source, and the source was in the middle of the enclosure (and its minimum distance from the wall restricted by engineered means), this dose rate could be reduced further by 1/3² (1/9) which would give us about 4.62 micro Sv/h. Then introduce the collimator from the earlier discussion on site radiography (6 HVT = 1.807 TVT = attenuation of 0.0156), and the dose rate on the external perimeter of the enclosure drops to 0.07 micro Sv/h (i.e. background). So Lead is possible, but consider the following:

• To be cost effective (and structurally sound) lead is only really suitable for smaller enclosures.
• The dose rates calculated above were for 370 GBq (10 Ci) of Ir-192, so the enclosure would need a site based license (or equivalent) for maximum allowable activity.
• Restricting source distance from wall needs engineered means (e.g. short source guide tube), but this might restrict the size and orientation of the radiographic object.
• Use of the collimator will rely on human factors (i.e. using it) - this needs to be carefully considered and justified, or the work may not meet enclosure radiography conditions (might still be considered site radiography).
• Access to a relatively small enclosure would usually require a sliding door rather than a maze. Suppose the lead in the door was 1.8m (high), 2.5m (width) to allow movement of radiographic objects in and out, and 0.06m (60mm) thick then the door (without its steel frame) would have a volume of around 0.27 m³. Since lead has a density of 11400 Kg/m³, the minimum mass of our door would be around 3000 kg (3 tonnes). That is a lot of door to slide back and forth, even with the help of pneumatic systems.

So lead is possible - but not for our open top enclosure. So let's now look in more detail at its shielding.

The shielding for this facility was designed with the aid of computer modelling software. For this blog we will keep things simple and consider just empirical factors. The shielding detail are as follows:

• Inner wall 21.5 cm of 1.9 density concrete block.
• Middle area 46 cm of 1.7 density compacted sand.
• Outer wall 21.5 cm of 1.9 density concrete block.

The height of the walls is 2.5 m (we will discuss potential sky shine later in this blog).

Standard density concrete is generally taken to be 2.35 g cm^-3 (we call this 2.35 density, where water would therefore have a density of 1).

Let the TVT for Ir-192 at 2.35 density concrete be 14.7 cm. This is taken from table 6.4, page 6-15 (Exposure & shielding from external radiation) from the publication - 'Handbook of Health Physics and Radiological Health' (3rd edition).

To a good approximation we can correct for density in concrete by considering ratio of densities. Therefore the TVT for 1.9 concrete with Ir-192 is inferred to be - 18.2 cm. For simplicity, we will assume the sand is equivalent to concrete at 1.7 density (whereas the actual shielding model considered sand explicitly). So we can infer that 1.7 density sand provides a TVT of 20.3cm.

[Ionactive comment. Use density ratios with care! Generally at reasonably high photon energy (> 511 keV) you can ratio densities to adjust TVT of the same material type. Don't ever try this using say lead and concrete density ratios as it will not work. For a fuller explanation please have a read of the following Ionactive resource: Can I use density ratios when working out radiation shielding thickness? Since we are increasing TVT with lower density, at worse we will overestimate the shielding required].

With the above data accepted, we can determine the total TVT provided as follows:

Inner wall - TVT is 21.5/18.2 = 1.2 TVT

Middle (sand area) - Total TVT is 46 / 20.3 = 2.3 TVT

Outer wall - is 21.5/18.2 = 1.2 TVT

So the total TVT offered by the wall for Ir-192 is 4.7 TVT.

Using the attenuation equations earlier, we can see that 4.7 TVT has an attenuation factor of 2 x 10^-5 (0.00002) (Note: attenuation in this example = 10^-4.7). We will now examine this attenuation in our shielding example.

[Sanity check on TVT - attenuation. Suppose we have a dose rate of 1,000,000 micro Sv/h. An attenuation of 0.00002 (2 x 10^-5) would reduce this to : 20 micro Sv/h].

We will now position a 370 GBq (10 Ci) Ir-192 source completely unshielded inside the enclosure and consider the shielding performance in terms of dose rate outside the wall (sky shine over the top will be considered later). The next graphic provides some dimensions we can work with.

Occupational exposure dose rates through enclosure shielding

We note that the distance between source and calculation point is 3.9m and this is through 4.7 TVT (0.00002 attenuation as shown above). We have specified the dose rate for 370 GBq (10 Ci) of Ir-192 as 41.82 mSv/h at 1m earlier in this article. Following the simple calculation methodology shown earlier we apply both the inverse square law and attenuation.

\[ Wall dose rate=\frac{1}{3.9^{2}}\times 0.00002\times 41820=0.055\mu Sv/h\]

Therefore dose rate is reduced to background levels through the wall. Note that the dose rate without the shielding wall would be 2750 micro Sv/h at 3.9m from the source. Also note that the wall shielded dose rate is for an unshielded and uncollimated source with no radiographic object.

[Ionactive comment: Depending on workplace circumstances, this dose rate could also be used to calculate non-occupational exposures to those not involved in the radiography].

Occupational exposure dose rates - shy shine (scatter)

We now consider dose rates outside the enclosure which are from sky shine (scatter) over the top of the shielding wall. The values reported below are from actual measurements taken around the enclosure. We will compare some of this data with calculated values - however we will not detail the calculation methodology for sky shine as it is way more complicated than anything so far considered and outside the scope of this article. For a more in depth consideration, please see this Ionactive resource: Skyshine – radiation scattering around and over shielding (we will use a picture from this resource which is shown below).

The following graphic illustrates some key dimensions for consideration outside the enclosure (these are not to scale - just believe them!). They show:

• 3m from dose point (green circle) to red perimeter line (no work above 2.5m in this zone).
• 3m from the red perimeter to the red platform (critical worker is located such that the trunk of their body is 3m above the ground).

The data shown below is real dose rate measurement results from skyshine for a 370 GBq (10 Ci) Ir-192 source. In this blog article we will only consider dose rates in one direction (as shown above). We could limit dose rates in other directions with a collimator, and this would reduce some of the radiation over the top of the bay. However, ultimately this facility is for radiography of large bore pipes with a panoramic collimator. This type of collimator will:

• Reduce radiation exposures either end of the bay.
• Reduce the scatter above the enclosure either side of our (green) measurement spot.

It follows that if we are aligned with the source (with or without panoramic collimation), the dose rates being considered will tend to a maximum.

Some initial observations.

• With the exception of the 3m height measurements, peak dose rate is somewhere between 4-6m from the shielding wall (there is a shadowing effect close to the shielding wall).
• For the 3m height measurements, highest dose rates are near the wall and drop of with distance (not surprising as the wall offers less of a shadowing effect at this height).
• Notwithstanding exposure time (considered shortly below), dose rates are significantly above 7.5 micro Sv/h in areas where employees external to the enclosure may work - this is not acceptable (or ALARP).
• Always consider skyshine and monitor further enough away from the shielding wall to ensure you capture the highest potential dose rates.
• The observations above could also be used for assessment of non-occupational (i.e. for other persons not involved with work involving ionising radiation).

We now consider the scenario where the Ir-192 source is inside an 18mm steel wall wellhead pipe. For this facility 18mm steel is the smallest thickness available, more common wall thicknesses available vary between 25-50mm.

The dose rate measurement results show an expected reduction - the overall pattern of dose rates is similar to the completely unshielded source. We now see that waist and head height dose rates are substantially below 7.5 micro Sv/h. The working at height (3m) dose rates, from 6m onwards are substantially < 7.5 micro Sv/h, and whilst there is no intention of working at height close to the enclosure (i.e. inside the red permitter line), dose rates 4-5m away are still significantly above 7.5 micro Sv/h. This would be likely rectified with thicker walled wellheads.

For example, suppose the steel wall thickness was 32mm (i.e. 32mm-18mm = 14mm thicker). We note from earlier that the HVT for Ir-192 with steel is about 13mm. Therefore, we would expect, at the very least, for a 1 HVT reduction in dose rates (i.e. at least 50% reduction in the dose rate values noted in the table above).

However, it was decided to build a shielding canopy to be manoeuvred over the radiographic object. This was built to run on rails either side of the wellhead to ensure that its use became an integral part of the radiographic procedure, rather than an afterthought. Early on it was decided that the canopy would be used for wellheads with wall thickness below 35mm steel, however once work in the enclosure was underway a decision was made to use the canopy regardless of wellhead wall thickness (this reduces the risk of the canopy not being used when required).

Have a look at the dose rate data below with the shielding canopy in place over the 18mm wellhead. The construction details of the canopy is given afterwards.

Let's explore the shielding canopy in more detail.

Much earlier in this article we have specified TVT Steel as 43 mm for Ir-192. We have also specified TVT lead as 20 mm for Ir-192.

The shielding canopy is made from 12mm steel, 6mm lead another 12 mm steel. So we have 24mm steel in total and 6mm lead. The attenuation provide by this canopy (in theory) can be expressed as follows below. [Note: In practice the shielding canopy should do better (or at least as well as) the calculated attenuation since it is dealing with lower energy scatter and not just primary radiation from the source].

The attention expressions for the lead and total steel present can be expressed as follows:

\[ Attenuation(Pb)=10^{-\left(\frac{6}{20}\right)}=0.501\]

\[ Attenuation(Fe)=10^{-\left(\frac{24}{43}\right)}=0.277\]

The total attenuation (lead plus steel) is the product of the values which is 0.14 (rounded).

How does this compare with the measured values when using the canopy? Pretty close it seems! Try using the 0.14 attenuation and applying to the pre-canopy table above.

[Ionactive comment: Throwing some additional shielding over the top of a radiographic object would be more appropriate for site radiography than enclosure radiography. However, in our example the canopy is engineered for a specific type of work and runs on rails either side of the laydown markers for the wellhead. Just as 'search and lockup' (looking for a person left in the enclosure upon exit) is a procedure (with associated human factors), so is the use of the canopy. Therefore in our specific example the canopy is an integral part of the process and use of the facility and so enclosure radiography is justified. This conclusion will be investigated further below when we consider potential exposures to those outside the enclosure when radiography is underway].

Likely exposures to those inside the enclosure setting up the radiographic shot and operating the system from the control panel outside the enclosure

This is clearly occupational exposure.

This can be estimated by looking back at our data for the radiographic projector and our discussion on open site radiography. However, the actual exposures will be much less due to:

• No exposure to flash dose rate during source deployment and retraction.
• Exposure very near background around the permitter of the enclosure and ground level (including the control desk area) during the exposure.

Our estimate (based on a workload which will be described below) is that a radiography technician operating only this facility for 8 hours / day, 5 days a week for a full working year, would not receive an occupational exposure measurable above background. This is an interesting conclusion when assessing exposures to other persons some distance from the enclosure which we will now consider.

Likely exposures to those working away from the enclosure and who have nothing to do with the radiographic work taking place in the enclosure

We will first consider some workload data. In the last table of data we have shown that with the shielding canopy, regardless of type of wellhead, instantaneous dose rates (IDR) are significantly < 7.5 micro Sv/h. With the 18 mm (wall thickness) wellhead, the highest IDR (in our direction of interest) is 3.5 micro Sv/h (at 3 m height in the red perimeter restricted zone). The highest waist level dose rate is 0.4 micro Sv/h (at 4 m from the enclosure wall) and the highest permitted working at height (3 m) dose rate is 0.6 micro Sv/h (at 6 m from the enclosure wall). This data might be easier to digest by simply looking at the last table of dose rate data provided. We will use these values to explore exposures in these areas.

Workload

It was determined that a panoramic shot for the 18 mm wellhead would need 100 Ci.min. Therefore, the duration of each shot will be 10 mins (based on the 370 GBq / 10nCi Ir-192 source).

It was further determined that each wellhead "weld" would need 3 shots, so total duration of radiography was 30 mins per wellhead.

Radiography technicians determined that the total set up time (i.e. craning wellhead into the enclosure, setting up the radiography, performing the radiography and craning wellhead out of the enclosure) would take 2 hours (120 minutes). Therefore the radiography 'source exposure' period is 30 minutes in every 120 minutes.

At full working rate there could be up to 90 minutes of radiography source exposure time in an 8 hour working day (i.e. 3 wellhead over 6 hours plus 2 hours of breaks / rest / maintenance / paperwork etc).

[Note: A typical 50mm walled wellhead will need up to 250 Ci.min, which would mean an exposure time of 25 minutes per shot. However, the additional attenuation of scatter from this well head will be significant. The additional steel is 32mm, so the additional attenuation is 10^(-32/43) = 0.18 (revisit earlier if you need to consider how this works). Whilst the exposure time for the thickest wellhead is 2.5 times more than for the thinnest wellhead, this is compensated by the additional attenuation (nearly 5 times more). For this blog article we will just stick with the thinnest wellhead considerations].

Dose assessment

We will present the dose assessment as a weekly exposure - this can be multiplied by 50 to provide an annual value.

Total exposure time in a week is [5 days / week x 90 minutes / day exposure] = 450 minutes / week (i.e. 7.5 hours / week).

• Consider Person CP₁ standing 4 m from the wall (see picture below). If they were located at this point for the whole week the weekly waist (trunk of body) exposure would be [0.4 micro Sv/h x 7.5 hours / week] = 3 micro Sv/week (150 micro Sv/ year). Exposure to the head (eyes) would be [0.5 micro Sv/h x 7.5 hours / week] = 3.75 micro Sv/week (188 micro Sv/ year). This assumes 100% occupancy which is not realistic for this work location (if an admin office was located at this position it might be more valid). So actual exposure will be substantially less than this.

• Consider Person CP₂ standing 6m from the wall where their trunk position is at an average 3m above the floor (see pictures earlier). If they were located at this point for the whole week the weekly waist height (trunk of body) exposure would be [0.6 micro Sv/h x 7.5 hours / week] = 4.5 micro Sv/week (225 micro Sv/ year). Since the height may vary, and due to lack of further data, we will attribute eye dose to the same value (225 micro Sv/year). The trunk dose is 75% of our dose constraint of 300 micro Sv/year (whole body dose). However, this assumes 100% occupancy of the critical person in this position which is not realistic. Actual weekly / annual exposure is likely to be considerably less than predicted here.

This blog article provides examples of an assessment process - if this was for real similar assessments would take place all around the enclosure. Before we move on it is appropriate to briefly consider again the occupational exposure potential of the radiographic technicians who are working outside the enclosure, in the region of the control panel.

Consider the picture below and the analysis which follows.

• The radiography technician is standing / sitting 0.5m from the wall next to the control panel. The average dose rate in the area was measured as 0.15 micro Sv/h (close to background). If they were located at this point for the whole week (after setting up each radiographic shot) the weekly whole body exposure would be [0.15 micro Sv/h x 7.5 hours / week] = 1.13 micro Sv/week (57 micro Sv/ year). This assumes 100% occupancy at this point which is a more reasonable assumption compared to those who are not occupational exposed (since they should be in close control of the work at all times).

The IDR specified above was 0.15 micro Sv/h - this is so close to background that many radiation monitor users would quote background (especially if the monitor had an analogue scale). Therefore the quoted exposure over a year is unlikely to be distinguishable from background.

What is interesting about this result is that likely exposure to the radiography technician outside the enclosure is less than the calculated exposures to those - not occupationally exposed - further away from the enclosure. Skyshine - it has a lot to answer for! That said, we have shown that for compliant use of the enclosure, all exposures outside the enclosure are less than our 300 micro Sv/year whole body dose constraint.

[Ionactive comment: a word about radiation monitoring. In this blog article we have used instantaneous dose rate (IDR) measurements to assess likely annual exposures for a number of scenarios. However, whilst the article is based (more or less) on a real facility, the monitoring conducted was significantly more rigorous than might otherwise be suggested. For example, accumulated dose measurements were used to support IDR measurements. A radiation monitor was set to measure 'dose' and not 'dose rate'. Over a complete morning of representative radiography the monitor accumulated a dose value - this was background corrected with another monitor placed in another building. This accumulated value was then integrated over a week and / or year. Furthermore, during the enclosure trials and after commissioning, passive environmental dosimeters were placed around the enclosure including positions at 3 m (as illustrated earlier for CP₂). Annual predicted exposures from accumulated dose were very similar to those calculated from IDR measurements. This is not surprising since the exposure periods were long (10 - 25 minutes) so there was little variation of dose rate over time which an IDR based measurement might miss].

What about exposures at the entrance gates to the enclosure? Note the following picture and the analysis which follows.

If you look at the above figures and picture you can see that the annual 'gate' whole body dose is predicted to be 57 micro Sv/ year. We have already stated that this is unlikely to be measurable above background. This would also assume 100% occupancy which for the two gate locations is nonsense. For 10% occupancy there is no accumulated exposure measurable above background in these areas.

We have arrived at the end of this section considering occupational exposures (and also non occupational exposures to other workers). We are now ready to consider accidental radiation exposures in industrial radiography - considering both open site and enclosure examples.

Route causes - why radiation accidents in industrial radiography?

There are some useful publications which have analysed this question. One such publication is IAEA Safety Report Series 7 - Lessons Learned from Accidents in Industrial Radiography (opens in a new window). This publication is from 1998 and we would hope that significant improvements in NDT safety have been made since. Accidents of the type described shortly are thankfully very rare in the UK. However, we have seen poor practice whilst on work trips to the Gulf well into the 2010's, so across the world there are still significant improvements to be made.

Another resource to take a look at is here: IAEA - The information channel on nuclear and radiological events (opens in a new window). At the time of writing this blog we note that the latest event was posted 6 December 2023 'Worker Exceeded Annual Whole Body Dose Limit' and involves a 2.33 TBq (63 Ci) Ir-192 source. This incident was due to poor training and supervision and resulted in a whole body dose of 7.5 mSv whole body dose and 258 mSv to the extremities.

[Ionactive note: We really do encourage you to take a look at the IAEA events database from time to time. Whilst it was designed originally for recording nuclear plant incidents (and such events are still recorded), you will note that many of the events are non-nuclear and are mostly concerned with accidental exposures during industrial radiography and use of ionising radiation in medical applications. This is where significant radiation exposures are being received, not from the nuclear industry].

Generally industrial radiography radiation accidents appear to have the following route causes (these are written in descending order of likely occurrence).

• Poor regulatory control.
• Not following procedures.
• Poor (or no) training.
• Poor (or no) maintenance.
• Poor (or no) use of monitors & dosimetry (this features in most of the other categories and examples below).
• Human error.
• Faulty radiographic equipment.
• Equipment design issues.
• Deliberate poor practice.

We will explore these in a little more detail and supply a working example of each. After this we will return to our own previously described sources and look at some accident scenarios involving them.

Not following procedures

Despite the fact that many radiography contractors around the world are regulated, there is still evidence that some do not follow the procedures they have written (or procedures written for them). Here are a couple of examples from this category.

• During 1993 a radiographer received 3 Sv (3 Gy) to his right hand and 12 mSv whole body dose following an incident. He was working with a 3600 GBq (97 Ci) source of Ir-192. Something was clearly wrong as his real time dosimeter (EPD) was alarming - however, for some reason he did not respond to this (as required by the procedures he was supposed to follow). He also appears to have ignored an off scale indication on his radiation survey monitor. He tried to correct the problem rather than contact the company radiation protection officer (also a requirement of the company procedures he failed to follow).

• During 1994 an industrial radiography technician was working on a night shift with a 780 GBq (21 Ci) Ir-192 source. He appears to have had difficulty in locking the projector into a safe state. His alarming EPD (active dosimeter) was reading off scale, but a defective radiation monitor was not responding. To achieve the projector locking he desired, he hit the device with a hammer. He then (presuming the projector was now safe) left the area to obtain a working radiation monitor. Upon returning to the radiographic area he noted the alarming EPD was still off scale, the replacement radiation monitor was also not working, and the locking mechanism on the projector was still defective. His passive radiation dosimeter indicated 8.5 mSv whole body exposure.

Clearly procedures were not followed in the above examples. However, they appear to demonstrate lack of training and maintenance of equipment.

Poor or inadequate maintenance

Thankfully the latest radiographic source projectors provide fail safe features to minimise unplanned exposures by ensuring the source remains securely attached to source drive cables etc. Whilst all equipment should be maintained as per manufacturer guidance, poor maintenance of older equipment can be particularly troublesome.

• During radiography the source assembly of a projector become stuck in a cut in its guide tube. This damage had not been detected prior to use. The radiographer disconnected the tube and shook it vigorously with both hands in an attempt to release the source. Three fingers on left hand and two on the right hand were exposed up to 8.8 Sv (8.8 Gy), requiring the injured fingers to be amputated. Estimated whole body dose was 0.1 Sv (100 mSv).

• A radiographer and assistant were working with a 3000 GBq (80 Ci) Ir-192 source. Upon completion of radiography the assistant dissembled the equipment, placed it in a truck and drove back to base. Upon removing projector from truck and carrying to storage facility, the source assembly fell out of the projector and onto the floor, as the unit was tilted and placed onto a shelf. A radiation alarm in the storage area immediately sounded and action was taken to recover the source (safely it appears). The subsequent investigation revealed that the unit had undergone no recent maintenance, and the spring loaded latch designed to secure the source in the shielded position was defective (the latch was jammed in the unlocked position). In addition, the projector shutter position was 'on' and the dust cap was not installed. A combination of poor maintenance, defective equipment and poor training resulted in the radioactive source falling to the floor. In this particular case use of the radiation alarm probably saved the individuals from further exposure as they reacted quickly and correctly to the situation upon the alarm sounding.

Faulty radiographic equipment

• A radiographer was using a 2700 GBq (73 Ci) Ir-192 which failed to retract into the expected safe shielded position. This was caused by a 'crank-out' failure where the gearbox of the winder separated from its inner liner. The situation was recovered by the operator cutting the housing near the gear box and then pulling the source (via the inner drive cable) manually back into the desired shielded position. No overexposure occurred and this incident was referred to the regulatory authority.
  
  
• Whilst a source projector was being wheeled on a trolley back towards it's storage room, a 200 GBq (6 Ci) Ir-192 source fell out due to a defective locking mechanism (the radiographer did not notice this). Two canteen workers found the source and picked it up - seeing the trefoil symbol they reported their discovery. Dose to their fingers was estimated at 8 Gy, whole body dose 200 mSv.

Deliberate poor practice / wilful negligence

It appears to be rare (thankfully) that a worker would deliberately be negligent with a radioactive source, especially if their training or knowledge highlights the significant harm from doing so. Unfortunately the same cannot be said for a small group of employers who would readily allow such bad practice due to work pressure, lack of resources, time constraints, contractor fees etc. Furthermore, there will be others with no particular knowledge of the risks who will steal equipment containing radioactive sources with little awareness of the health effects to themselves or others they pass the equipment on to.

• In the early 1990's an organization deliberately allowed untrained individuals to perform industrial radiography at their site (this was a breach of known procedures and local rules). Following a radiographic disconnection between source assembly and drive cable, the untrained individuals did not act as would have been expected if trained and following procedures. This resulted in a whole body exposure of 20 mSv to each, with the hand of one individual having a calculated exposure of 900 mSv. No individual was wearing an alarming dosimeter and they had received no training on how to use a radiation dose rate monitor.
  
  
• During 1991 a 26 GBq (0.7 Ci) Ir-192 source assembly stored in a lead shield was stolen from a storage pit, together with the unloaded source projector. The theft was unnoticed for several days and the authorities did not identify the thieves. The source assembly changed hands several times and ultimately was discovered at a scrap dealers shop. Scrap dealers estimated whole body dose was 200 mSv, members of the public were estimated at a maximum of 35 mSv.

This ends our examples of radiation incidents / accidents involving industrial radiography. Whilst we have only provided a few it seems apparent that:

• Most of the incidents involve open site radiography, rather than enclosure radiography.
• Most incidents involve use of radioactive sources rather than radiation generators (e.g. x-ray sets).
• By far the most popular radionuclide involved is Ir-192.
• Compared to our own industrial radiography case studies given earlier, the activities are much higher. Our own Ir-192 source under consideration is 370 GBq (10 Ci), whereas Ir-192 sources feature in the above examples were up to 7400 GBq (200 Ci).

We will now complete this blog article by returning to our own featured radiography source projector (for open site radiography) and radiography and consider the dose rates / doses involved in certain accident / incident scenarios.

Accidental radiation exposure / incident - site radiography

Firstly, a quick reminder of what we are working with.

• 370 GBq (10 Ci) Ir-192 radioactive source in a projector.
• 41.82 mSv/h at 1 m from totally unshielded source.

Note that the activity here is considerably less than some of the sources featured in our review above. Recall that you can scale the activity and dose rate - so a 3000 GBq (80 Ci) Ir-192 will yield a dose rate of about 335 mSv/h at 1m (all other things being equal).

A reminder of the workplace situation is given in the picture below.

We are not going to feature source recovery methods in any significant detail (qualified NDT technicians will know all about this). We will consider the concepts involved, and in particular when they are not followed. Generally where a high activity sealed source (HASS) is in an uncontrolled or unsafe situation the following concepts will always be employed:

• Minimise exposure time so far as is reasonably practicable.
• Maximise distance from the source so far as is reasonably practicable.
• Use shielding (either local shielding placed over the source, or shielding features such as staying behind the source projector if this provides a shielding advantage).

Even for very high activity sources (i.e. much higher than the 370 GBq / 10 Ci source featured in our example), the literature review shows that whole body exposures leading to deterministic radiation effects (radiation sickness) are still thankfully rare. Unfortunately the same cannot be said for localised exposures, particularly to the extremities (fingers), and there are many cases where fingers have been amputated following severe radiation damage. So as illustrated below, it is absolutely vital that industrial radiography sources (or the guide tube containing them) are never directly handled.

A small word about the inverse square law

All the calculations so far , and those soon to feature, use the simple inverse square law which can be stated as:

\[\frac{1}{r^{2}}\]

where r is the distance between the radioactive source and the point of interest. It can also be stated in words as follows:

• Doubling the distance between the point of interest and the source will quarter the dose / dose rate.
• Halving the distance between the point of interest and the source will lead to four times the dose / dose rate.

It is important to note that this assumes a point source. Whilst the above concept is simple enough, care should always be taken to ensure you have a point source (if calculating scenarios for risk assessments etc). If you want to consider this further the following Ionactive resource might be of interest:

• Inverse Square Law - when is a source a point source?

Essentially the above link states that you can consider inverse square law mathematics to 'work' where the distance between the source and point of interest is 10 times the largest dimension of the source. Typically the outer capsule dimension of an Ir-192 source used in industrial radiography will have a diameter of about 6 mm and length about 19mm. The 'inner' (radioactive) capsule will have a diameter of about 5 mm and length of about 6.5 mm. So we might use 6.5 mm as our critical source dimension which would imply the inverse square law is good enough for distances down to 65 mm (6.5 cm) from the source. This is adequate for whole body dose / dose rate assessments, but might not be so for extremity dose / dose rate calculations. However (!) at distances less than 6.5cm (for our specific example) you will over estimate dose rates. So generally, at distances < 10 times the longest dimension of the source you will overestimate the dose / dose rate (typically by 30% up very close - we will get someone else to look at the maths!). This is therefore a cautious approach (literally!).

[Ionactive comment - in a real event never rely on calculation alone. Calculation is good for creating scenarios, training, risk assessments, writing contingency plans and blog articles. In a real event you must always measure using a suitable 'tested' (i.e. IRR17 annual test) radiation monitor. When using a radiation monitor up very close (or upon) a radioactive source you may significantly underestimate the true dose rate (particularly if you were trying to evaluate extremity dose). Much of this is to do with geometry, the size of the source compared to the size of the detector, and how close you can actually get the detector (inside the monitor casing) to the source. So please do not mix up our discussion on point source calculations and real time radiation monitoring - calculations can overestimate, monitoring up close can underestimate!].

Stuck radioactive source

Recall our 'flash dose' discussion from earlier in this blog. This was the transient dose rate caused by the Ir-192 source moving from the source projector to the collimator (and back again at the end of the exposure). A reminder of the geometry is shown below.

Recall earlier that we said dimension A was 7.8 m. We noted that the dose rate from a 370 GBq (10 Ci) Ir-192 source was 41.82 mSv/h at 1m. So with simple inverse square (equation given earlier) we can show that the dose rate at the barrier will be 687 micro Sv/h. However, this is no longer a transient dose rate or flash dose rate, the source has become stuck at point A due to a undiscovered kink in the source guide tube. So we have a constant dose rate of 687 micro Sv/h at the boundary and it's not going away - the controlled area has therefore moved and you are now standing inside it! Whilst you stand for a moment considering what has just happened you are acquiring a whole body radiation exposure at the following rates:

• 11.45 micro Sv / minute

• 0.19 micro Sv / second

[Ionactive note: If any qualified industrial radiographers are still reading this blog at this point, we thank you very much for staying with us! The next few stages of this example may be far from what you would actually do - there is no suggestion you would demonstrate poor practice. However, for the purposes of this blog we are particularly interested in practice that falls below required standards since this is will most likely to lead to accidental radiation exposures].

Let's think about moving the barrier back to the 7.5 micro Sv/h point. This is probably not that practical and would require consideration in all four directions. We can borrow an expression from earlier :

\[d_{2}=\sqrt{\frac{D_{1}d_{1}^{2}}{D_{2}}}\]

In the expression we have:

d₂ = the distance from the source to the barrier (required)

d₁= the current distance between source (A) and barrier = 7.8m

D₁- the dose rate currently at the barrier (687 micro Sv/h)

D₂- required dose at the barrier (7.5 micro Sv/h)

Note that distance (B) - see above graphic - was the original distance between the exposed source (collimated at the radiographic object) and the the original barrier position (9.3 m). You might want to revisit this earlier in the blog.

Plugging the values in we see that d₂= 74.6m !

Reality check - don't believe the above answer? We know the dose rate at 1 m unshielded is 41.82 mSv/h (41820 micro Sv/h). So just try a 1/d² relationship as follows :

\[ Dose rate(7.5\mu Sv/h)=\frac{1}{74.6^{2}}\times 41820=7.5\mu Sv/h\]

As you see, we get 7.5 micro Sv/h as expected at 74.6 m.

At point (A) we are moving from 7.8 m from the source to 74.6m from the source, so an increase of 66.8m in all directions.

Totally unworkable in most workplaces. Something has to be done, and the above analysis shows that rather than move away from the source (which works but does not fix anything), the technician will have to move closer to the source to solve this situation.

Let's walk towards source - here are some expected dose rates as we move nearer. Note there is some rounding in the figures presented.

• 7.5 micro Sv/h at 74.6 m from the source.
• 10.2 micro Sv/h at 64 m from the source.
• 2.61 mSv/h at 4 m from the source.
• 10.46 mSv/h at 2 m from the source.
• 41.82 mSv/h at 1 m from the source.

Assume that a technician is standing at 2 m from the source observing the scene to see if there is anything obvious (such as something heavy fallen on the guide tube). Consider the exposure rate:

• 174 micro Sv/minute.
• 2.9 micro Sv / second.

Assuming an active dosimeter is being worn then it would (should) be alarming vigorously at this point (i.e. dose rate alarm). It would also be accumulating at a significant rate as illustrated. Arms out stretched (perhaps holding a radiation monitor) would receive a higher dose rate, but we are still in the region of whole body exposure at this point (rather than considering extremity exposure specifically).

Consider a move to 1 m from the source for a closer look. This is now the time to be very careful since if we assume an outstretched arm is 65 cm, then the finger tips could be within 35 cm of the source where exposure rates are becoming dramatic. Whole body exposure at this point would be as follows:

• 696 micro Sv / minute.

• 11.6 micro Sv/ second.

[Ionactive comment 1. At this point the technician might throw some shielding at the problem. For example, they could consider using a bismuth shielding blanket with a reported HVT of 1.25 (as noted earlier). This would reduce dose rates to about 42% (0.5^1.25 = 0.42) which is helpful but not a dramatic improvement at these higher dose rates. Use of the blanket also potentially obscures the problem - so for the time being we will assume no temporary shielding is being used].

[Ionactive comment 2. We were once asked "...would it be worth wearing a leaded apron like they do in hospitals ?" The answer is a NO! A typical heavy duty shielding apron in a hospital x-ray facility might offer a lead equivalence of perhaps 0.7 mm. Let's assume the TVT for lead and Ir-192 is 20 mm (references given earlier), the apron provides 0.035 of a TVT (0.7/20=0.035). This provides an attenuation of 0.92 (10^-0.035 = 0.92) which is hardly impressive (8 % reduction) and not very worthwhile - whilst adding wear fatigue. So no, industrial radiography technicians do not carry 'leaded aprons' in their emergency kit bag!].

Let's explore cumulative exposure whilst considering our problem source.

Assuming the barrier has still not been moved, we know from above analysis that the dose rate at 7.8 m from the stuck source is 687 micro Sv/h. We also know that the dose rate at 1 m from the source is 41.82 mSv/h. Imagine the following situation:

• Technician stays static at barrier for 1 minute collecting thoughts and checking equipment. [Stage 1]
• They then walk to within 1 m of the source - a distance of 6.8 m towards source. We assume this takes about 7 seconds (that is slow careful walking speed). [Stage 2]
• They then stay at 1 m from the source for 15 seconds determining the potential problem (during this time they notice the kink in the source guide tube). [Stage 3]
• They then swiftly about turn and walk back to the barrier (distance of 6.8 m) at a slow walking pace taking 7 seconds. [Stage 4]
• They wait a further 1 minute at the barrier - collecting tools, radiation warning signs and similar. [Stage 5]
• They then walk (move) the barrier back to the 7.5 micro Sv/h point (74.6 m from the source). They do this at a slightly faster walking pace of about 1.5 m/s (so taking about 1 minute 7 second, or 67 seconds). [Stage 6]

We could go further with this assessment but will leave it at this point - you get the idea. Also note that without additional shielding the barrier in all directions would need to be moved out - for the moment we will assume this can be done (but probably unlikely for many workplaces undergoing site radiography).

Note that stages 1,3 and 5 above are easy to calculate - a fixed dose rate for a fixed period of time. Note that with stages 2,4 and 6 the dose rate is changing with time and distance (i.e. dose rate is changing as the technician moves to and from the source). Let's do the easy bit first, then ponder the changing dose rate with distance and time.

• Stage 1 : (60/3600) x 687 micro Sv/h = 11.45 micro Sv.
• Stage 3: (15/3600) x 41820 micro Sv/h = 175.25 micro Sv.
• Stage 5: (60/3600) x 687 micro Sv/h = 11.45 micro Sv.

So far so good (or bad!). We have an accumulation of 197.15 micro Sv.

We have noted that the technician should (would) have an alarming and accumulating active dosimeter which will accumulate total dose during this period (so similar figures are expecting as compared to the maths above and below). We now need to assess dose accumulation during stages 2,4 and 6. For this we need calculus (!) - but our good friend Chris Robbins has done the hard work for us. To look at how this all works consider visiting this Ionactive resource: The accumulated dose when moving up to a source (written by Chris Robbins, opens in a new tab).

It turns out we can use the following expression (this is the same as the linked resource, but we have just changed the letters used to fit with this blog article):

\[ Cumulative[\mu Sv]=\frac{D_{1}T}{d_{s}d_{f}}\]

In this expression we have:

D₁- Dose rate at 1m from the source (important, this particular expression needs this)

T - Total time of exposure (h)

d_s - start distance (from source in m)

d_f - finish distance (from source in m)

The total exposure time for the 'trip' can be estimated from the distance travelled and a chosen steady walking pace as we have estimated above. We will try this with stage 2 - written in full.

\[ Cumulative[\mu Sv]=\frac{41820\times\frac{7}{3600}}{7.8\times 1}=10.4\mu Sv\]

Stage 4 is calculated in exactly the same way (it is just the trip in reverse) so also has an accumulated dose of 10.4 micro Sv.

Stage 6 is walking the barrier backwards to the 7.5 micro Sv/h point (74.6m from the source, at about 1.5m/s so taking 67 seconds). Recall that the original barrier position is 7.8m from the stuck source. Writing in full we have:

\[ Cumulative[\mu Sv]=\frac{41820\times\frac{67}{3600}}{7.8\times 74.6}=1.3\mu Sv\]

The above stage 6 calculation shows the significant advantage of increasing distance and not hanging around whilst you do it!

The total exposure for moving towards source, observing, moving back to barrier, collecting tools and signage and moving the barrier further back is (in micro Sv) : 197.15 + 10.4 +10.4 +1.3 = 219.25 micro Sv (so about 219 micro Sv). Not trivial by any means.

Returning the stuck radioactive source to its shielding (using cv reachers)

Recall that this particular blog is looking at incident / accidental exposures, so as highlighted earlier the aim is not necessarily to show best practice in source recovery - rather it is designed to show the potential exposures possible in such circumstances.

It has been decided that another technician (T2) will approach the source and try to 'bend the kink' in the guide tube the opposite way to free up source movement, allowing it to be returned to the source projector. This means for a short duration they will be very close to the source and their hands closer still. They will first try this using a pair of 1 m length Cee Vee reachers, but concede they may need to get their hands directly on the guide tube to bend the kink out. Meanwhile, the first technician (T1) will try and wind in the radioactive source using the handle (still at the original location since they do not have a longer source drive cable available). This is roughly (!!) depicted as shown below.

Note that the body of T2 is about 1.7 m from the source this allows the 1 m cee vee reachers to just about reach each side of the source guide tube (assuming arms of atom man are about 0.7m length).

• The 'work trip' for T2 is to travel from 74.8m to 1.7m from the source (i.e. a distance of about 73.1m in 73 seconds). They spend one minute attempting to manipulate the source guide tube, before returning back to the 7.5 micro Sv/h barrier (set at 74.8 from source).
• The 'work trip' for T1 is to travel from 74.8m to 7.8m from the source (i.e. distance of about 67m in 67 seconds). They spend 73 seconds at this location, trying to use the source winding handle (note they started off from the barrier at the same time, hence the slightly longer stationary duration as T2 has to travel further). They then return back to the 7.5 micro Sv/h barrier.

Unfortunately this procedure fails to move the source. However let's analyse the dose uptake in this scenario. We will jump over some of the calculations steps for this example and just provide the accumulated doses, the methodology is exactly the same as outlined in our calculation on accumulated exposure earlier. You just need to recall that the dose rate at 1 m from the unshielded Ir-192 source is still 41.82mSv/h.

• T2 whole body dose for 'trip' - 6.7 (towards source), 241 (at source), 6.7 (away from source) = 254.4 micro Sv.
• T1 whole body dose for 'trip' - 1.3 (towards source), 13.9 (at handle), 1.3 (away from source) = 16.5 micro Sv.

Note that the exposure to the hands of T2 (approximately 1 m from the source) whilst they manipulate the cv reachers over a period of 60 seconds will be 697 micro Sv.

Returning the stuck radioactive source to its shielding (using hands to manipulate source guide cable)

Remember (!) - this article is about accidental exposures, not necessarily what a trained industrial radiography technician would do. The exposure assessment coming up will demonstrate the importance of what happens if you do not take a suitable approach to source recovery.

T1 and T2 choose NOT to swap roles (should they have dose shared ??).

T2 decides that they will 'quickly' manipulate the source guide tube, either side of the source, by hand to remove the 'kink' and free the source. They estimate that their body may be 0.7 m from the source and their hands 20 cm from the source either side.

T1 has the same role / position as before at the source winding handle. This is roughly (!) depicted below.

• The 'work trip' for T2 is to travel from 74.8m to 0.7m from the source (i.e. a distance of about 74.1m in 74 seconds). They spend 30 seconds attempting to manipulate the source guide tube with their hands, before returning back to the 7.5 micro Sv/h barrier (set at 74.8 from source).
• The 'work trip' for T1 is to travel from 74.8m to 7.8m from the source (i.e. distance of about 67m in 67 seconds). They spend 33 seconds at this location, trying to use the source winding handle (note they started off from the barrier at the same time, hence the slightly longer stationary duration as T2 has to travel further). They then return back to the 7.5 micro Sv/h barrier.

For this part of the assessment we have to carefully consider the whole body and extremity cumulative dose for T2. For T1 the "trip" exposure is very similar to the previous cv reacher example.

It so happens that this procedure does not work either - but let's still analyse the exposures. As before we will skip some of the mathematical steps (these were explained in detail earlier).

• T2 whole body dose for 'trip' - 16.4 (towards source), 711 (at source), 16.4 (away from source) = 744 micro Sv
• T2 extremity dose to fingers - 8.71 (at source), = 8.71 mSv (to & fro exposure ignored as trivial)
• T1 whole body dose for 'trip' - 1.3 (towards source), 7.1 (at handle),1.3 (away from source) = 9.7 micro Sv.

As you see the extremity dose to the fingers of T2 is now substantial at 8.71 mSv for the 30 second exposure.

Returning the stuck radioactive source to its shielding (touching source by mistake)

As we have noted above, nothing has worked so far. We are now going to go one stage further, and this is only about T2. By miscalculation, T2 actually places his right hand on the source (nominally 1 cm from the source within the guide tube). What are the exposure implication of this? This is depicted in the picture below.

To a reasonable approximation the body of T2 is still 0.7 m from the source - so for the 30 second exposure the dose is 711 micro Sv (as before, less the transit to and fro doses). Whilst whole body exposure is still important, the critical exposure is now going to be the extremity dose to the right hand.

The dose rate at 1 cm from the source is going to be 418 Sv/h. For a 30 second exposure, the dose to the fingers of the right hand will be about 3.5 Sv (rounded). We could actually write this as 3.5 Gy as this is now in deterministic radiation injury territory. Reddening of the skin can be expected, but at these (estimated) distances and durations, only a slight deviation (in the wrong direction) could make things even worse. For example:

• A duration of 60 seconds at 1 cm from this source would yield a dose to the right hand of 7 Gy (most certainly injury territory).
• If a finger of the hand was actually at 0.5 cm from the source, then the dose rate might be 1672 Sv/h and a 30 second exposure would yield just under 14 Sv (14 Gy) - a radiation injury is likely to occur, and possible loss (amputation) of one or more fingers could be the outcome.

Also note that whilst the above calculated extremity doses are severe, the whole body dose (at 0.7 m) is 711 micro Sv. This is a significant accidental exposure over such are short duration of 30 seconds, but is stochastic in nature, where as the extremity exposure is deterministic and very likely to cause a physical radiation injury.

Further note that all the above calculations are based on our relatively modest Ir-192 source activity of 370 GBq (10 Ci). Consider the likely extremity finger exposure at 1 cm from the source for 30 seconds if the source activity was 7400 GBq (200 Ci). All other factors being the same, such a larger activity would yield an extremity exposure of 70 Gy to the finger / hand - a likely scenario here is that T2 would lose their hand completely. Their whole body exposure would be of the order of 14 mSv (a serious accidental exposure, but still within the dose limits and nowhere near deterministic injury).

Note that the extremity doses reported above e.g. 7 Gy, 70 Gy (7 Sv, 70 Sv), are massively in excess of the legal limit (which is 500 mSv / per year in the UK, i.e. 0.5 Sv / year).

So what actually did happen ?! We don't know, please let this story end as you would like it to end! Perhaps the source wind-in with cv reacher manipulation worked? Perhaps the radiography technicians T1 and T2 decided to cut the source guide tube and cable and recover the source, on the end of its wire and place in a shielded container / emergency shielding pot. Perhaps T1 and T2 used local shielding whilst undertaking this work, certainly possible but as shown above, substantial shielded is required to make a material difference in dose reduction, and this has to be set against the problems of accessibility and visibility and also the potential dose uptake in setting the shielding in position.

What we can say, based on the list of incidents we reviewed earlier from the IAEA publication, is that our design based accident / incident is reasonably foreseeable.

To conclude we will consider an incident involving our radiography enclosure featured earlier in this article.

Mark Ramsay

Radiation Protection Adviser (RPA)

Chartered Radiation Protection Professional

December 12, 2023