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Update and context to my PhD research:

I was asked to verify whether or not the assumed value of dry deposition used in theoretical modelling was adequate and fit for purpose. I determined extremely high fluxes of radioactive materials including highly enriched plutonium hot particles. The currently assumed value of dry deposition used by radiological modellers throughout the world will under-estimate deposition fluxes of radioactive materials within the first 5000 metres from its emission source. I carried out my Ph.D fieldwork at the Sellafield nuclear reprocessing plant in Cumbria. During the course of my fieldwork within the barbed wire fence of Sellafield I determined the existence of a source or sources of fugitive on site radioactive emissions. I called these materials 'hot particles' and their inventory included aged plutonium, americium, curium and a characteristic suite of aged and freshly released gamma-emitters including 134Cs. I was unable to pinpoint the exact location the sources(s) of these hot particles yet I was able to determine that hot particles were present at various points within the site. The biological danger of plutonium-enriched hot particles to on-site and off-site personnel is obvious. At one point my project supervisors threatened to shut my work down and/or deflect my original findings into another avenue of investigation. The worth of any investigation is in asking the right questions rather than seeking answers to questions you already know. Is science true because its powerful or powerful because its true. My direct experience at the Sellafield plant favours the former.

                                          "Is science powerful because its true or true because it's powerful"

Prior to leaving Westlakes Research Institute I had to hand over all my Ph.D research documents and electronic files to Professor Steve Jones and signed a document to the effect that I would not publish my findings on my Sellafield hot particle work which formed the basis of my Ph.D. To cut a long story short, I was awarded my Ph.D in 1998 but the findings of my Ph.D work were embargoed for the next 8 years and not allowed into general and academic circulation via the British Library. Imperial College also colluded in these acts. Suffice to say, I fail to understand why these events took place - Science after all is impartial, is it not??

The main outcomes of my work, (1) representative air sampling of a suite of stack-discharged radionuclides including caesium-137, cobalt-60, americium-241 and plutonium isotopes show dry deposition velocities well in excess of the commonly assumed value of 1 micron AMAD, (activity median aerodynamic diameter). The implications of these very high values determined in my work of dry particle Vg suggest that all the past and current atmospheric dispersion modelling (both near and far field) are wrong. The second outcome of my work carried out at the Sellafield nuclear reprocessing plant relate to the high number of hot particles I detected and characterised both in air and soil samples. These hot particles certainly contributed to the high values of dry particle deposition velocity but the real issue is where do they come from? Do they originate from fugitive emissions from somewhere on site or from stack emissions? I determined the isotope ratios for many of these hot particles and they were characteristic of materials discharged during the 1950s i.e. nuclear weapons production. I was not very popular when I presented these data to my supervisors and I had to literally fight to include some of the data on these hot particles in my Ph.D thesis. So much for the impartiality of Big Science and its stated objectivity!!


Official PhD Abstract

Environmental characterisation of particulate-associated radioactivity deposited close to the Sellafield works (1998). Imperial College of Science, Technology & Medicine (University of London)

The calculation of dry particle deposition velocity (Vg) for small particles in field studies is subject to much variability between theoretical and measured values. This work will assess some of the factors which may influence the calculation of Vg. The other key parameter which is used to define the near-surface exchange of material is the resuspension factor (K) and some of the variables in the derivation of K will also be assessed. The certainty of using a Vg which is representative of small particles typically 1 um requires clarification.

During September 1993-1994, four size-selective air and deposition experiments were carried out at an on-site and off-site sampling location close to the BNFL Sellafield nuclear reprocessing plant in Cumbria UK. Dry and total (wet & dry) particle deposition velocities were determined for 137Cs, 239+240Pu and 238Pu. The dry deposition velocities of naturally-occurring 7Be were also determined and used as a ‘marker’ for small particle transport processes. Resuspension factors K (m-1) for radiocaesium and plutonium were also derived. Automated individual particle characterisation (AIP) using scanning electron microscopy coupled to energy-dispersive x-ray analysis (SEM-EDXA) was used to determine the particle size distribution and associated elemental composition of material deposited to Frisbee collectors. Enhanced alpha-emitting hotspots from surface soils and material deposited to Frisbee collectors at 1 m above ground level were isolated using nuclear track detector film, LR-115.

Dry particle deposition velocities for both 137Cs, 239+240Pu and 238Pu were commonly in excess of 1E-03 m s-1 by more than two orders of magnitude. Total deposition velocities (dry and wet) were even higher with values ranging from 0.2 m s-1 - 1 m s-1 for both radiocaesium and plutonium. Derived high values of dry deposition velocity for radiocaesium and plutonium were to some extent influenced by air sampling artefacts Air sampling artefacts result in an under-estimation of airborne activity for radioactivity associated with particle diameters > 10 um. Values of Vg for 7Be were consistent with literature values of sub-micron sized particles with evidence of a seasonal spring maximum. Resuspension factors K (m-1) for radiocaesium and plutonium agreed well with literature values of weathered weapons fallout values which ranged from 2E-08 to 5E-11 (m-1) for plutonium and radiocaesium respectively.

Differences in dry particle deposition velocities for radiocaesium and plutonium between the on-site and off-site locations varied by no more than a factor of three. Correlations between dry particle deposition velocities for radiocaesium and plutonium with wind speed, wind direction and precipitation rates were not found. Size-specific air sampling show that most of the plutonium was associated with the > 11 um aerosol size fraction. The association between activity and large particle size suggest this material was probably attached to large soil-derived particles. Plutonium isotope data indicate this material originated from the nuclear weapons programme of the late 1950’s-early 1970’s. Radiocaesium was equally distributed between the < 10 um and > 11 um aerosol fractions for two out of the four runs. This suggests that historically deposited 137Cs and current emissions contributed to measured airborne activities. Enhanced alpha-emitting hotspots were isolated from bulk surface soils and in material deposited to Frisbee collectors. Rare particles such as these with atypical activities of Pu may lead to misleadingly high deposition fluxes. The calculation of Vg in the field is therefore sensitive to the presence of these particles because they are not representative of the aerosol flux and their size > 50 um precludes their collection by the Pm10 air sampler. Hotspot particles were typically rod-shaped and approximately 70 um long. Plutonium isotope data suggest these particles were discharged in the late 1950s-early 1970’s.

The main conclusion of this work indicate that the very high values of Vg sometimes measured in the field are strongly influenced by large resuspended soil particles and consequent air sampling artefacts. The inclusion of large particles within the deposition flux is confirmed by a novel size selective mass-based Vg using scanning electron microscopy.

My final thoughts on this work:

It’s strange thinking about my experiences of some years back when I was a PhD student. I honestly thought at that time that if you were asked to do a job of work and you do it, surely, that should be that. But it wasn't. Still, I have no regrets and no hard feelings towards anyone. It’s clear to me now as it was then that other people have their own agendas on the why and how of the type of research they want carrying out. This work was carried out at the Westlakes Research Institute and I was one of the first of two research students to carry out PhD work. To put the work of this Institute in context it was staffed by mostly ex-BNFL personnel and financed mostly by BNFL as well. I came in to do this work and within a year I had a pretty good idea on what was going on but other people had other ideas on what I should be looking at. Strangely, after working for almost 7 days a week for three years (nerdy PhD student habit) I left the Institute in a deafening silence, not that it really bothered me that no-one came to say goodbye or bid farewell because in effect, I had served my time. Still, its all a bit weird from my point of view when I occasionally remember my time there. During and subsequent to this period working at Westlakes I have always kept my eyes on the prize and that was to do a good bit of work. How these (Westlakes) and other people at BNFL interpret my findings is down to their particular model of the world.

In conclusion alpha-emitting hot particles that arise from the nuclear fuel reprocessing cycle and particularly from weapons-grade plutonium are a problem within the immediate environs of the Sellafield nuclear reprocessing site because they are small, highly radioactive, highly toxic and can be (a) resuspended to a height of at least 2 metres above ground and inhaled into the lung (b) there are sites of fugitive emissions within Sellafield. That being the case, these are unregulated and therefore illegal emissions to atmosphere.

When you consciously set out to find hot particles, it is difficult though not impossible to separate them from the matrix on which they may adhere or be absorbed onto. If the activity of these materials were expressed on their true mass basis, the reported activities would be orders of magnitude higher than what they are currently reported as e.g. soil (kg) basis. Hot particle (alpha emissions) track damage (yellow bits) to cellulose nitrate alpha particle detector film from soils and air samples collected at the Sellafield nuclear reprocessing plant Cumbria UK

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