Background to Sellafield's Hot Particle Problem

Nuclear track detection and isolation of alpha-emitting hot particles in surface soils around the BNFL Sellafield site perimeter

Experimental

As detailed earlier in chapter 4 twenty sites around the Sellafield perimeter were assayed for gamma-emitting and alpha-emitting nuclides. Surface soil scrapes were taken to a depth of 1 cm for sites 1 - 13 and road dust sweepings were taken for sites 14 - 20 with an additional road dust sample taken at site 5 which is the Met. Station.

A sub-sample of surface soils (approximately 50 g) from each site was evenly spread within an aluminium container and a hand-operated alpha, gamma and beta contamination meter was traversed over the sample to identify above-background activities of alpha and gamma-emitters (see photograph 6.2.1). Beta-emitting particles were not analysed in this work.

Areas of high activity relative to the bulk of the sample were isolated successively by quartering and coning until approximately 2 - 3 g of material was separated from the bulk sample. This material was exposed to LR-115 alpha-sensitive cellulose nitrate film to exposure periods of between 27 days and 3 months.

Following LR-115 analyses the more active samples which is a purely operational definition (defined by intense LR-115 track damage relative to background) were re-assayed for gamma-activities using HpGe detectors and qualitative gross alpha-activities using silicon surface barrier detectors. Three of the isolated alpha-emitting hot particles were subjected to a heavy liquid separation using di-iodomethane and alcohol (Cotter-Howells 1993) to separate the heavy elements from the light elements. This stage eliminated more than 90-95% of sample mass and the resulting heavy particles were either black in colour or of a shiny lustre. Following heavy liquid separation the particles were analysed by SEM-EDXA for particle shape, size and associated element composition.

Simple screening method using a portable contamination meter to identify alpha , beta and gamma-emitting particles

It was necessary to pass all particles through a 125 um sieve to minimise the vertical distance of the sample to the track detector film. This was especially important for road dusts because they contained large numbers of man-made materials such as glass and plastics etc. Areas of high alpha-activity were found using a portable contamination meter and a small sub-sample were subsequently exposed to LR-115 analysis for a period of 31 days.

A 5 cm by 5 cm square of a Pm₁₀ air filter with about 9 days sampling activity was also analysed for alpha-activity for run 2 by placing the air filter and track detector in a Perspex hand press for an exposure period of 31 days. The hand press was necessary to ensure intimate physical contact between the air filter and track detector.

Results for the nuclear track isolation of alpha-active particles in surface soils

The results for the LR-115 nuclear track data for all sample sites is presented in Table 6.2.1. Locations with the highest per unit area of alpha-emitting hot particles were found to the south west and to the east of the site perimeter. Sites 5 and 6 contained material with easily defined LR-115 alpha-active hot particles with four and two particles respectively. In contrast, no hot particles were found in the majority of road dust sites to the south and south east of the plant. Regions of interest (ROI’s) are defined as much smaller spots of alpha-track damage which may be due to the presence of fragments from the main body of alpha-activity.

Table 6.2.1

Occurrence and density of alpha-emitting hot particles found in surface soils around the Sellafield perimeter

SAMPLE TYPE	SAMPLE SITE	HOT PARTICLES	REGION OF INTEREST (ROI)		SITE LOCATION
SOIL	1	0	0		SSW
SOIL	2	2	3		SSW
SOIL	3	3	3		SW
SOIL	4	0	5		SW
SOIL	5	4	0		W
SOIL	6	2	12		W
SOIL	7	ND	ND		ND
SOIL	8	ND	ND		ND
SOIL	9	0	2		N
SOIL	10	0	1		N
SOIL	11	0	1		N
SOIL	12	0	16		E
SOIL	13	7	2		E
ROAD DUST	14	2	0		E

PICTURE OF A HOT PARTICLE FOUND AT SELLAFIELD

The alpha autoradiograph below illustrates the extent of track damage to an LR-115 nuclear track detector film from an hot particle collected from site 2 surface soils. Site 2 is situated to the south of Sellafield near the coast. The ²³⁸Pu/^239+240Pu ratio for the bulk sample for this soil was 0.08 which is characteristic of mid 1960’s processing when plant throughput was dominated by low irradiation material (Gray et al 1995). It can be seen from figure 28 that track damage is roughly spherical. Track pattern shows no evidence of directional screening, which may occur if the main body of alpha-activity was overlain by other material. It is possible to measure a track ‘burnout’ size of approximately 550 - 600 m um diameter for this hot particle.

One of the most exciting things about this particle were the track-damage trails which were big enough to be seen by the naked eye!! The film was re-positioned over the sample hot spot area and this area was punched out using a 5 mm biopsy punch. The area of sample containing the hot spot was carefully placed in an alpha-spectroscopy counting chamber. As soon as I switched on the machine I knew this was something quite special because I saw instantaneously that a number of counts (alpha emissions) started to appear in the energy channels of between 4.8 - 8 MeV. You must bear in mind that the picture below shows the peak resolution from this sample without radiochemical separation. The alpha-spectrum from this particle shows exceptional resolution three sharply defined peaks corresponding to energies at 5.2 MeV, 5.5 MeV and 5.9 MeV. The peak at 5.2 MeV is characteristic of ^239+240Pu, with the peaks at 5.5 MeV being characteristic of ²⁴¹Am + ²³⁸Pu and the peak at 5.9 MeV attributable to ²⁴⁴Cu. The sample was counted for a relatively short time to eliminate background interferences from naturally-occurring radioactive material.

LR-115 isolated hot particle from surface soils