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

SSEM/2011/4730 June 2011Histogram and probability plots of the SL actinide ratio data for the Magnox and Oxidesubgroups show that the ratios are normally distributed with a standard deviation of 20% ormore. To assess the extent of this uncertainty a scatter plot was generated that showed theerror bars based on the analytical measurement uncertainties (Figure 3.14). The axis valuesof this plot have been removed to avoid restricting this document. Their absence does notaffect the conclusion that the Magnox subgroup is relatively well resolved and the Oxidesubgroup less so. The error bars for the unresolved group could place them in either theMagnox or Oxide groups, which could imply that there are few, if any, mixed Magnox andOxide finds. To check if a more refined assessment could be undertaken using the NPLanalysis the 16 alpha-rich finds from the second and third tranches were plotted (note thatthe 238 Pu uncertainty values were not available for the first tranche. No noticeableimprovement was seen using the NPL data compared with the SL data, with the SL databeing better for the Magnox subgroup. Figure 3.15 shows the same data without error barsso the find positions on the graph are more easily seen.Beach Find Origin AssesmentOxide origin findsAm241 / Pu239 RatioMagnox origin findsFinds outside ovals are unresolved or of mixed Magnoxand Oxide originsPu238 / Pu239 RatioOxide origin Magnox origin Unresolved or mixed origin NPL DataFigure 3.14. Alpha-rich Find Subgroups Based on their Actinide Activity Ratios© Nuclear Decommissioning Authority 2011. 41