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

SSEM/2011/4730 June 2011The data used for this initial assessment were limited to Sellafield beach areas because findrates at other beaches were too low for meaningful analysis, but supports the hypothesis thatrepopulation is significant for the Sellafield area for alpha-rich finds. Linking this conclusion tothe low interchange rate between the beach and sub-sea proposed earlier would suggestthat finds are predominately moved around from other areas of the beach by tidal action.Further work is planned on automating the GIS overlap assessment tool to help refine thisconclusion.Two reports have been received on statistical techniques that can be employed for theanalysis of beach monitoring data. The first report (Scott and Tyler, 2011a) givestechniques that can be employed to characterise beach finds and the second (Scott andTyler, 2011b) gives guidance on the best 'statistical' monitoring system for future beachmonitoring programmes. The reports' recommendations and the initial SL response aregiven in the Table 3.17 and Table 3.18 below.Table 3.17. Statistical techniques report recommendations.Recommendations1 Systematic sampling areas required. The spatialheterogeneity of particle distribution may begoverned by sedimentological processes ofbeach accretion/erosion and particle selectivitythrough systematic processes of sorting (or lackof) of grain size. Non systematic sampling ofbeaches may lead to bias in the estimatedparticle density. In reporting the sampling effort,and to provide a basis for assessing thesystematic coverage of monitoring, it is importantthat the monitoring effort be reported as afootprint and as a proportion of the total availablearea on the beach. Comparison of successivefootprints will provide a basis for assessingsystematic coverage, i.e. contiguous areas arecovered and the frequency with which areas ofthe beach are sampled.2 Systematic temporal sampling of beachesrequired. The temporal heterogeneity of theparticle population may change with time,specifically the seasonality of tidal and waveenergy. Estimating particle density without atemporal perspective may result in an under oroverestimate in particle abundance.SL initial responseThe current reporting system is based on analpha-rich find footprint (i.e. it is judged thatthe 5cm to 10 cm turnover of sand over asingle tide will provide fresh area with respectto monitoring for 241 Am; which has a limit ofdetection of 1E+05 Bq at 5 cm depth). Thisrecommendation will be reviewed with respectto reporting radionuclide specific footprintsover radionuclide specific monitoring periods.There is little evidence of a seasonal affect inthe current data. The 2011/12 monitoringprogramme includes repeat areas forSellafield, St Bees, Braystones and Seascalebeaches. Sellafield beach, which isprogrammed for the most frequent monitoringand has the highest find rates, will providedata that should enhance the seasonalassessment capability. The repeat monitoringat St Bees (x5), Braystones (x3) and Seascale(x3) will provide additional supporting data.Future monitoring programmes will takeaccount of any seasonal affects found.© Nuclear Decommissioning Authority 2011. 59