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

SSEM/2011/4730 June 2011Table 3.10. Beta-rich Caesium Isotope Decay Factors.Time After Reprocessing(years)Percentage Activity Remaining137 Cs134 Cs137 Cs / 134 Cs Ratio0 100 100 11 97.72 71.46 1.372 95.49 36.50 2.623 93.32 13.32 7.014 91.19 3.47 26.255 89.11 0.65 137.6310 79.41 0.02 3530.3815 70.76 1.46E-04 4.86E+0520 63.05 1.76E-07 3.59E+0825 56.19 3.95E-11 1.42E+1230 50.07 1.66E-15 3.02E+16Table 3.11 gives an estimated age range for beta-rich finds based on Magnox reference fueltype.Table 3.11. Beta-rich Find Estimates.137 CsActivity (Bq)Corresponding 134 CsActivity (Bq) 1Estimated Age(years)137 Cs Mean 1.20E+04 2.24E+01 Greater than 15.7137 Cs Maximum 2.92E+05 1.06E+02 Greater than 20.9137 Cs Minimum 4.75E+02 3.71E+01 Greater than 3.7 21. The age estimates are greater than the calculated value as all the 134 Cs values are at the analysis limitof detection.2. This was an excess beta-rich find with a low 137 Cs activity, which resulted in the low age estimate.A similar assessment to the alpha-rich correlation with particle volume was undertaken forbeta-rich particles. In this case the 137 Cs activity was found to be more strongly correlated toparticle surface area than to particle volume. As the difference between the surface area andvolume correlation was small, the assessment was extended beyond the use of all availabledata to finds with a volume greater than 1 mm 3 and finds with a volume less than 1 mm 3 . Thecorrelation R values for all three assessments were consistently higher for surface areacompared to volume by a small amount for each of these groups (i.e. R SA /R V = 0.55/0.54,0.59/0.50 and 0.80/0.76 respectively). The reasons for the small difference can be postulatedas being due to finds created in layers or the activity absorbed towards the centre of theparticle in very porous materials. A further limitation of the analysis was that it was based ondata from only 20 finds due to the difficulty in estimating particle dimension for smallparticles. This analysis will be updated when the tranche 3 analysis data becomes available. In a similar way to the alpha-rich finds an obvious starting point for a beta-rich finddistribution assessment is to look at simple histogram plots of log transformed 137 Cs activitydata. In this case there are significant numbers of both stone and particle beta-rich finds so© Nuclear Decommissioning Authority 2011. 49