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

2. MODEL PARAMETERISATIONThe approach taken in this study to investigate the impact on detection limits of anybackground 137 Cs activity is similar to that used in the earlier phase of this study (Sanderson& Cresswell 2010). Data from surveys of the beaches in the vicinity of Sellafield have beenexamined to attempt to determine typical distributions of 137 Cs activity, which have then beenused to generate 137 Cs background spectra at ground clearances of 15m and 75m. The earlierreport describes the rationale for the selection of the beach material characteristics used in thesimulations and the selection of the code. The same parameters for the beach material areused in this work.In the earlier work, three natural background levels representative of the environment on thebeaches near Sellafield were used. The work presented here will describe the impact ondetection limits of 137 Cs backgrounds relative to the medium level background from thatwork.2.1 Background 137 Cs Activity Values137 Cs backgrounds that are representative of the range of the beaches in the vicinity of theSellafield site where particulate activity may be present have been selected by examination ofsurvey data from the beaches. The data used were collected by SUERC using airbornesurveys in 1990 and 2000 (Sanderson et.al. 1990, 2000), backpack systems comprising 3x3”NaI(Tl) detectors with digital spectral processing and integrated GPS (Cresswell et.al. 2010)and Nuvia Ltd using the Groundhog systems.The airborne surveys conducted by SUERC show 137 Cs activity along the beaches of WestCumbria. Figure 2.1 shows the frequency distribution of 137 Cs activity per unit area for thesection of coast between St Bees and the Duddon Estuary, excluding the salt marshenvironments and sections where the radiation signal included direct shine from the Sellafieldsite and the Drigg waste repository. The mean activity per unit area determined in the March2000 survey was 17.4 kBq m -2 , with a total 137 Cs activity of 1.3TBq.2520Frequency (%)1510500 10 20 30 40 50137 Cs kBq m -2Figure 2.1: Histogram of 137 Cs activity per unit area distribution for the coastline between StBees and the Duddon Estuary from the March 2000 survey, excluding salt marshes and directradiation from the Sellafield and Drigg sites.2