review of respirable particle size range - nda

Review of Respirable Particle Size Range

To: NDA RWMD

Date: 22 April 2013

From: AMEC

Your Reference: RP0605-86C

Our Reference: D.00005.01/P001 Issue 6

D.00005.01/P001Issue 6

Title Review of Respirable Particle Size Range

Prepared for

NDA RWMD

Your Reference

RP0605-86C

Our Reference

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Contact Details AMEC Building 150 Harwell Oxford Didcot Oxfordshire OX11 0QB United Kingdom Tel +44 (0) 1635 280300 Fax +44 (0) 1635 2280305 amec.com

Name Signature Date

Author(s) H Connelly R G Jackson

19 April 2013

Reviewed by

D Charles

22 April 2013

Approved by

D Holton

22 April 2013

Transport Flask Photograph courtesy of Magnox Electric Ltd Submarine Photograph by: Mez Merrill; © Crown Copyright/MOD, image from www.photos.mod.uk. Reproduced with the permission of the Controller of Her Majesty’s Stationery Office

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CONDITIONS OF PUBLICATION

This report is made available under the NDA Transparency Policy. In line with this policy, the NDA is seeking to make information on its activities readily available, and to enable interested parties to have access to and influence on its future programmes. The report may be freely used for non-commercial purposes. However, all commercial uses, including copying and re-publication, require permission from the NDA. All copyright, database rights and other intellectual property rights reside with the NDA. Applications for permission to use the report commercially should be made to the NDA Information Manager.

Although great care has been taken to ensure the accuracy and completeness of the information contained in this publication, the NDA can not assume any responsibility for consequences that may arise from its use by other parties.

© Nuclear Decommissioning Authority 2013. All rights reserved.

BIBLIOGRAPHY

If you would like to see other reports available from NDA, a complete listing can be viewed at our website www.nda.gov.uk, or please write to the Library at the address below.

FEEDBACK

Readers are invited to provide feedback to the NDA on the contents, clarity and presentation of this report and on the means of improving the range of NDA reports published. Feedback should be addressed to:

Dr Elizabeth Atherton, Head of Stakeholder Engagement and Communications, Nuclear Decommissioning Authority, Radioactive Waste Management Directorate, Curie Avenue, Harwell Oxford, Didcot, Oxfordshire, OX11 0RH, UK.

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Executive Summary This report has been prepared in response to an invitation from NDA RWMD to carry out a review of the particle size ranges which should be considered when calculating the dose from the inhalation of radioactive particles, to workers and members of the public, in both normal operational conditions and as a result of potential accidents. The review has also addressed the size range of particles which can be considered respirable within the human respiratory tract. Particle size distributions measured during normal operations within a range of nuclear workplaces

gave AMAD values of approximately 5 µm. This value is considered to be appropriate in the case of accidents as larger particulate will be removed or settle out before reaching workers. Consideration of the environment within a Generic Geological Disposal Facility (GDF) does not suggest that the size of the particles generated there should differ significantly from those reported in the survey of other nuclear workplaces.

It is recommended that RWMD adopt a particle distribution with a median of 5 µm AMAD for workers. This is also the default value recommended by the International Commission for Radiological Protection where specific information on radioactive particle size distributions is not

available. The assumption of the 5 µm AMAD aerosol will determine where the inhaled radioactivity is deposited within the various regions of the respiratory tract. These deposition patterns, the associated clearance and biokinetic models held within the ICRP respiratory tract model allow the calculation of inhalation dose coefficients for a wide range of radionuclides for use in safety assessments. For members of the public, doses from releases of radioactive material to atmosphere in normal

operations should be based upon the inhalation dose coefficients for a default AMAD of 1 µm. This value reflects the fact that discharges from nuclear facilities to the environment are normally subject to abatement using high efficiency filters which remove particles of a larger size. In the event that a release should occur directly to the environment from a GDF without being subject to prior filtration, for example as a result of an impact scenario at the surface, particles with a larger mean aerodynamic diameter would be expected to be encountered at distances where members of

the public are expected to be located, though it is not clear whether it would be closer to 5 µm than

1 µm. Factors that would influence the choice of which value would be appropriate for a specified release would include the distance of travel to the most exposed individuals, the prevailing atmospheric conditions and the nature of the release (i.e. release resulting from an impact or drop, and the height to which the resulting airborne material may rise). As part of this work, the recommendations contained in radiological assessment guidance issued by a number of other UK Nuclear Site Licensees and other international bodies involved in radiation protection have also been reviewed. Licensees have developed methodologies for assessing doses to workers which use dose coefficients for either 1 or 5 µm AMAD particle size distributions or, in some cases, a worst case combination of both. Inhalation dose coefficients for a

1 µm AMAD particle size distribution are generally adopted when calculating doses to members of the public, whether from normal (assumed filtered) or accidental (unfiltered) situations; however, in

the case of resuspended aerosols one Licensee recommends a value of 5 µm AMAD (the dose coefficients for members of the public are only published in tabular form by ICRP for 1 µm AMAD particle size distribution; for other AMADs it is necessary to download software from the ICRP web

site and obtain the dose coefficients for the relevant nuclides). In some cases the 1 µm AMAD inhalation dose coefficients are put forward in the calculation of doses to workers. Such an airborne particle distribution will result in the highest level of deposition within the sensitive areas of the lung ensuring that the inhalation doses calculated are generally conservative for particulate in insoluble chemical form. This may be desirable in terms of ensuring high standards of radiological

protection but does not suggest that a 1 µm AMAD particle distribution is representative of radioactive aerosols in the workplace.

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Contents

1 Introduction 6

2 Review Process 6

3 Generation and Behaviour of Airborne Particulate 7

3.1 Measured Particle Size Distributions within Nuclear Workplace Environments 8 3.2 Measured Particle Size Distributions for Accident Conditions 9 3.3 Generic Geological Disposal Facility (GDF) Environment 10

4 Inhalation of Particulate Material into the Human Respiratory System 12

4.1 The ICRP Human Respiratory Tract Model 13 4.2 Influence of Particle Solubility 17

5 Guidance Issued by other UK Nuclear Licensees on Particle Size Distributions 18

6 Conclusions 19

7 Recommendations 19

8 References 21

Appendix 1 Particle size distributions of radioactive aerosols measured in workplaces 25

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1 Introduction NDA RWMD asked AMEC Power and Process to carry out a short review on respirable particle size ranges and from this to recommend a value or values which should be adopted for use within their safety assessments for workers and members of the public. This report, summarising the review, will record the evidence and reasoning behind the recommendations so that a clear evidence trail is provided to the choice of respirable particle size.

2 Review Process The approach to the review has followed a number of strands. Firstly the review focused on the generation of airborne particles and summarised the information about the general particle size ranges produced by different process. The scientific literature has been examined for reported measurements within relevant nuclear workplaces which provide information on particle size distributions both under normal operating and, where possible, accident conditions. In many cases the size distribution of freshly generated particulate is different to the size distribution which reaches the mouth or nose of a worker within the release vicinity and so the influences of the most important mechanisms by which particulates may be removed from the air prior to inhalation have also been reviewed. Some information about the generic repository environment and the type of expected accident scenarios has also been considered to establish whether it is likely that there is anything specific about the operations or the environment which would result in unusual particle size distributions requiring more specific information. For those particles which remain suspended and are then inhaled the review has examined the information around deposition and clearance of airborne particles within the human respiratory system and the biological models which have been developed to represent the dose which may be received from the inhalation of radioactive particulate. In terms of respiratory deposition modelling the starting point for the review is the Human Respiratory Tract Model developed by the International Commission for Radiological Protection (ICRP) and published in 1994 [1]. As some time has passed since the release of ICRP Publication 66, this review has also examined the literature for any documented shortcoming of the model, proposed updates or alternative models, particularly related to the particle size distribution and deposition of inhaled material. The review also reports on the position taken by other nuclear licensees within the UK regarding respirable particle sizes and that adopted by other key organisations active in radiological protection and nuclear safety. In the conclusions section the review pulls together the information gained from the work described above and uses this as the basis for a recommendation regarding the choice of a respirable particle size range for safety assessment purposes. The structure of this document follows the description of the review process above.

� Section 1 provides an introduction and explains the purpose of the review.

� Section 2 outlines the work carried out as part of the review.

� Section 3 covers the generation and behaviour airborne particles, including some information on the generic repository environment.

� Section 4 addresses the deposition of airborne particulate in the respiratory tract giving information on key deposition and clearance mechanisms, the models used to translate particle deposition to calculated radiological dose and the importance of particle solubility.

� Section 5 gives the position of other UK nuclear licensees and key organisations in the nuclear industry relating to airborne particle size distributions.

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� Section 6 draws conclusions from the reviews and uses these to make recommendation to RWMD regarding the use of particular airborne particle size distributions in safety assessment

A section of references follows the main text and Table 1 in Appendix 1 presents the information on measured workplace particle size distributions.

3 Generation and Behaviour of Airborne Particulate Airborne contaminants occur in the gaseous form (gases and vapours) or as aerosols. In scientific terminology, an aerosol is defined as a system of particles suspended in a gaseous medium, usually air in the context of occupational hygiene [2]. Airborne dust or particles are always present in the air we breathe and yet unless we enter a visibly dusty environment we are often not aware of them. These airborne particles are formed in a variety of ways by both man-made and natural processes. Man-made processes include physical processes such as grinding, crushing, and drilling, combustion processes and secondary processes like resuspension from surfaces by sweeping and walking. Within a nuclear workplace, airborne activity can be present due to minor releases of radioactivity caused by the migration of contamination through seals, small releases during posting and glove change operations, and the later resuspension of this material. Dusts are solid particles, ranging in size from below 1 µm up to at least 100 µm, which may be or become airborne, depending on their origin, physical characteristics and ambient conditions [2]. In referring to the particle size of airborne dusts a common adopted measure of particle size is the aerodynamic diameter which is defined as “ the diameter of a hypothetical sphere of density 1 g/cm

3 having the same terminal velocity in calm air as the particle in question regardless of its

geometric size, shape and true density” [2]. Within any aerosol produced the airborne particles present will have a wide range of sizes and mathematical functions are often used to describe this distribution. The function most often used to describe aerosols is the lognormal distribution which is a close fit for such skewed distributions where mean values are low, variances are large and only positive values are possible [3]. The distribution is described by the median diameter, which is the 50

th percentile particle size and the

geometric standard deviation (GSD or σg) which is the ratio of the 50th percentile size to the 16

th

percentile size (or the 84th percentile size to the 50

th percentile size).

As this review is concerned with radioactive airborne particles another term, the Activity Mean Aerodynamic Diameter (AMAD) also needs to be defined. The AMAD is defined as the value of aerodynamic diameter such that 50% of the airborne activity in a specified aerosol is associated with particles smaller than the AMAD, and 50% of the activity is associated with particles larger than the AMAD [4]. The range of particle sizes formed is largely dependent on the types of particle formation mechanisms present, which in turn relates to the type of activities being carried out or the events

occurring. Combustion particles typically occur in the size range 0.01-0.05 up to around 10 µm.

Although combustion processes can generate particles smaller than ~0.5 µm these particles often

agglomerate to form larger particles of up to around 10 µm [2]. Particles generated by mechanical

processes typically occur in the size range 0.5 up to 100 µm [2]. Measured particle sizes in nuclear workplaces are discussed in Section 3. Following generation, airborne particles in the sub-micron and micron range can be removed from the air via a number of processes including:

� gravitational settling

� inertial impaction

� charge effects

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Settling of airborne particles occurs under the influence of gravity and so the larger and heavier particles will tend to drop out first. It is generally accepted that particles with an aerodynamic

diameter greater than 50 µm do not usually remain airborne for very long [2]. This can be shown using Stokes Law to calculate the time various particle sizes would take to settle a particular

distance in still air. For example, a particle with an aerodynamic diameter of 100 µm would take

less than 6 seconds to fall through 1.5m of still air, a 10 µm particle would take 8.2 minutes to fall

the same distance and a 1 µm particle would take 12 hours [5].1 The terminal velocity of a 1 µm

particle is about 0.03 mm/sec, so movement within the air is more important than sedimentation through it [2]. It is important to bear in mind the relatively rapid removal of larger particulates from any release when assessing the fraction that is likely to reach a worker or member of the public in the context of hazard and safety assessments. This is particularly important in the case of potential dropped packages (Section 3.2 below), where the information available on the basis of the

experiments conducted relate to particles smaller than 100 µm, and only a very small proportion of

the resulting release comprises particles larger than 10 µm. Particle inertia impaction, whereby airborne particles can be deposited on surfaces, is another removal mechanism. When an obstruction causes the air stream carrying the particles to change direction, particles with sufficient inertia will continue to move in a straight line, colliding with the surface of the obstruction where they will tend to be trapped and held. Causing aerosols to impact on a cascade system in which the aerosol is separated into fractions of successively smaller sizes is a widely used sampling technique for measuring airborne dust levels and size distributions [6]. Particles can be become positively or negatively charged depending on their generation process. These charged particles can then become attracted and captured onto surfaces due to electrostatic forces.

When airborne particles have a size of less than 0.1 µm, Brownian diffusion becomes a significant factor and particles may not settle to any significant degree. For all but the smallest particles, these processes will act to reduce the size fraction of any released material that will reach the breathing zone of an exposed individual. Another important factor is clearly the exposure distance. The further away an individual is from the point of the release the more likely it is that they will only be exposed to smaller particle sizes as a significant fraction of the larger particles may be removed through settling.

3.1 Measured Particle Size Distributions within Nuclear Workplace Environments A key review paper in this area was published by Dorrian and Bailey in 1995 [7]. This paper collated the results of 52 earlier papers where aerosol measurements had been made in a range of workplaces. The Dorrian and Bailey review paper covers a wide variety of workplaces associated with the nuclear industry, including those associated with nuclear power, research centres, uranium mills, fuel handling facilities, reprocessing plants and a range of other facilities. Their work did not exclude papers or results unless they were clearly not relevant to the study, for example papers reporting activity size distributions of radon progeny aerosols. All the reported studies carried out air sampling in the workplace, with over 60% using a cascade impactor device to make AMAD measurements. Most air sampling devices tend to underestimate

AMADs due to intake efficiencies decreasing with increasing particle size above about 5 µm due to particle inertia; however this problem is recognised and can be compensated for [7].

1 Interestingly, this work also suggests strongly that the timescale for particles settling in still air

within a typical room (~2.5 m high) will be similar to that of the ‘dry deposition half life’ for these particles from turbulent air in the same room.

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Dorrian and Bailey looked for differences between workplaces and for differences with regard to the dates of the studies themselves. When the measured values of AMAD from all the workplaces were plotted they largely conformed to a log-normal distribution, see Figure 1. In Figure 1 it can be

seen that results below about 1 µm are inconsistent with a single log-normal function and suggest a bimodal distribution. No systematic change with the general age of the studies was uncovered, although there were relatively few papers before the 1970s.

Figure 1 Distribution of measured AMADs for all workplaces [7]

The median AMAD value reported in their paper for all workplaces was 4.4 µm. Dorrian and Bailey

recommended the choice of a 5 µm AMAD as the default value for occupational exposures due to the tendency towards underestimation in the measurement of airborne particle sizes [7]. The current study has extended this earlier review with a small number of additional references which are discussed below and reported in Appendix 1. Ansoborlo et al [8] measured the particle size distribution of uranium aerosols in the French nuclear fuel cycle. The measurements reported had been collected over a 10 year period from four uranium cycle facilities. The average measured

AMAD is reported as 5.5 µm, with a standard geometric deviation of 2.6. Cheng et al [9] were able to characterise a plutonium aerosol collected during an accident at Los Alamos. The measured

AMAD of the aerosol was 4.8 µm with a standard geometric deviation of 1.5. This large group of measurements across a wide range of industries gives confidence that the

suggested figure of 5 µm AMAD is indeed typical of airborne particle sizes within nuclear

workplaces in normal operating circumstances. The ICRP therefore recommend the use of 5 µm AMAD aerosol for assessing worker doses to airborne particulate.

3.2 Measured Particle Size Distributions for Accident Conditions When undertaking safety assessments it is also necessary to focus some attention on potential accident conditions where much larger inhaled doses could be received due to releases from primary containment systems. Most of the reported data on measured particle size distributions in the workplace, described above, relates to normal operating conditions and hence there is much less evidence and certainty regarding the size distribution of airborne activity in the case of an accidental release. In the GDF, a drum, package or glovebox may be challenged by a significant drop or vehicle impact. Prior to the impact the particulate material is likely to be relatively static but could be made airborne from the energy of the impact. Particulate material will also be generated during

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progressive damage, such as the release of surface contamination or the propagation of cracked surfaces. If the containment system were to remain intact, no matter how much material is made airborne by the impact, there would be no airborne release. For particulate material to be released, the impact must both damage the containment system and make the particulate material airborne. However, in this case the distortion of the container would help to reduce the loading passed to the material inside and even in a damaged state the containment would act to contain airborne activity, most likely removing larger particles. An experimental study carried out in 1982 attempted to characterize the fragments generated when brittle radioactive waste glasses and ceramics were impacted. A falling weight was directed onto a sphere of material (glass or ceramic) held in a metal chamber. Their experiments showed that in all cases less than 1% of the fragments produced had

a size fraction of less than 10 µm, i.e. material which, if released, could remain airborne for more than a few minutes [10]. A study for UK Nirex Ltd, RWMD’s previous organisation, on break-up and airborne release from different cemented materials subjected to impact was carried out in 2005 [11]. This work carried out impact tests on nine different cemented materials to provide impact performance data. The generation of airborne particles with aerodynamic diameters smaller than

100 µm were collected following the impact. In one sample the entire airborne fraction was classified by size. For nearly all specimens a linear relationship was found between the mass of

airborne material generated with particle sizes below 100 µm and the specific energy input. The particle size distribution of the analysed sample showed that only a small fraction of the released

material was of a size fraction less than or equal to 10 µm (~0.6 wt%). However, due to the settling velocities associated with larger particles, see Section 3, only these smaller particles would be expected to remain airborne for a sufficient length of time to allow inhalation. The DoE Handbook for non-reactor nuclear facilities [12] is also a valuable information resource for information of this kind. In conclusion, although there is not the same level of experimental evidence to support a choice of airborne particle size distribution under accident conditions compared to normal operations, the discussion above shows that in many cases the types of containment deployed in the nuclear industry will result in the preferential removal of larger particle sizes and hence lower the median of the resulting airborne material significantly. Potential exposure to larger particle sizes could occur only in cases where a pressurised release results or where a release occurs at height with particulate material falling through a workers breathing zone. Determination of whether this could be an issue would depend on the type of scenarios that could occur and the physical separation of the personnel from the incident concerned.

In the light of the discussion above it is concluded that the use of a 5 µm AMAD default for workplace accidents as well as normal operations is appropriate.

3.3 Generic Geological Disposal Facility (GDF) Environment Although the measurements of particle size distributions reported in the previous section indicate

that a 5 µm AMAD is an appropriate default value for aerosols generated in a range of nuclear workplaces, it is important to consider whether anything specific in either a GDF environment or the envisaged fault scenarios within could justify an amendment to this conclusion. At present, no site for a GDF in the UK has been identified and therefore RWMD’s safety assessments are focused around three different representative concepts – based on disposal within higher-strength rock, lower-strength sedimentary rock and evaporite formations. The temperature within a GDF within which personnel may require access will range from 20 up to a

maximum of 50 degrees centigrade (°C). With the exception of the evaporite concept which would be expected to be relatively dry and surrounded by rocks with low permeability, humidity within a GDF could be relatively high. A key requirement would be to prevent the corrosion of emplaced packages and this may set limits on permitted relative humidity, but at present the only specified constraint is that the relative humidity should be below 90% to ensure the correct operation of extract filters [13]. Aerosol particles with a high content of soluble material are known to be hygroscopic and in conditions of high humidity, therefore, any soluble particles that may be present may absorb water, resulting in an increase in particle mass and size. The extent of water

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absorption would depend on the proportion and makeup of the airborne material that is water soluble [14 and15]. This effect will tend to encourage particle size growth for some particulate material and their subsequent removal by deposition. Therefore, in the human respiratory tract, the near-saturation conditions will have the effect of promoting an increase in size of particles, altering the deposition profile along the airways, enhancing or reducing deposition within the deeper regions of the lung [16]. This is a factor which should be given further consideration once more definite information on the relative humidity within a facility becomes available. As mentioned above it is also important to consider whether any of the fault scenarios could result in airborne distributions with higher proportions of larger particle sizes. Examples would include scenarios in which a pressurised release occurs directly into an operator’s breathing zone or where a release could occur above an operator’s head with particulate being inhaled during the process of gravitational settling in addition to dispersion. Fault scenarios included in this study involve impacts and drops from a maximum height of 15 m. Faults that may occur at surface facilities of a GDF include low level drops, and loss of containment events (including releases following a seismic event). This high level consideration of the faults that can be postulated within a GDF, together with their context, does not suggest that the distribution of particle sizes should be significantly different to other workplaces within the nuclear industry.

3.3.1 Particle Size Distributions for Assessing Doses to Members of the Public

Members of the public could be exposed to airborne particulate released from a GDF in two main ways: through air extracted from sub-surface facilities of a GDF and then discharged into the environment and through the more direct release of particulate material above ground from surface facilities. Airborne material released within the sub-surface regions of a GDF would have to travel some distance before being discharged to atmosphere. This period of time will result in the larger particles depositing from the air due to gravitational settling. In addition all extracted air from the facility would pass through several stages of High Efficiency Particulate in Air (HEPA) filtration prior to discharge. The performance levels which must be achieved by different HEPA filters are defined in BS EN 1822-5:2009 [17].HEPA filters used for applications in the nuclear industry (taken as type

H14) are required to capture a minimum of 99.97% of contaminants at 0.3 µm in size. HEPA filters

have higher efficiencies in removing particles that are larger than 0.3 µm. Efficiencies are generally

quoted around the 0.3 µm value as this particle size presents the greatest challenge to the operation of such equipment. Any airborne material released from a GDF which is able to travel as far as the extract filters would mostly be captured there and hence any release should be in the sub-micron particle range. In Section 13 (paragraph D196) of ICRP 66 [1], a reference AMAD of 5 µm is recommended as being representative for occupational exposure settings. However, it then states that for exposures in the general environment, it states that a smaller value of 1 µm AMAD may be appropriate. The age dependent inhalation dose coefficients for members of the public in ICRP 72 [18] have therefore been based on an AMAD of 1 µm. While there is no further explanation, as the dose coefficients are aimed at assessment of consequences from normal operations, the justification is assumed to be that all environmental airborne activity will either pass through a filtered discharge point or be described as adventitious discharge as it is released via building fabric. Both processes will reduce the AMAD. This assumption would not apply in the case of an accidental release where particulate material is released directly to the environment with little or no filtration. The assumption, within the generic Operational Safety Case [13], that members of the public will be at least 100 m, and typically much further, away from any airborne releases means that the larger particles would be preferentially removed by settling. As the maximum particle size remaining airborne is reduced, the competing effects of turbulence and gravitational settling are likely to limit the loss by this mechanism. However, turbulence will continue to bring particulate close to the ground where interception and

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impaction will preferentially remove larger particulate and thermodynamic deposition will further deplete the airborne concentration and modify the particle size distribution. On the basis of the particle settling rates that can be obtained using Stokes Law (Section 3, page 8), only a small percentage of particles larger than 10 µm in diameter will remain airborne at distances where members of the public could receive inhaled doses as a result of releases from a GDF. It is therefore reasonable to base the airborne concentration on the fraction of the release which has a particle size less than 10 µm. It is, however, not possible to predict the AMAD of the residual aerosol. While it must be lower than 10 µm it is not clear whether it would be closer to 1 or 5 µm. Factors that would influence the choice of which value would be appropriate for a specified release would include the distance of travel to the most exposed individuals, the prevailing atmospheric conditions and the nature of the release (i.e. release resulting from an impact or drop, and the height to which the resulting airborne material may rise). Age dependent inhalation dose coefficients for members of the public provided in ICRP 72 [18] are based on a 1 µm AMAD aerosol as suggested in ICRP 66. Ahead of the forthcoming publication of the ICRP document Compendium of Pre-ICRP Publication 103 Dose Coefficients for Use in Radiological Protection of Workers and Members of the Public, ICRP has made available downloadable inhalation dose coefficients for members of the public for 5 µm AMAD aerosols [19]. As ICRP point out, however, although these data have been made generally available, the ICRP Publication mentioned above will be the definitive and referenceable compendium of these coefficients. A comparison shows that the dose coefficients for 1 and 5 µm AMAD aerosols are generally within a few tens of percent of one another at most, for both workers [18] and members of the public [19]. It is therefore suggested that decisions on the most appropriate dose coefficients for members of the public from specified releases to the environment are informed by sensitivity analyses where considered appropriate, with judgement influenced (outlined earlier), by the proximity of the most affected population, the radionuclide composition of the release, the chemical form, and the physical characteristics of the potential incident.

2 These factors should be taken into account in

addition to the degree of conservatism considered appropriate.

4 Inhalation of Particulate Material into the Human Respiratory System Only particles that are small enough to remain airborne for a reasonable period of time will be inhaled and deposited in the human respiratory tract. The largest inhaled particles, with

aerodynamic diameter greater than approximately 30 µm, are deposited in the airways of the head that is the air passages between the point of entry at the mouth or nose and the larynx [2]. For the particles that enter the respiratory tract, the human respiratory system has a number of mechanisms to reduce the proportion of airborne particles that can be transported into the lower regions of the respiratory tract, potentially causing harm. As air is inhaled it is drawn in through the nose and/or mouth into the upper respiratory system, which consists of the nasal passages, trachea, and the airways conducting the flow down to the lungs comprising the bronchi and bronchioles. The inhaled air becomes moist and makes numerous twists and turns through the nasal passages and branching airways. Particles in the range of 10 to 100 µm in size are unable to make the turns and impact on the nasal hairs, nasal mucosa, or the mucus-covered ciliated epithelium in the bronchi and bronchioles. Soluble particles trapped in this area will simply dissolve, while insoluble particles are transported up the conducting airways by the ciliated epithelium and swallowed or expelled from the body by coughing or nose blowing.

2 . The review of extant guidance issued by other UK Nuclear Licensees on particle size

distributions presented later in Section 5 shows that in most cases, values based upon an AMAD of 1 µm is adopted, consistent with currently definitive advice from ICRP.

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Smaller particles, less than 10 µm in size, are generally able to travel into the pulmonary part of the lungs (the respiratory bronchioles, alveolar ducts, and alveolar sacs), where gas exchange, or respiration, occurs. Particles that reach this part of the lungs are called respirable particles and, if deposited, are typically removed by particle-ingesting cells called macrophages if they do not dissolve first. The macrophages transport the insoluble particles either to the lymphatic system or to the ciliated epithelium in the bronchioles. The description above is a summary compiled from many literature sources regarding the current understanding regarding the behaviour of particulate matter within the human respiratory tract. Although an exhaustive search has not been carried out as part of this review, it is clear that particle deposition within the human respiratory tract is well understood and has been subject to extensive study in the wider medical community, principally in relation to the environmental health issues associated with inhalation of environmental airborne particulate, e.g. fuel combustion particles and the development and targeted delivery of aerosolised drugs. A number of definitions regarding inhalable material are given by the European Standards Organisation (CEN) and published in the UK in BS EN 481:1993 [20] and these are reproduced below. Inhalable fraction – defined as the mass fraction of particles which can be inhaled by nose or mouth. Since there are no experimental data on inhalable fraction of particles with an aerodynamic diameter greater than 100 µm, particles in this size range are not included in the inhalable convention. Thoracic fraction – defined as the mass fraction of particles that pass the larynx. The median value of the distribution of particle sizes able to penetrate into the respiratory tract beyond the larynx is 11.64 µm with a geometric standard deviation (GSD) of 1.5. It has been shown that approximately 50 % of the particles in air with an aerodynamic diameter of 10 µm belong to the thoracic fraction. Respirable fraction – defined as the mass fraction of particles that can reach the alveoli. The median value of the distribution of particle sizes in this category is 4.25 µm with a GSD of 1.5. It has been shown that 50 % of the particles with an aerodynamic diameter of 4 µm will be in the respirable fraction. This gives a clear understanding of what range of airborne particulate can be taken as respirable, i.e. reaching the sensitive regions of the lung.

4.1 The ICRP Human Respiratory Tract Model The International Commission on Radiological Protection (ICRP) provides information and guidance to the world wide nuclear community to inform radiological protection practices. Fundamental to that function is the provision of reference dose coefficients for workers and members of the public. The ICRP publish dose coefficients for both inhalation and ingestion of radionuclides. In order to calculate these dose coefficients the entry and behaviour of radionuclides within the human body needs to be well understood and then modelled appropriately. Since the 1960s the ICRP have developed three inhalation models, each increasing in sophistication, taking account of progressively more information on the physiology of the respiratory tract. The first model was published in ICRP Publication 2 [21], the second in Publication 30 [22] and the third, and current model, referred to as the Human Respiratory Tract Model (HRTM), in Publication 66 [1]. The deposition and clearance models included within HRTM enable the amounts of activity throughout the respiratory tract to be calculated for any time after intake. The dosimetric models then enable the resulting doses to each part of the lungs to be calculated. Both soluble and insoluble material are represented in the ICRP model and both will give a dose to the body, with the exception of material deposited in the front of the nose, see below for more details.

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The mathematical models have been validated against experimental data where human subjects have inhaled aerosols of differing sizes and the resulting deposition within the airways is evaluated from measurements of the distribution of particles in the expired air. The HRTM is described in detail in ICRP Publication 66 and other references [23] and only a summary is provided here. Computer programs have been developed to facilitate calculations using the HRTM model [24, 25].

Figure 2 The human respiratory tract showing the various regions used within the ICRP

Human Respiratory Tract Model (HRTM) As shown in Figure 2 the respiratory tract is modelled as five regions, based on differences in radio-sensitivity, deposition and clearance.

� The extrathoracic (head and neck) airways (ET) is sub-divided into regions termed ET1, the front of the nose and ET2 which represents the posterior nasal and oral passages, the pharynx and larynx.

� The thoracic regions is sub-divided into three areas: the Bronchial region (BB), which includes the trachea and bronchi, the bronchioles (bb) and the alveolar-insterstitial (Al), which is the main region in which gas exchange takes place.

The deposition model calculates where the airborne particles will deposit within each region of the respiratory tract. The ICRP’s task group on lung dynamics concluded that respiratory tract deposition was related to the median of the distribution, in particular the activity median aerodynamic diameter (AMAD) and the breathing characteristics of the person, i.e. whether

breathing takes place through the mouth or nose. It is worth remembering that a 5 µm AMAD

aerosol is actually a distribution of particle sizes around a median size of 5 µm, and larger particles will deposit preferentially within the upper airways. Table 1 below shows the calculated deposition

of a 5 µm AMAD in each of the modelled regions of the respiratory tract.

D.00005.01/P001Issue 6

Table 1 Regional deposition of inhaled 5 µµµµm AMAD in reference workers carrying out light work [1, Table 28]

Region Deposition % (Male)

ET1 33.9

ET2 39.9

BB 1.8

Bb 1.1

AI 5.3

Total 82

It is worth noting that 18% of inhaled particulate is not deposited and is therefore present in the exhaled air. Once deposited, the clearance of particles is modelled by a number of processes within the ICRP HRTM:

• physical removal, i.e. nose blowing;

• particle Transport (movement of particles into the gastro-intestinal tract and lymph nodes);

• absorption into blood.

This is shown below in Figure 3.

Figure 3 Routes of clearance from the respiratory tract modelled in the HRTM

The relative importance of these clearance mechanisms differs for the different part of the respiratory tract.

4.1.1 Clearance: Particle Transport

Figure 4 below shows a representation of the compartment model used in the HRTM to model particle transport from each respiratory tract region. The rates of transport are shown alongside the arrows in units of d

-1.

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Figure 4 Compartment model representing particle transport within the HRTM

4.1.2 Clearance: Absorption to blood

For a given particle size distribution defined by the AMAD a fraction of the activity will be deposited within each part of the respiratory tract. The solubility of a radionuclide affects the rate of transfer after deposition and therefore the target organs and residence time. These are all taken into account in the ICRP modelling. The ICRP model uses three default absorption parameters to account for particle solubility and absorption in the blood. These absorption parameters relate to the speed of absorption of radionuclides into the body. The ICRP recommends that material-specific rates of absorption should be use where reliable experimental data exists. Where this is not the case default values of parameters are recommended according to whether absorption is considered to be fast (Type F), moderate (M) or slow (S). The absorption rates are expressed as approximate biological half-times. The amounts of particulate deposited in each region that are dissolved and reach blood are summarised below, taken from [26].

• Type F (Fast) Selection of this absorption type means that 100% of the material is absorbed, with a biological half-life of 10 minutes. There is rapid absorption of almost all the material in the lungs (BB, bb and AI model compartments) and 50% of the material deposited in the ET2. The other 50% of the material deposited in ET2 is cleared to the gastrointestinal tract by particle transport. Examples of radionuclides where this absorption parameter would be valid are compounds of iodine and caesium which are both highly soluble.

• Type M (Medium) 10% of the material deposited is absorbed with a biological half-life of 10 minutes and 90% with a biological half-life of 140 days. There is rapid absorption of about 10% of the deposit in the BB and bb compartments and 5% of material deposited in ET2. About 70% of the deposit in AI eventually reaches body fluids by absorption.

• Type S (Slow) 0.1% of the material deposited is absorbed with a biological half-life of 10 minutes and 99.9% with a biological half-life of 7000 days. There is little absorption from ET, BB or bb and about 10% of the deposited material in AI eventually reaches body fluids by absorption.

For all three absorption types all the material deposited in the ET1

region (the front part of the nose)

is removed by physical means, such as nose blowing and does not result in a dose. In the other

D.00005.01/P001Issue 6

model regions most of the deposited material that is not absorbed is cleared to the gastrointestinal tract by particle transport. The small amount transported to the lymph nodes continues to be absorbed by body fluids at the same rate as that selected for the respiratory tract.

4.1.3 Later Work

A few years after ICRP Publication 66 [1] was published, the National Council on Radiation Protection and Measurements (NCRP) published its own model for the deposition of airborne radioactive particles inhaled into the respiratory tract [27]. A comparison of the particle deposition calculated by the two models was carried out by Yeh et al. [28]. Their results showed that the deposition fractions calculated by the two models are very similar except for particles smaller than

10 µm, where the ICRP Publication 66 model predicts a lower tracheobronchial deposition and a higher pulmonary deposition than the NCRP model. It was noted that this could have a significant effect on the calculated dose estimates of inhaled ultrafine particles. Recently the ICRP has been working on a revision of the dose coefficients for workers [29]. This has involved looking again at both the human respiratory tract model and the human alimentary tract model. Revisions have been made to both models to take account of more recent data and information which has become available since the original ICRP publication was issued. In the consultation document [29] one change proposed to the HRTM is a transfer from ET1 to ET2. If adopted this change will result in particle transport rates of 0.6 d

-1 from ET1 to the environment and

1.5 d-1

from ET1 to ET’2. Thus material deposited in ET1 (soluble and insoluble) would result in

some systemic uptake and, if adopted, this change is likely to increase dose coefficients in many cases.

4.2 Influence of Particle Solubility A report produced in 1998 by Jackson [30] recommended that for radiological protection purposes no explicit account should be taken of the respirable fraction of an aerosol (that which is able to penetrate into the alveolar region of the lung) unless it can be demonstrated that the radionuclides are in predominately insoluble form. The key point made by the report is that when particles containing significant quantities of soluble materials are inhaled, the dose received from large

AMAD aerosols, e.g. 100 µm AMAD, is only slightly less (a factor of 2) than the dose received from

small AMAD aerosols, e.g. 1 µm AMAD. Were both to comprise insoluble material, the committed

dose from a 100 µm AMAD aerosol would be very much less per unit activity inhaled when

compared to a 1 µm AMAD aerosol. This effect would clearly be important if the size distribution characteristics of the aerosol of interest were significantly larger than the default value recommended by the ICRP and contained a high proportion of soluble particulate but it does not preclude the application of appropriate reduction factors to the initial release to account for gravitational settling or other mechanisms which would mitigate the consequences of inhalation of airborne activity from an accidental release. Few safety assessments are supported by measurements and analysis of the airborne particulate associated with the scenarios under consideration. Most have to make assumptions about the nature of the airborne particulate and as shown in Section 3.1, measurements of airborne particulate made

within a large number of nuclear workplaces support the use of a 5 µm AMAD aerosol. ICRP

therefore recommend the use of 5 µm AMAD for workers, and in the absence of specific

information to the contrary, suggest a corresponding value of 1 µm AMAD for members of the public. In the context of a generic GDF there is no compelling evidence to suggest that fault sequences will result in individuals being exposed to a significantly different particle size distribution from the recommended default values should be used in assessments of the radiological consequences of material released to air. In conclusion, the particle deposition and solubility aspects addressed within the ICRP Publication 66 model and the consequent dose factors in ICRP 68 for workers and ICRP 72 [18] for members of the public should be sufficient to avoid underestimation while ensuring that the resulting doses are not unduly pessimistic.

D.00005.01/P001Issue 6

5 Guidance Issued by other UK Nuclear Licensees on Particle Size Distributions Internal guidance documentation issued by Nuclear Licensees for the conduct of radiological assessments in general do not make explicit recommendations on particle size distributions for material released in routine and accidental situations. However, the choice of dose coefficient for workers and members of the public is inextricably bound up with assumptions about the respirable fraction. Table 2 below summarises the position taken by a number of Licensees.

Organisation Exposed Group Assumptions regarding default particle size distribution, AMAD

Dounreay Site Restoration Ltd (DSRL)

Worker

1 or 5 µm is assumed for different radionuclides based on selecting the maximum dose factors for different lung clearance classes for both accident and normal operations scenarios [31].

Member of the Public 1 µm assumed for all scenarios

Research Sites Restoration Ltd (RSRL)

Worker

1 or 5 µm is assumed for different radionuclides based on selecting the maximum dose factors for different lung clearance classes for both accident and normal operations scenarios [31]

Member of the Public 1 µm assumed for all scenarios

Atomic Weapons Establishment (AWE)

Worker Data presented for 1 µm and 5 µm for assessment of doses from normal operations and accidents [32].

Member of the Public Data presented for 1 µm and 5 µm

Sellafield Ltd Worker 1 µm assumed for accident conditions [33]

Member of the Public 1 µm

Table 2 Assumptions regarding inhaled particle size distribution

From the table above it can be seen that in the absence of more specific information, most UK Nuclear Licensees have adopted the ICRP recommended or suggested values (as appropriate) for mean particle sizes to calculate doses to workers and members of the public exposed to airborne radioactive particles. This is clear from the reported dose coefficients for use in their safety assessments which are taken from ICRP Publication 68 [34] and ICRP Publication 72 [18] respectively. In contrast with the other licensees, AWE present inhalation dose coefficients for an

assumed AMAD of 5 µm in addition to 1 µm for members of the public. The tabulated data for

5 µm, which are specified to be used only for exposures cause by resuspension, however, are derived from a draft internal document [35] and not from the ICRP tabulation referenced earlier [19], and are confined to values for isotopes of uranium, plutonium and americium.

Cases where licensees have selected values other than 5 µm for use in workplace assessments reflect the desire to ensure that the doses calculated as a result of potential accidents are conservative. This would apply even where measurements within the workplace have suggested

that 5 µm is an appropriate value to use because of the uncertainty associated with particle size distributions related to accidental releases. For assessments of doses to members of the public,

the extant guidance issued by all licensees apart from AWE indicates that a 1 µm AMAD particle size is to be used both for routine and accidental releases.

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The International Atomic Energy Agency (IAEA) publishes a series of dose coefficients and Derived Air Concentrations (DACs) [36] for radionuclides that are likely to be of interest in a number of workplaces where no information is available. The published inhalation dose coefficients and associated guidance are based upon the assumption of a default median particle size for the

workplace of 5 µm. As discussed earlier, in the absence of specific information to the contrary the

suggestion by the ICRP for particle AMAD for releases to the environment is 1 µm [1], although

data published after Publication 72 [19] does provide corresponding values for a 5 µm AMAD particle distribution.

6 Conclusions On the basis of the review undertaken the following conclusions can be drawn

� Measurements of airborne particulate in a wide range of nuclear workplaces have shown that activity is distributed on airborne particulate such that the Activity Mean Aerodynamic Diameter (AMAD) is in the region of 5 µm.

� From the information available there are no obvious reasons why the particulate produced within a facility such as a GDF should have a significantly differing activity distribution to that measured in a wide range of other nuclear workplaces. Conservatively this is represented by

the mass fraction of particles <10 µm aerodynamic diameter.

� While the initial release of particulate from an accident may have a large AMAD, factors such as residual containment decontamination factor (DF) and gravitational settling will reduce the

particle size distribution to which workers are likely to be exposed in a GDF, to 5 µm AMAD,

and that to which members of the public are exposed to between 1 µm and 5 µm AMAD. This will be dependent on release and environment related factors.

� In terms of predicting dose from inhaling radioactive particulate the AMAD is the key parameter for determining deposition within the respiratory tract.

� The mass fraction of airborne particulate which can deposit in the alveolar region of the lungs is centred on particles with an AMAD of around 5 µm with a geometric standard deviation of 1.5.

� The ICRP recommends adoption of an AMAD of 5 µm as a default workplace particle size where no specific information is available.

� In the absence of specific information to the contrary, the ICRP suggests that adoption of an

1 µm AMAD as a default particle size for airborne exposure of members of the public may be appropriate

� In the assessment of doses from inhaled activity, Nuclear Licensees within the UK adopt default AMADs of either 5 or 1 µm to quantify airborne particulate released under normal or accident conditions. This information is then used in conjunction with values tabulated by the ICRP for these sizes to select dose coefficients for the radionuclides of interest in the safety assessments.

7 Recommendations It is clear from Table 2 that some other UK Nuclear Licensees have adopted a conservative

approach when calculating worker doses and have assumed the aerosol inhaled has a 1 µm AMAD value rather than using the more justifiable value of 5 µm [7, 8].

Adoption of inhalation dose coefficients for a 1 µm AMAD aerosol size in safety assessments more generally would ensure that deposition of material to the most radiosensitive areas of the lungs, and hence in general the dose received, is maximised. This approach is more commonly taken when assessing the inhalation dose received by members of the public and when more conservative assessments are warranted.

D.00005.01/P001Issue 6

In the case of workers involved in accidents, the scarcity of relevant measured data from historical

incidents is a significant issue. Notwithstanding, the use of dose coefficients based on a 1 µm AMAD is still considered to be too cautious as the assumptions and parameter values adopted in such assessments are already subject to considerable caution to ensure that the doses are not underestimated. Further conservatism could result in over-engineering and unnecessary assurance effort. Currently, it is felt that there is insufficient definitive evidence to the contrary to recommend the NDA RWMD adopt anything different to the inhalation dose coefficients for the standard default AMAD values recommended by the ICRP which are used extensively within the nuclear industry. Therefore:

It is recommended that NDA-RWMD adopt a 5 µm AMAD particulate size inhalation dose coefficients as the default value to characterise the respirable fraction of aerosols released within the workplace environment in normal operational conditions and in the case of potential accidents.

It is recommended that NDA-RWMD adopt a 1 µm AMAD particulate size inhalation dose coefficients as a default value when undertaking safety assessments to calculate doses to members of the public from an airborne release of radioactive material, whether it occurs as a result of normal operations or from potential accidents. However, as corresponding values for a

5 µm AMAD particulate size have also recently been made available by ICRP, where releases have not been subject to prior filtration (i.e. for accidents), NDA-RWMD may consider it appropriate to conduct a sensitivity analysis to inform judgements on which values of inhalation dose coefficients to adopt in specific circumstances. The factors that would influence this decision are highlighted in the report. It is recommended that NDA RWMD assumes that, in the case of particulate arising from an

accidental release, members of the public are exposed to the mass fraction of <10 µm aerodynamic diameter.

D.00005.01/P001Issue 6

8 References

1 International Commission on Radiological Protection. Human Respiratory Tract Model for Radiological Protection. ICRP Publication 66 (Oxford Pergamon Press) Ann. ICRP 24(1-3) 1994.

2 World Health Organisation. Hazard Prevention and Control in the Work Environment:

Airborne Dust. WHO/SDE/OEH/99.14, 1999. 3 Limpert, E. et al, 2001. Log-normal distributions across the sciences: keys and clues.

Bioscience. Vol 51. No5, May 2001. Available at http://stat.ethz.ch/~stahel/lognormal/bioscience.pdf.

4 International Atomic Energy Agency, 2006. IAEA Safety Glossary. Terminology used in

Nuclear, Radiation, Radioactive Waste and Transport Safety, V2, 2006. 5 Baron, P. Generation and Behaviour of Airborne Particles (Aerosols). National Institute for

Occupational Safety and Health. Available at http://www.cdc.gov/niosh/topics/aerosols/pdfs/aerosol_101.pdf.

6 HSE, 2000. Methods for the Determination of Hazardous Substances. 14/3 General Methods

for Sampling and Gravimetric Analysis of Respirable and Inhalable Dusts. HSE/MDHS/14_3, Feb 2000.

7 Dorrian, M. D. and Bailey M. R. Particle Size Distributions of Radioactive Aerosols measured

in Workplaces. Rad. Protect. Dosim. 60, 119-133, 1995. 8 Ansoborlo, E. et al, 1997. Particle Size Distribution of Uranium Aerosols measured in the

French Nuclear Fuel Cycle. Radioprotection, 32(3) 319-330, 1997. 9 Cheng, C.Y. et al, 2004. Characterisation of Plutonium Aerosol Collected during an Accident.

Health Physics, 87(6), 596-605, 2004. 10 Jardine, L.J., Mecham, W.J., Reedy, G.T. and Steindler, M.J. 1982. Final Report of

Experiemental Laboratory Scale Brittle Fracture Study of Glass Ceramics. Argonne National Laboratory, ANL-82-39, 1982. Available at http://osti.gov/bridge/servlets/purl/6554121-yLQurn/6554121.pdf.

11 Determination of Breakup and Airborne Release for different Cemented Materials when

subject to Mechanical Impact for United Kingdom Nirex Limited. Fraunhofer-ITEM Report:1129331, 2005.

12 DOE-HDBK-3010-94, 1994. US DOE Handbook: Airborne Release Fractions/Rates and

Respirable Fractions for Non-reactor Nuclear Facilities. 13 Nuclear Decommissioning Authority, Geological Disposal, Generic Operational Safety Case

Main Report, NDA/RWMD/020, 2010. 14 Birmili, M et al. Measurements of humified particle number size distribution in a Finnish Boreal

forest: Derivation of Hygroscopic particle growth factors. Boreal Environment Research. 14: 458-480, 2009.

15 Hu, D. et al. Hygroscopicity of Inorganic aerosols: Size and relative humidity effects on the

growth factor, Aerosol and Air Quality Research, 10: 255-264, 2010.

D.00005.01/P001Issue 6

16 Broday, D. and Georgopoulos, P.G. Growth and Deposition of hygroscopic Particulate Matter in the Human Lungs. Aerosol Science and Technology, 34:144-159, 2001.

17 British Standards Institution, 2009. High Efficiency Air Filters (EPA, HEPA and ULPA).

Determining the efficiency of filter elements. BS EN 1822-5:2009. 18 International Commission on Radiological Protection 1995. Age-dependent Doses to the

Members of the Public from Intake of Radionuclides - Part 5 Compilation of Ingestion and Inhalation Coefficients. ICRP Publication 72. Ann. ICRP 26 (1).

19 International Commission on Radiological Protection, www.icrp.org/page.asp?id=145, 201. 20 British Standards Institution, 1993. Workplace atmospheres. Size fraction definitions for

measurements of airborne particles. BS EN 481 1993q 21 International Commission on Radiological Protection (ICRP), 1960. Recommendations of the

ICRP, Report of Committee II on permissible dose for internal radiation. ICRP Publication 2. 22 International Commission on Radiological Protection (ICRP), 1979. Limits for intakes of

radionuclides by workers. ICRP Publication 30. Part 1. 23 Bailey, M.R. et al, 2003. Practical Application of the ICRP Human Respiratory Tract Model.

Radiat Prot Dosim 105, pp71-96. 24 Birchall, A. et al, 2003. IMBA Expert. Internal Dosimetry Made Simple. Radiat Prot Dosim,

105, pp421-425. 25 IMBA (Integrated Modules for Bioassay Analysis) Professional Plus: Estimating Intakes and

Calculating Internal Radiation Doses. Health Protection Agency. 26 Bailey, M.R and Puncher, M., 2007. Uncertainty Analysis of the ICRP Human Respiratory

Tract Model applied to Interpretation of Bioassay Data for Depleted Uranium. Health Protection Agency Report. HPA-RPD-023. ISBN 978-0-85951-591-7.

27 National Council on Radiation Protection (NCRP), 1997. Deposition, Retention and Dosimetry

of inhaled radioactive substances. NCRP Report 125. 28 Yeh, H. C. et al, 1996. Comparisons of Calculated Respiratory Tract Deposition of Particles

Based on the Proposed NCRP Model and the New ICRP66 Model. Aerosol Science and Technology, 25, 134-140.

29 ICRP Draft Report for Consultation ‘Annals of the ICRP, ICRP Publication XXX, Occupational

Intakes of Radionuclides Part 1’. ICRP ref 4828-2081-0510, February 2012. 30 Jackson, R G, 1998. Implications of ICRP 66 on the Definition of Respirable Aerosol. AEA

Technology Report. AEAT-SM(N)-59136001-R01. 31 UKAEA Safety Assessment Handbook – Inhalation Committed Effective Dose Factors,

UKAEA/SAH/D7, Issue 3, Dec 2006. 32 AWE Company Safety Procedures, CSP 865, Section 8. Dose Coefficients for use in

Assessments. 33 BNFL Technical Guide. Choice of Parameters Influencing Internal Dose Coefficients for

Safety Assessment Purposes. D3.20/SD5, 2001. 34 International Commission on Radiological Protection 1994. Dose Coefficients for Intakes of

Radionuclides by Workers. ICRP Publication 68. Ann. ICRP 24 (4).

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35 AWE, Dose per Unit Exposure Coefficients for Uranium and Plutonium Releases, Draft Report, 2000.

36 International Atomic Energy Agency, Methods for assessing operational radiation doses due to

intakes of radionuclides, Safety Series No. 27, 2004.

D.00005.01/P001Issue 6

Appendices

Contents

Appendix 1 Particle size distributions of radioactive aerosols measured in workplaces

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Appendix 1 Particle size distributions of radioactive aerosols measured in workplaces

Contents

Table 3 Size Distributions of Radioactive Aerosols in Working Environments

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Table 3 Size Distributions of Radioactive Aerosols in Working Environments – a selection from Dorrian and Bailey, 1995 [7], Ansoborlo et al [8] and Cheng [9]

Type of Workplace Process Radionuclides AMAD

µµµµm σσσσσσσσ

Dorrian and Bailey [5]

Nuclear Power industry

Nuclear power station(Magnox)

Cooling pond enclosure 239+240

Pu 5.65 4.5

238

Pu 5.8 4.6

214

Am 5.3 4.7

137

Cs 5.95 3.6

60

Co 4.15 3.2

106

Ru-Rh 5.3 2.8

Nuclear Power station (PWR) Steam generator tent

51Cr,

54Mn,

59Fe,

57Co,

58Co,

60Co,

95Zr,

95Nb,

103Ru,

106Ru-Rh,

141Ce,

144Ce-Pr

7.2 3.6

238

Pu, 239+240

Pu, 241

Pu, 241

Am, 242

Cm, 243, 244

Cm 6.8 3.7

51Cr,

54Mn,

59Fe,

57Co,

58Co,

60Co,

95Zr,

95Nb,

103Ru,

106Ru-Rh

141Ce,

144Ce-Pr

5.25 4

238

Pu, 239+240

Pu, 241

Pu, 241

Am, 242

Cm, 243, 244

Cm 5.35 5.3

Nuclear Power Station (BWR) Sand blasting of steam turbines

60Co 3.5 2.3

131

I 2.8

137

Cs, 140

Ba, 140

Ca, 141

Ce 3−4

Nuclear Power Station (BWR) Sand blasting of steam turbines

60Co 3.4 2.3

131

I 2.9 2.4

137

Cs 3.0 2.3

140

Ba 3.1 2.3

140

La 3.1 2.3

141

Ce 3.1 2.4

Thermal arc cutting of waste concentration vapour bodies

60Co 3.2 1.9

54

Mn 3.1 1.9

134

Cs 3.2, 0.29 1.9, 2.2

137

Cs 3.2, 0.28 1.9, 2.2

Grinding of heat exchanger pipes

51Cr 4.1 2.0

54

Mn 4.2 2.0

58

Co 3.9 1.9

59

Fe 4.2 2.0

60

Co 3.7 2.0

Off-gas sump room 89

Rb 0.41 1.8

138

Cs 0.43 1.9

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Steam jet air ejector room

Gross beta 0.44 1.9

Reactor building 60

Co, 95

Zr, 140

Ba 4

Re-tubing of reactor 14

C 2-5

Decommissioning:

Segmentation of steel pipes:

(i) Plasma torch 2

(ii) Band saw 7 1.8

(iii) Reciprocating saw 6

Dismantling reactor pressure vessel: arc saw cutting device

Actinides, fission products, activation products

0.45 17

Research Centres

Research Establishment Incinerator house Alpha 5.0 2.6

Beta 5.0 2.6

Filter house Alpha 6.0 3.0

Beta 6.25 3.05

Decontamination area Alpha 3.5 4.3

Beta 5.0 3.4

High activity handling building

Beta 4.7 2.9

Research Centre Weighing of PuO2 and rearrangement of materials in glove box

Plutonium 1.5 1.5

Research Reactor/centre Rod cutting building beta 5.73 2.16

Research Institute Hot cell Operations

Decontamination of hot cells

Alpha 6.0-6.4 1.6-1.9

Beta 2.5-11 1.7-3.2

Changing exhaust filters Beta 7-10 1.7-2.6

Overhauling hot cells 6.5-8 1.8-2

Handling contaminated container

8.5-12 1.8-2.8

Handling irradiated graphite

15 2.8

Dismantling hot drain pipe

9.0 1.7

Overhauling hot cells (including welding)

0.41-1.2 7.1-12.5

Research establishment Posting of material in glove boxes

238Pu,

239Pu,

214Am 5

Uranium mills

Uranium mill After 2h inactivity Uranium (yellowcake) 3-6

Drum loading 6-12

Powder sampling 12-20

Lid sealing 12-20

Uranium mill Product packaging area Uranium 13 4.1

Uranium mills Final product packing: Uranium

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Mill 1- drum filling 9.1 3.2

Mill 2 – drum filling 5.8 2.7

Small abnormal

dust fall from overhead hopper.

14 4.1

Filtration 0.72 2.5

Packaging 4.5 2.3

Powder sampling 5.7 2.2

Packaging area: no activity

0.5 2.8

Uranium mills Uranium/thorium 7

Other uranium facilities Uranyl fluoride production

Uranium 1-9

Uranium ore concentrates preparation

5

Slag crushing 4 – 8

Annealing and inspection of uranium metal rods

3.5

Weighing of UF4/magnesium pellets

2.5

Uranium plant Saw operation Uranium (natural) 0.3

Oxide burner operation 0.12

Uranium plant Burning of uranium cuttings

Uranium 6.0 0.49

Uranium metal fabrication plant

>1

Uranium mines stopes Uranium 5

Crusher-rock breaker (underground)

Uranium/thorium 7

Uranium mine Normal mining operation

Long lived alpha emittors 11.7

Uranium mine

Ore crushing and transportation operations (underground)

238U,

232Th,

228Th,

226Ra,

224Ra

3

Fuel Handling Facilities

Research establishment Fuel fabrication laboratory: drilling out end of Al fuel can

239Pu,

240Pu

241Pu,

242Pu 6

Enrichment plant Uranium 8

Fuel fabrication plant Pellet press Uranium 6.1 2.1

Fuel fabrication plant Fuel pellet loading Uranium 5.2 1.8

Grinding pellets 5.7 2.1

Sintering furnace area 5.3 2.1

Waste treatment area 5.65 2.0

Pellet press 3.9 2.0

Powder blending 3.7 1.8

Power drum handling 5.2 1.9

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Plutonium facilities

Manufacture of plutonium oxide/uranium oxide fuel

Plutonium

(i) Normal conditions 9

(ii) Incident conditions 16.5

Decontamination of respirator face pieces

11

Reprocessing plant Chemical operations area:

Plutonium

Shaking of glove box glove

Non-prefiltered area 7 2.5

Prefiltered area 1.3 2

Chemical analysis 2.1 1.6

No activity 4 1.5

Mechanical operations 3.5 1.9

Decontamination operations:

3.5 1.9

Cleaning work area Plutonium 1.7 2.6

Americium 1.6 2.5

Sandcasting of walls Plutonium 4.5 2.6

Contamination control decommissioning

Cutting of plexigas glove boxes with:

Plasma-arc torch 3.5 1.65

Oxy-arc torch 1.28-1.9 and

2.3-3.8

Fuel reprocessing plant Plutonium reconversion laboratory:

Normal operations 5.6

Special operations (glove changing, minor decommissioning, ventilation failure)

7.0

Maintenance (incl. major decommissioning)

8.9

Ansoborlo et al [8]

Uranium cycle facilities UO2, U3O8 4.7 2.7

UO2, U3O8 3.9 3.0

UO2, U3O8 5.8 2.4

UO2, U3O8 6.1 3.1

Um, UO2, U3O8 7.6 2.5

UF4 8.0+ 2.6

U3O7(NH4)2 7.9 2.0

U3O8 5.8 2.2

UO3 3.1 2.6

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Cheng et al [9]

Research area for fuel fabrication

Incident condition PuO2 4.8 1.5

review of respirable particle size range - nda

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