Minutes of the 7th Meeting of the NMS Airborne Radioactivity Monitoring Users' Group
NPL – 14 November 2007
Attendees:
Derek Brazer AWE Peter Burgess NPL Julian Dean NPL Stuart Fannin UKAEA James Forde-Johnston Canberra Harwell Ken Frost Southern Scientific Ed Holden Berthold Technologies UK Ltd Dominic Jones AWE Steven Judge NPL Alfred Klett Berthold Technologies UK Ltd Piers de Lavison NPL Maria Marouli NPL Fred Martin British Nuclear Group Frank McGurty UKAEA Trevor Nicholls Lab Impex Systems Yoshimune Ogata NPL Hilary Phillips NPL Jeff Rivers Lab Impex Systems Gareth Roberts RWE Nukem Granville Roberts Environment Agency Dale Robinson BNFL Debra Rook British Nuclear Group David Ryden Canberra Harwell Austen Short Southern Scientific John Simpson RWE NUKEM David Tattam GE Healthcare Bern Warr GE Healthcare Eliot Williams UKAEA
1. Chairman’s Welcome, Previous Minutes and Actions Arising
1.1 The chairman (Steven Judge) welcomed delegates to the 7th ARMUG meeting and briefly summarised the day’s agenda. Many of today’s presentations would pick up on, or develop, topics raised at earlier meetings, including a proposed GPG on workplace air monitoring, progress on the proposed GPG on Tritium-in-air monitoring, and the next stage in the development of an NPL calibration service for positron-in-air monitors. There would also be a talk from the Environment Agency on their requirements for radioactivity-in-air monitoring. In the afternoon session there would be a ‘workshop’ session which would be an opportunity for delegates to discuss instrument and measurement problems.
1.2 There were no corrections to be made to the previous minutes and they were agreed to be an accurate record of the 2006 meeting.
1.3 Previous Actions
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.’ David Ryden, John Simpson and Pete Burgess had kindly volunteered at the 2006 ARMUG meeting, but the secretary had not yet arranged an initial meeting. The action stands (see Action A7.1).
Action A6.2: ‘Members are invited to complete the ARMUG questionnaire which is available on the ARMUG webpage (www.npl.co.uk/armug) and to return to the secretary.’ No questionnaires had been returned since the 2006 meeting. The action stands (see Action A7.2).
Action A6.3: ‘Members wishing to discuss uncertainties, or any other topic at a future meeting should contact the secretary.’ No requests or suggestions had been received. The action stands (see Action A7.3).
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.’ This had been done. A consultation draft had been put on the ARMUG webpage and delegates were asked to send comments to the secretary by 30th November 2007 and the final version would be put on the website in December (see Action A7.4, with revised deadlines).
Action A6.5: ‘Secretary to contact Trevor Birkett, convene working group and progress compilation of Tritium monitoring Good Practice Guide.’ This had been done, with significant progress made. Trevor Birkett was unable to attend but John Simpson has prepared a summary for presentation later in the morning.
Action A6.6: ‘NPL to consider producing guidance on the positioning of air monitors in the workplace.’ Pete Burgess had discussed this with Stuart Fannin and they would give a joint presentation later in the morning.
Action A6.7: ‘Members with available supplies of low-activity air filters possibly suitable for a comparison to contact the secretary.’ Some supplies had been identified and a project to compare real and apparent activities on filters had been included in the current NPL programme. Members with further supplies were asked to contact the secretary (see Action A7.5).
2. Invited talk: Discussion on the feasibility of a Workplace Air Monitoring Good Practice Guide - Stuart Fannin, UKAEA, and Pete Burgess, NPL
2.1 This talk developed the presentation from ARMUG 2006 which had been given by Pete Burgess on Stuart’s behalf. It was based on an AWE project to investigate and clarify requirements for workplace air sampling.
2.2 Estimating the dose from radioactivity in particulates is complex because of the dependence on the dynamics of the laboratory air. The AWE study had approached this by modelling of plume dispersions to define bounding cases and had also involved a literature review and consideration of operating experience. The factors affecting air-monitoring regime detection capabilities had been considered (e.g. location, level of dilution and alarm levels).
2.3 Bounding cases were those assuming (i) turbulent diffusion and (ii) isotropic expansion. Intermittency (i.e. the probability of the plume hitting the detector), Downwind and Crosswind effects, body heat from the worker and cellular air-flows within the laboratory were taken into account. It was concluded that a two-stage air-monitoring regime was needed – a primary measurement close to the release point (and above the worker’s head), and a secondary measurement in a remote position. The merits of the two monitoring positions, and their respective detection levels and response times, were compared. The study resulted in the issue of an AWE Radiation Protection standard.
2.4 Pete Burgess proposed that a Measurement Good Practice Guide on this topic should be written. Several delegates supported this proposal and were willing in principle to join the Writing Group. NPL will consider including such a guide in its current NMS programme (Action A7.6).
2.5 It was apparent that there may be a need to develop a novel personal air sampler. The chairman pointed out that funding (or part funding) or consultancy may be available via one of the DIUS Knowledge Transfer initiatives (e.g. the Measurement for Innovators programme, Joint Industry Projects and secondments). NPL is happy to discuss this with any members who are interested.
3. Invited talk: Environment Agency Requirements for Monitoring Radioactivity in Air - Granville Roberts, Environment Agency
3.1 Granville summarised current standards and guidance relating to both ambient (workplace) and stack discharge air monitoring, and some future requirements. Current standards include the process industry standards ‘MCERTS’ (which includes continuous emission monitoring of stacks) and guidance documents M1 and M2 (which help with sampling positions). M1 and M2 can be found on the EA website. M11 (‘Monitoring of radioactive releases to atmosphere from nuclear facilities’) is also relevant.
3.2 There is general unfamiliarity with MCERTS among the users. The Radiological Monitoring Standards Working Group (RMSWG) was recently formed to develop and promote good practice in this field, including development and revision of the above guidance. For gaseous effluent monitoring, there are EU drivers for standard reporting of discharges to air and there may be future MCERTS standards on gaseous flow and analysis as well as revisions to M11 and a new ISO standard on sampling from stacks and ducts. For the radiological monitoring of ambient air, again there may be an MCERTS standard and the RMSWG may produce guidance on sampling.
3.3 Some standard authorisation conditions were cited which place requirements on the operator regarding sampling, the maintenance of equipment, its suitability for purpose and its calibration. Typical problems include users not adhering to agreed methods in their local procedures and experimental methods being changed without the procedure being amended accordingly.
4. NPL talk: Tritium Good Practice Guide - John Simpson, NUKEM Ltd
4.1 John described progress on a new Measurement Good Practice Guide ‘The Calibration and Testing of Tritium-in-Air Monitors’, including why it is needed, its scope, problems with measuring Tritium and approaches to calibrating monitors.
4.2 Measurement difficulties arise from the short range of the Tritium beta particle and the low observed current per unit activity in ion chambers.
4.3 The guide is needed because there are very few UK calibration facilities for Tritium in air and they do not cover the full range of the monitors; moreover, calibration at high activities is impractical anyway as this could leave the monitor irreversibly contaminated. The use of a surrogate radionuclide (e.g. 85Kr) is impractical owing to the corrections which would need to be applied.
4.4 An alternative approach is to use a gamma calibration facility with sealed sources and traceable doserates. An equivalence of 1 MBq m-3 @ 3 μSv h-1 can be assumed. Collated results from 52 calibrations using responses to both Tritium and gammas were presented and the observed ratio of Tritium to gamma response was 1.01 (standard deviation 14.7 % at 1σ). The gamma method was deemed practical for calibrating monitors across their full range as long as Tritium is also used for Type Testing and for confirming the gamma data.
4.5 The new GPG will be similar in layout to GPG 82, incorporating sections on instrument testing regimes, instrument displays, specific tests on the air circuits and radiation detection assemblies, facilities and traceability and the certification of tests. A rough draft has already been prepared and a consultation draft should be available to members in May 2008.
5. NPL talk: A Transfer Instrument for the Calibration of Positron-in-Air Monitors - Pete Burgess, NPL
5.1 Pete discussed the requirements for a transfer instrument for calibrating positron-in-air monitors in situ at UK PET facilities. The instrument will be calibrated by comparison with NPL’s primary standard for positrons-in-air (which is based on NPL’s internal gas proportional counters). PET radionuclides are low-Z and neutron-deficient and have short half-lives. They are produced in large activities in accelerators, and the possibility of stack discharges (in the case of gaseous species) means that stacks must be monitored.
5.2 As well as the positrons themselves, annihilation gammas are produced. Approaches to monitoring include using a NaI(Tl) scintillation detector outside the duct to detect the 511 keV photons (with shielding to minimise external interactions) or a plastic scintillator-based detector, within the duct, to detect positrons (this method would be positron-energy dependent but the low gamma response would mean less shielding would be required). The latter approach is to be adopted. This should give a low background and a response of the order of 100 cps Bq-1 m3. There would be no need for a thin window and no need for a light-tight window for a detector positioned within the duct. It will be important to minimise the path length to minimise the number of positrons stopped by the air in the detector volume.
6. Workshop on Instrument and Measurement Problems
6.1 The aims of this session were: (i) to identify and discuss users’ current problems regarding instruments and measurements, and (ii) to either find a solution today or to action NPL or ARMUG members to address the problem later.
6.2 The delegates split up into small groups and one person from each acted as spokesperson and timekeeper. Each group spent about 20 minutes listing their instrument or measurement problems and identifying two or three common issues. The spokespersons then each gave a five-minute talk to the whole meeting describing the issues identified and how they might be addressed, and inviting comments and suggestions. The main points arising are listed below.
6.3 The first point concerned sampling. A member asked if reducing the number of air samplers in an area could be justified. Some members felt it could be but that such a reduction would need to be balanced against the increased risk of underestimating doses. It was felt that advice on this should be given in the proposed Good Practice Guide (see Action A7.6). The positioning of monitors was a problem for some users and the guide will address this as well.
6.4 The use of ‘in-house’ standards for air-filter measurements was discussed. One member uses such an alpha ‘standard’ based on a ‘real’ contaminated glass-fibre filter which had also been measured by gamma spectrometry. It is used alongside some commercial solid alpha sources. However, others pointed out that any ‘filter standards’ should be used with great care; the source may lose activity, added to which the detection efficiencies of other samples will vary with levels of dirt or dust, not only on the filter surface but also within the pores of the filter. NPL is happy to advise on this.
6.5 Compensation for radon and gammas is a problem, especially for radon. There will be a session on radon compensation at the next ARMUG meeting to address this.
6.6 Regarding positron-in-air measurements, one member had found that measuring F-18 in stacks was problematic in that if it is measured too far from the point of generation, the activity may have been diluted and be too low to measure, whereas if it is measured nearer the point of manufacture false coincidences can arise from activity adsorbing onto nearby surfaces.
6.7 A related issue was the position of flow-rate meters; should flow-rate be measured near the filter or near the pump? Some members felt that it should be measured downstream of the filter; however, if it can be demonstrated that there are no leaks in the system, the flow-rate can in principle be measured at any point in the circuit.
6.8 Choice of instrument is sometimes a problem. Members were encouraged to canvass other users before adopting a particular model to ensure the instrument is fit for its intended purpose. The manufacturers would welcome feedback on the application of their instruments and any limitations or problems arising.
7. Any Other Business
7.1 James Forde-Johnston suggested inviting a speaker from the CEA to the next ARMUG meeting. A session on radon compensation was also suggested.
8. Actions arising from this meeting
Action A7.1: Secretary to arrange meeting of working group to produce guidance on adsorption factors and to ensure that guidance is published on the ARMUG web page
Action A7.2: Members are invited to complete the ARMUG questionnaire which is available on the ARMUG web page and to return to the secretary.
Action A7.3: Members wishing to discuss uncertainties, or any other topic at a future meeting should contact the secretary.
Action A7.4: Members to send the secretary any comments on the draft list of Frequently Asked Questions on the ARMUG web page by 28th February 2008 and secretary to finalise the list by 31st March 2008.
Action A7.5: Members with available supplies of low-activity air filters possibly suitable for a comparison to contact the Secretary.
Action A7.6: NPL to decide whether to include a Measurement Good Practice Guide on Workplace Air Monitoring in its current NMS programme and to begin work accordingly.
Julian Dean
Secretary, ARMUG
29th January 2008