Annual Report for 2010/11 and Forward Programme - Sellafield Ltd

3.1.2.2 Detailed Small Area SurveyFor smaller areas, more detailed surveys are possible. These would be conducted at lowerground clearances where possible, and reduced speed and line spacing. For this work, asurvey speed of 15 knots (5 m s -1 ) and a line spacing of 20m has been used as detailed surveyparameters. This corresponds to the survey height and speed for the detailed beach surveyconducted in March 2000 (Sanderson et.al. 2000). This would be sufficient to cover a2kmx200m area of beach in a one hour survey.Figures 3.10-3.12 show the significance of transects across superficial and buried 137 Cs andsuperficial 241 Am sources for 1s measurements at 15m height. As noted, increased integrationtime reduces the measurement uncertainty with optimal integration time corresponding to thetime taken to cross a feature. At 15m ground clearance, 90% of the observed signal isobserved within 25m of the source, giving an optimal integration time of 10s at 15 knots.Table 3.2 gives the significance of 9s measurements for the different source activities anddepths and background conditions for figures 3.10-3.12. It can be seen that for these detailedsurvey parameters, the significance of single 9s measurements would exceed 2 for superficialsources of 200kBq ( 137 Cs) and 1.2MBq ( 241 Am) and buried 1.0MBq 137 Cs sources.The earlier work had shown that reducing ground clearance increases peak count rate, with137 Cs peak count rates at 10m twice those at 15m and at 5m the count rate increases by afactor of 10. Thus, detection limits for very low surveys conducted at even slower speedswould be lower than these.SourceDepth137 Csbackground137 Cs0.1 MBq137 Cs0.2 MBq137 Cs0.5 MBq137 Cs1.0 MBq137 Cs2.0 MBqSurface 0 Bq kg -1 1.49 2.64 5.14 8.0012.5 Bq kg -1 1.36 2.45 4.92 7.7825 Bq kg -1 1.26 2.30 4.72 7.5850 Bq kg -1 1.11 2.07 4.38 7.22Buried 0 Bq kg -1 0.40 0.77 1.73 3.03 4.9912.5 Bq kg -1 0.36 0.69 1.59 2.83 4.7625 Bq kg -1 0.33 0.64 1.48 2.67 4.5750 Bq kg -1 0.28 0.55 1.31 2.41 4.23241 Am241 Am241 Am241 Am10 MBq1 MBq 1.2 MBq 5 MBqSurface 0 Bq kg -1 1.95 2.31 7.78 12.9112.5 Bq kg -1 1.89 2.24 7.64 12.7525 Bq kg -1 1.84 2.18 7.50 12.5950 Bq kg -1 1.75 2.08 7.25 12.28Table 3.2: Significance of single 9s measurements centred on different sources indifferent 137 Cs background concentrations for detailed survey parameters.18