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8/11/2009 - Unintended exposure of a worker

This is taken from the IAEA event log.

The Incident (29/09/2009)

 

The French Nuclear Safety Authority (ASN) has rated at level 2 on INES the accidental exposure on the 29 September 2009 of a radiographer of the ABC Company (HORUS holding) during a welding test using gamma radiography at the Flamanville NPP operated by EDF. Through violation of the working procedures, the worker entered the controlled area and remained in during several seconds while the high activity source was not yet in its safe store position.ASN was first quickly informed by EDF. Later on, ABC declared the event as "significant incident" according to French technical guidelines.

The worker received a dose of 5 mSv due to this incident. The medical officer in charge of monitoring the worker was informed. He did not consider it necessary to conduct a medical examination after the incident.

This accidental exposure has been caused by the non-compliance with basic radiation protection procedures. Based on a misinterpretation of the actions performed by the radiographer operating the radioactive source, the exposed worker, who was observing from the distance, believed the exposure was completed. He then entered the controlled area while the radiation source was still exposed without waiting for the formal confirmation of the end of operations and without any survey meter to check the end of the exposure.

A joint inspection by ASN and the Labour Inspectorate was carried out on 29 October 2009. Deviations from radiation protection approved standards were confirmed. The corrective actions implemented by ABC and Horus were considered as satisfactory. ASN will ensure the follow-up of the implementation of these actions.

ASN has rated this incident at level 2 on INES (October 2008 version) due to the non respect of the comprehensive set of safety rules to access the controlled area which has led to a significant unintended exposure of a worker.

The device used is a gamma radiographer containing a high activity radioactive source of Ir 192 (1,73 TBq)

The incident log can be read at the following link: IAEA News (You may need to log in as a guest).

Ionactive Comment

As always we provide analysis for general information and interest only - we may not always have all the relevant facts.

Let us assume that ‘several seconds' is 3 or more - so let that be 3 seconds. Assuming the 5mSv dose suggested above is ‘whole body' (effective dose) then that is quite a considerable exposure based on the limited duration. It is true that the dose rate contributing to the entire exposure is unlikely to have been constant throughout as the worker may have entered the area after the exposure had commenced (so dose rates would have increased with decreasing distance to the source, and decreased if he was moving away from the source).

Since we do not have detailed information we will assume that the worker was static during the exposure. The dose rate from a point source of Ir-192, of activity 1.73 TBq, will be in the order of 202 mSv/h at 1m. This assumes the source is not collimated or otherwise shielded. The dose rate per second is therefore about 0.056 mSv/s - so not enough to produce the exposure in the individual over the understood exposure duration (at distances of 1m or more).

For an exposure of 5mSv in about 3 seconds, a dose rate of around 1.67mSv/s would be required (6000 mSv/h). This implies that the distance from the exposed source would be somewhat less than 20 cm (18.3cm actually). This is shown in the graphic below.



The area of interest is shown in this expanded graphic:



Therefore this implies (noting our assumptions above) that the individual would have been very close to the source indeed - probably right over the source projector or within the region of the actual object being radiographed.

7/11/2009 - Radiation Risk Calculator



Radiation Risk Calculator

Came across this interesting site located at www.xrayrisk.com.

This site claims to provide a risk calculation service which will determine your additional cancer risk, i.e. over and above that risk that you receive from exposures from everyday life (cosmic, radon etc), from a number of medical diagnostic procedures. In fact the site goes further than just x-rays because it also considers a number of nuclear medicine procedures (i.e. whole body PET scan).

I have not verified the data, and comparisons with everyday exposures are from a US perspective (but references are given for you to explore if you wish). For example, the site states that average US exposure is 6.2 mSv/y (all sources) whereas the UK is around 2.7mSv/year (UK Ref: HPA-RPD-001) - therefore you need to take that into account when looking at risk comparisons.

Taking a quote directly from the site is informative:

While the need for education in this area has clearly been established, there are no widely available resources that provide information to both patients and health care providers about the increased risk of cancer from medical imaging. X-RayRisk.com is an educational website that focuses on estimating this risk. One of the site's main features is a web based calculator that allows users to track their imaging history and estimate their personal risk, while providing answers to frequently asked questions.

The site plays is quite straight and there is no particular attempt to either provide reassurance or enhance concern (which is good). We would emphasise (to anyone who tries out this resource) that the whole point of using any of these diagnostic procedures is to ensure the benefit of the procedure clearly outweighs the risks. So the early detection of breast cancer using mammograms is still shown to save more lives when compared to 'lives lost' through cancer which might be attributed to the actual radiation exposure.

Calculation Method

The calculation methodology adopted appears to be fairly standard and relies on published data in the literature - in other words the calculator is not creating risk values from first principles, rather it is a database of pre-defined values which are selected on the basis of your inputs (e.g. age, type of procedure etc). The key reference used is: Biological Effects of Ionizing Radiation (BEIR) VII Phase 2 Report - typical data is shown in the table below (from xrayrisk.com).



The above data is then converted into a % attributable risk of cancer for males and females against age. As one will expect the risk tends to be higher in the young where there are more years ahead of them (to realise a potential radiation induced cancer).

The calculator also uses data from ‘Effective Doses in Radiology and Diagnostic Nuclear Medicine: A Catalog (Mettler FA)'. Some typical data from this publication is shown below (the link in this paragraph will take you to the actual report).


A risk Calculation

Using the risk calculator is straight forward. In the example below I have selected a chest x-ray, selected gender and entered age and number of scans, and then pressed calculate. You can also build an x-ray risk report by adding the risks from a number of separate procedures together.



Final Comments

Overall this is a nice piece of online resource. Like everything on the web, one must be cautious when using any online data or information. The right thing to do is to question your medical practitioner about any medical radiation exposure you are about to receive - and get them to explain the risks to you - for your own specific case.

However, talking to family members and friends who have undergone such procedures, does reveal that many practitioners (radiologists / x-ray operators) are not always geared up to explain the key concepts. It is not that they do not know the answer, it is more likely that they are not often asked the question and so do not have a strategy to answer simply.

7/11/2009 - Know your Schrodinger from your Bohr ?
Ionactive Consulting Radiation Protection Training Resource - C-14

Or your Dirac from your Curie?

Solvay Physics Conference of 1927

Well the Solvay Physics Conference of 1927 gives you insight into to some of the most famous scientists of that period. The resource below, brought to you by freesciencelectures.com, provides a fascinating near 3 minute ‘home video' taken by Irving Langmuir.

Some of those shown in the video include: Ervin Schrodinger, Niels Bohr, Werner Heisenberg, Auguste Piccard, Paul Dirac, Max Born, Wolfgang Pauli, Louis de Broglie, Marie Curie, Hendrik Lorentz and Albert Einstein.

Apparently it was at this time that Einstein was quoted as saying "God does not play dice" in response to Max Born's statistical interpretation of the wave function (the Bohr model of the atom, itself a quantum physics-based modification of the Rutherford model, would never be quite the same!).



It is great to see such resource available for the masses, and meets Ionactive's objective of providing free radiation Protection / Physics online.

6/11/2009 - X-rays 'top scientific invention'

Ionactive Training Resource - X-ray Tube

Earlier on in the week we spotted that the public had voted the X-ray machine as the best invention, this being ahead of the Apollo 10 space capsule and Stephenson's Rocket. The story came to light on the BBC website and can be found here.

We now have our x-ray tube animation on YouTube which can be accessed directly below.

Ionactive X-Ray Tube Training Resource



This resource is also available for playback via a WMV file which can be found on our Radiation Protection Video (x-ray) page.

The resource looks at the major parts of the x-ray tube and considers Bremsstrahlung:

Ionactive Training Resource: Bremsstrahlung

 ... and Characteristic x-rays:


Ionactive Training Resource: Characteristic x-ray

It might also interest you to know what other inventions were in the top 10. The list was as follows:

  • 1st place - X-ray machines
  • 2nd place - Penicillin
  • 3rd place - DNA double helix
  • 4th place - Apollo 10 capsule
  • 5th place - V2 Rocket Engine
  • 6th place - Stephenson's Rocket
  • 7th place - Pilot ACE Computer
  • 8th place - Steam Engine
  • 9th place - Model T Ford
  • 10th place - Electric Telegraph 


Other X-ray Tube Resource

Please note: the resource below is provided for your interest only - Ionactive does not endorse this material.

For some much older x-ray tube resource, but no less useful, why not watch this 1940's X Ray Physics Documentary By William D Coolidge! This should play directly from the player below.



I would love to have an ion chamber radiation dose rate monitor in William D Coolidge's lab - might be quite interesting!

It is then time for a 'do not try this at home' (and I mean it). Check out this chap below measuring doserates from an x-ray tube - neither sensible or clever under the circumstances illustrated, but interesting none the less.



I could not quite make out the scale in the above video, but I think the instrument read 1R/h at one point. For our purposes that would give you an ambient dose equivalent of something near 10 mSv/h - not a trivial dose rate by any means!!

And finally, if you cannot find an x-ray machine it appears that you just go and make one yourself. Please note I do not condone this at all, what you see in this video below is potentially dangerous if not fatal - and that is from high voltage as well as radiation. However, I kind of get the impression that this chap knows what he is doing (even though I believe it to be wrong - there are no obvious safety interlocks for example). If I read the monitor correctly then the dose rate at the point of measure is 50R / h (for our purposes about 0.5 Sv/h).



That is all folks!

3/11/2009 - Worker Overexposure

This is taken from the IAEA event log.

The Incident

 

A radio-pharmacist was preparing Fluorine-18 (F-18) doses for use, when a manipulator malfunction occurred. The radio-pharmacist continued to prepare the F-18 manually instead of stopping the process. This led to a potential dose to the radio-pharmacist's right hand of greater than 0.5 sieverts (50 rem). This dose is a rough estimate from whole body dose values and reconstruction of the event due to the fact the radio-pharmacist was not wearing any dosimetry on the extremity. The regulatory authority continues to assess this event and a final dose estimate will be determined as part of that assessment.

The incident log can be read at the following link: IAEA News (You may need to log in as a guest).

Ionactive Comment

F-18 is a Positron Emitter as described in the diagrams below.

As the F-18 decays it releases a Positron (an electron with a positive charge).



The positron soon meets its fate by annihilation with an electron. During this stage a rather exotic entity is formed known as Positronium.



This has a mean lifetime of 125 picoseconds and decays into two gamma rays with an energy of 511 keV each. The detection of these photons is used in the medical diagnostic procedure Positron Emission Tomography (PET).



So the radiation safety aspects of working with F-18 are not trivial nor are the shielding requirements. Certainly in the UK we have noticed a significant increase in F-18 production which is allowable because of the greater availability of PET scanners. Looking at the process it is clear that the production of the PET Radio-Rharmaceutical is probably the point where highest exposures could occur because of the activity involved. This is because whilst a single dose to a patient might be of the order of 400 MBq, since the half life is only about 110 minutes then a single dose at the production end could easily be 4 GBq or more.

Those most at risk of exposure are not the production radio-chemists but rather those involved in the quality assurance side of things - mainly because they may have to manipulate single doses and take samples for HPLC analysis and similar. Whilst good radiation safety practice can minimise occupational exposures to very low levels (when compared to other occupationally exposed groups), in QA much of this relies on the user rather than automated systems. The same can be said for those involved in preparing the end user doses.

In the above event record it is not clear if this incident occurred at the hospital end (probably) or at the production end - although in many cases the two are effectively combined where the F-18 production cyclotron is physically located very near the end user.

The dose rate (due to the gamma rays) from 400MBq of F-18 at 5cm will be about 22mSv/h rising to well over 500 mSv/h at 1cm. Clearly, all things being equal, a 4GBq dose would produce dose rates of over 220 mSv/h and over 5000 mSv/h respectively, at the same specified distances. So you can see that the potential handling (extremity) dose rates are not trivial by any means.

The radio-chemist / pharmacist would probably be working behind a lead shielding wall or perhaps leaded glass - however the hands would most likely be located the ‘hot' side of the shielding, and certainly so if manual manipulations are being performed. The Tenth Value Layer (TVL) for lead with F-18 is about 13mm.

The above event record reveals that no extremity dosimetry was being worn and that extremity doses were reconstructed from whole body exposure data. This is possible using the type of data I have noted above, but it is a bit hit and miss when trying to estimate dose rates (and resulted exposures) from sources that are very close to their target.

Radiation protection training is clearly a significant driver in ensuring that these types of exposures cannot occur.

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This is the company blog of Ionactive Consulting Limited, a Radiation Protection Adviser consultancy. Visit here often to read our views on radiation protection and related matters. You can contact our director and RPA directly at mark.ramsay@ionactive.co.uk

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