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

SSEM/2011/4730 June 2011Table 3.15. Overlap find numbers summary for Sellafield beach monitoring in 2010/11.Number of instances of finds found in areasOverlap Survey Typethat were surveyed before or after the findrecoveryAlpha-rich Beta-rich TotalBefore overlap survey 20 4 24Before and after overlap surveys 4 2 6After overlap survey 18 9 27No finds were found in all 51 cases in the overlap areas in the 'after overlap' monitoringperiods, which would support a low repopulation rate. To investigate this in more detail anassessment was made of the height difference between the find depth in the sand and theheight of the sand surface during a previous monitoring period (i.e. the before). This wascompared to the limit of detection for the find activity, which indicated if the find should havebeen found during the previous survey. The outcomes to this assessment are:• The find could possibly have been in an existing stratum that was below thedetection capability of the equipment during the previous survey (i.e. outside LODrange).• The find repopulated the area after the previous survey.These options are shown in Figure 3.31 and summarised in Table 3.16 below.Scenario 1 - Previous beach level higher than find level by more than LODScenario 2 – Previous beach level higher than find level but less than LODDifference greater than LODFind levelDifference less than LODScenario 3 - Previous level lower than find levelFigure 3.31. Find Overlap Outcomes.Table 3.16. Before Overlap Survey Analysis for Finds within Equipment LOD Range.Reason for find being in locationAlpha-rich 1 Number of findsBeta-rich 2 TotalFind possibly in existing stratum - scenario 1 5 0 5Find could be either in existing stratum or2 3 5repopulated area - scenario 2 or 3Find repopulated area - scenario 3 13 1 14Total 20 4 241. Sand difference values for alpha-rich finds - Scenario 1 greater than 5 cm, Scenario 2 greater than 0 cm ANDless than or equal to 5 cm, Scenario 3 less than 0 cm.2. Sand difference values for beta-rich finds - Scenario 1 greater than 30 cm, Scenario 2 greater than 0 cm ANDless than or equal to 30 cm, Scenario 3 less than 0 cm.© Nuclear Decommissioning Authority 2011. 58