Lr-115 Nuclear Track Detection of 'Hot' Particles collected at the Sellafield Nuclear Reprocessing Plant, Cumbria (1993)

The Author is standing next to the 5m inverted Frisbee wet deposition collector mast. The golf-ball building in the background is the AGR, Advanced Gas-Cooled Reactor. The tall chimney to the right of the image is one of the Windscale Piles. The air and deposition sampling site consisted of an Pm10, a Harwell, AEA-designed Large Particle Impaction Rod Sampler, and a Rainhood for dry inverted Frisbee deposition collectors. My sampling site was located inside the Sellafield nuclear reprocessing Plant, Cumbria (1994). My soil sample sites included a short transect from here to the AGR building.

Characterising alpha-emitting material using LR-115 (nuclear track detection method)

During the second year of my PhD research and at 3 monthly intervals, I collected air and soil samples from within the Sellafield site and an off-site located at Petersburgh Farm, Beckermet, Cumbria. I then prepared my environmental samples for alpha and gamma-emitting radioactivities I had no idea I would find so many soil and airborne particles having extremely high alpha-emitting activities. This short article details the analytical method I used to initially screen my samples for 'unusual' radioactivity.

A prime objective of this work was to isolate alpha-emitting hot particles to assess their number frequency, Pu activities and ratios. Sub-samples of surface soils and deposited Frisbee material to a height of 5 m above ground were analysed from each of the four runs to assess any seasonal variation in the occurrence of these particles. Each sub-sample was exposed to a cellulose nitrate track detection film LR-115 which was supplied by Kodak France.

The principle of nuclear track detection or alpha-autoradiography is based upon the fact that when alpha particles impinge upon the surface of LR-115 a narrow trail of radiation damage is induced into the plastic (Hamilton 1978). After a suitable exposure period the film is etched to preferentially reveal track damage clusters. The density and size of cluster damage to the plastic is proportional to the activity of alpha particles from actinides such as plutonium, americium and curium.

LR-115 analysis detects both natural and anthropogenic alpha particles and one way of distinguishing between both types is to expose the film to periods ranging from hours to as long as 18 months. As explained in an earlier chapter, the soil type around Sellafield is not indicative of enhanced natural alpha-emitters from granite inclusions of uranium, thorium and their decay products, as found in parts of Cornwall, Devon and the Midlands of the UK. Thus, exposure times were kept relatively short to minimise interference from natural alpha - background activities.

LR-115 EXPOSURE METHOD

A sub-sample of surface soil was oven-dried at a temperature of 40°C. to constant mass and passed through a 125 m m sieve. The sample was sprinkled evenly onto a 10 cm x 7 cm adhesive tape mounted on Benchkote. A sticky surface is recommended for keeping material in one place because of the high electrical charge on most of the particles (Hanson 1975). A suitably sized piece of LR-115 (the correct side facing sample) was placed on top of the sample and to ensure intimate contact between sample and film, placed in a perspex press. Exposure periods for some of the samples ranged from 24 hours to 5 months. After exposure to field samples the track detector was etched in a strongly alkaline solution.

Etching conditions:

A water bath with good temperature regulation was prepared with the temperature set to 60°C. A 2 M solution of sodium hydroxide was made up in a large crystallising dish and placed in the water bath. This was left for about 15-20 minutes to ensure that the temperature of the solution remained constant at the pre-set temperature.

The exposed film was taken off the sample and lightly washed with distilled water and placed in NaOH solution using plastic forceps. The sample was left for a period of 1.5 to 2 hours to etch. At the end of this period the film was taken out and placed in a solution of 10% methanol for about 3-5 seconds. The film was lightly washed with distilled water and allowed to air dry. The film was viewed for track-damage analysis using a 40x optical microscope with green filter. Each film was subsequently photographed and detail logged for track-burn out diameters, overall track characteristics and number of discrete particles or hot-spots. A blank was run on all occasions and a number of soils were randomly tested for comparative activity assessments.

Figure 3 illustrates track damage features from a Frisbee deposition sample collected at 1 m above ground via dry deposition processes. The total length of track burnout was 250 m m with a diameter of 100 m m. The Frisbee sample was exposed to LR-115 for 5 months. Even with this relatively long exposure period, only one intense track damage area was identified. Natural background alpha- emitters were limited to a few random tracks of between 3-6 m m in length. The presence of these types of particles with 'abnormal' activities and large particle size help to explain the extremely high rates of Vg for material collected within the Sellafield complex.

Frisbee deposition sample showing a -track damage of 250 m m by 100 m m. The oblong 'spot' in the middle of the automicrograph was caused by intense activity from an alpha particle, possibly plutonium, americium or curium. The red area surrounding the particle is a measure of the low background activity for this sample.