NPL – 7 November 2006

 

Attendees:
		George Abbott	AWE
		Derek Brazer	AWE
		Pete Burgess	NPL
		Chris Davies	URENCO
		Piers De Lavison	NPL
		Julian Dean	NPL
		Mike Dolan	AWE
		Perani Francois	MGP Instruments
		Derek Hammond	Health Protection Agency
		Robert Huggett	Zinsser Analytic UK Limited
		Simon Jerome	NPL
		Bob Johnson	Berthold Technologies
		Steven Judge	NPL
		Mario Marouli	NPL
		Fred Martin	British Nuclear Group
		Richard Monaghan	Lab Impex Systems
		Bruce Morris	Lab Impex Systems
		Trevor Nicholls	Lab Impex Systems
		Hilary Phillips	NPL
		Mark Rainbird	AWE
		Jeff Rivers	Lab Impex Systems
		Dale Robinson	BNFL
		Debra Rook	British Nuclear Group
		David Ryden	Canberra Harwell
		John Simpson	NUKEM
		Bern Warr	G E Healthcare
		David Watt	Canberra Harwell
		Eliot Williams	UKAEA

 

1. Chairman’s Welcome, Previous Minutes and Actions Arising

1.1 The chairman (Steven Judge) welcomed delegates to the 6th ARMUG meeting and briefly summarised the day's agenda. In particular, he announced that Measurement Good Practice Guide 82 ('The examination and testing of equipment for monitoring airborne radioactive particulate in the workplace') was now complete and available either as a free download from the ARMUG website or as a hard copy, for a small charge. After thanking the Working Group for their time and effort in producing this new guide, he then invited delegates to introduce themselves.

1.2 There were no corrections to be made to the previous minutes and they were agreed to be an accurate record of the 2005 meeting.

1.3 Previous Actions

Action A5.1: Members should send any information on adsorption factors (e.g. papers, reports or other documents) to Julian Dean.

Nothing had been received, although David Ryden had kindly volunteered to help with producing some guidance. Pete Burgess and John Simpson offered their help as well. It was agreed that the guidance should be published on an ARMUG webpage in the first instance. The secretary was actioned to arrange an initial meeting of this working group (see Action 6.1).

Action A5.2: Members should send any suggestions for a speaker on shrouded sampling probes to Julian Dean.

Lab Impex Systems had kindly volunteered a speaker (Jeff Rivers) who would speak on this subject later in the meeting. Action complete.

Action A5.3: Members are invited to complete the questionnaire enclosed in the delegates' packs and return to Julian Dean. Electronic copies will also be sent out.Only one return had been received. Action stands (see Action 6.2).

Action A5.4: Members wishing to discuss uncertainties, or any other topic at a future meeting should contact Julian Dean.

Nothing had been received. Action stands (see Action 6.3).

Action A5.5: Secretary to complete list of Frequently Asked Questions and answers with help from John Simpson and Peter Burgess.

The working group had met in July. The secretary had yet to finish compiling a second draft for this group. When complete and redrafted by the group, the final draft will go on the ARMUG website for members' comments (see Action 6.4).

Action A5.6: Chairman to check if NMS funding is available for a Tritium monitoring Good Practice Guide.

Funding was available. Trevor Birkett (not present) had agreed to chair a working group to compile the GPG. John Simpson volunteered to help. The secretary asked delegates if the scope should be wider than the measurement of gaseous tritium but there was no immediate interest in doing this (see Action 6.5).

1.4 The chairman then gave a short presentation on the formulation of the next DTI/NMS programme in ionising radiation metrology, due to run from October 2007 to September 2010. This programme would be aimed at meeting users' needs for standards, reference materials, calibration services and knowledge transfer in ionising radiation metrology. The formulation process had now reached the user consultation stage and delegates were invited to feed back, as soon as possible, their anticipated needs for the next 6-7 years. Further details are given in the above link to 'Chairman's Welcome' and on the ARMUG webpage.

2. Invited talk: "Comparison of alpha-in-air monitors in a challenging environment" - Eliot Williams, UKAEA

2.1 Eliot's presentation was based on his M.Sc. dissertation entitled 'An analysis of the available methodologies for radon compensation'. The purposes of the study had been (i) to understand the monitoring problems within the Harwell solid waste storage facility, (ii) to investigate radon compensation methods, and (iii) to investigate possible improvements to the waste store's monitoring systems, if possible without replacing the instruments currently in use.

2.2 Eliot reported that Pete Burgess (then UKAEA) had looked into the monitoring problems in the waste store and had concluded (i) high radon levels had been found and they were probably 'natural' and not due to stored radium, (ii) AB96 (MGPi) and CMS2000AB (Lab Impex Systems) monitors performed similarly, but iCAM monitors (Canberra Harwell) would probably give the same false-alarm rate at a lower DAC setting, and (iii) monitors should be positioned close to the source and in the air flow from the source (positioning being particularly important when the alarm setting is high).

2.3 Eliot had trialled three different instruments and radon compensation types. Alarm levels were increased to reduce false alarms. The modification of existing equipment was considered but none of the monitors met the monitoring requirements of the waste store. Suggestions for future trials included using different filter papers, or a radon screen, on the CMS2000, reducing the 'long time' constant of the iCAM, and reviewing available information on the radon screen for typical particle sizes.

2.4 David Ryden pointed out that, in real-time monitoring, spectral shape improves with time. Relative levels of polonium nuclides can change rapidly.

3. Invited talk: "The shrouded probe" - Jeff Rivers, Lab Impex Systems

3.1 The shrouded probe was designed, tested and patented by Texas A&M University. It is used for taking samples from an air stream and can be used alone, whereas previously isokinetic sampling using several probes would have been required. Due to the shape of the probe and its inner nozzle, the gas is sampled at nearly the same aerosol particle concentration as the 'undisturbed' flow upstream of the shroud. Shrouded probes have the advantages (over non-shrouded probes) of giving lower internal wall losses, better 'off-angle' performance, lower sensitivity to flow-stream turbulence, and the ability to operate in both fixed and modulating flow-rates.

3.2 At a sample rate of 57 L min^-1 and a flow-rate range of 1 - 24 m s^-1, the probe conforms to the recommendation of ANSI N13.1 that a probe's transmission ratio (i.e. the aerosol concentration at the exit plane divided by that at the probe's location) be in the range 0.8 - 1.3. Tests are being carried out on a new range of probes suitable for lower flow-rates than those used in USA.

4. NPL talk: "Comparison of gas monitor responses to ⁸⁵Kr and Tritium" - Hilary Phillips, NPL

4.1 Hilary presented data from recent evaluations of Tritium-in-air monitors at NPL. This work had involved measuring the responses of each of two types of monitor to both Tritium and ⁸⁵Kr over a range of activity concentrations. One type of monitor had a second, isolated ion chamber next to the main detector (for background and g compensation) whereas the other was of the 'Kanne' style, consisting of three concentric cylinders, with the region between the outer and intermediate cylinders acting as an ion trap. Both have ²²²Rn suppression functions.

4.2 The monitor calibration procedure involves trapping a known volume of previously-standardised gas (e.g. Tritium) in a fixed volume located within a gas-handling manifold, quantitatively diluting the gas by mixing it with dried air in the manifold, then exposing the monitor to this diluted gas. When several calibration points are required, the process can be repeated by trapping diluted gas in the fixed volume, flushing the rest of the manifold with inactive dry air and then carrying out a second (and subsequent) dilutions.

4.3 In this study, each of the chambers was exposed to Tritium (0.5 - 20 MBq m^-3) and ⁸⁵Kr (0.5 - 500 MBq m^-3). The responses to ⁸⁵Kr (using the Tritium 'settings') were approximately five times the supplied activity, owing to the higher b-particle energy of ⁸⁵Kr. The responses of both monitors to both gases were linear, but there was a significant difference between the Tritium responses, possibly due to their being traceable to different national standards. The response to ⁸⁵Kr of the 'Kanne' chamber fell significantly at around 2000 - 2500 MBq m^-3 when the circulating pump was on and this was possibly due to leakage from the monitor or adsorption of krypton by materials within the monitor. The response of the 'isolated background' chamber appeared to vary slightly with time in a cyclic fashion, possibly due to small local variations of activity concentration within the gas manifold.

5. NPL talk: "Initial stages of the development of a UK primary standard for positron emitters in gas" - Maria Marouli, NPL/Surrey University

5.1 Maria gave a brief update on this project, the objective of which is to develop a national standard and a monitor calibration service for measurements of positrons in gas.

5.2 Positron Emission Tomography (PET) is an imaging modality which is increasingly applied in medical diagnostics. PET radionuclides are produced in cyclotrons and include ¹¹C, ¹³N, ¹⁵O and ¹⁸F, which have half-lives of less than 2 hours. There are 15 PET scanners installed in the UK at present and this is planned to rise to 23. When the radionuclide is prepared as a gas, there is the possibility of accidental release of the gas to the environment, possibly as high as 4 GBq. It is a legal requirement that any such discharges be monitored, using a suitably calibrated instrument.

5.3 The project will involve simulating the responses of the NPL internal gas proportional counters to positrons and their annihilation g-emissions (using the Monte Carlo code PENELOPE) and then validating the simulation using gaseous ¹¹C previously standardised (as a liquid) using the NPL ionisation chambers. Modelling has been used to investigate cross-talk between the counters (with and without lead shot between them), and also dose distribution and energy distribution in the counter materials. A point source of ⁸⁵Sr has been used to provide 514 keV photons (approximating to 511 keV annihilation photons) to check the modelling of the cross-talk, and the results confirm the simulations. A procedure is being developed to convert ¹¹C (as NaHCO3) to ¹¹CO₂, to dry it and transfer it to the NPL gas counters.

6. NPL/UKAEA talk: "Workplace air sampling" - Pete Burgess, NPL/Stuart Fannin, UKAEA

6.1 Pete gave this presentation on behalf of Stuart, who was unable to attend the meeting. He explained the types of monitoring regimes in use, their applications, factors affecting their performance, the merits of different monitor locations, and how to optimise a monitoring regime.

6.2 The two types are: real-time air sampling (e.g. to provide prompt alert to losses of containment so immediate safety action can be taken) and static air sampling (e.g. for monitoring low-level chronic releases and control of low-risk operations). Real-time sampling is appropriate when working with material within containments and for protected operations, whereas static sampling is used in tandem with real-time monitoring, or when RPE is used, or to check areas prior to de-restriction.

6.3 Performance is affected by air-flow patterns within the laboratory, source-to-detector distance, and alarm levels. For real-time sampling, separate monitors must be deployed close to, and remote from, the activity release point. Static samplers must cover the whole work area and be in positions where they are most likely to detect activity. Alarms should be set to sound when low levels of activity are detected. Monitoring poorly-ventilated buildings, or ones with high radon levels, is difficult.

6.4 Following the talk, there was some discussion on the positioning of air monitors and various papers and contacts were mentioned. There was an apparent need for NPL to produce some guidance (see Action 6.6).

7. Any Other Business

7.1 Pete Burgess had suggested a comparison of contaminated air filters. This would include radiochemical analysis of some filters to determine response factors for air monitors. Loss of activity in transit was noted as a practical problem in such an exercise. Delegates were asked if anyone could supply such filters, ideally with low activities (see Action 6.7).

7.2 The next meeting is likely to be held in November 2007 but this is dependent on the outcome of programme formulation. Members will be informed as soon as the date has been set.

8. Actions arising from this meeting

Action A6.1: Secretary to arrange meeting of working group to produce guidance on adsorption factors and to ensure that guidance is published on the ARMUG webpage.

Action A6.2: Members are invited to complete the ARMUG questionnaire which is available on the ARMUG webpage and to return to the secretary.

Action A6.3: Members wishing to discuss uncertainties, or any other topic at a future meeting should contact the secretary.

Action A6.4: Secretary (with help from John Simpson and Pete Burgess) to finalise the list of 'Frequently Asked Questions', put out for consultation and publish on the ARMUG webpage.

Action A6.5: Secretary to contact Trevor Birkett, convene working group and progress compilation of Tritium monitoring Good Practice Guide.

Action A6.6: NPL to consider producing guidance on the positioning of air monitors in the workplace.

Action A6.7: Members with available supplies of low-activity air filters possibly suitable for a comparison to contact the secretary.

Julian Dean

Secretary, ARMUG

12^th December 2006