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

SSEM/2011/4730 June 2011Table 3.9. Basic statistics data in 3.8 converted from the natural log values back to activities.Radionuclide =>241 Am Activity(Bq)239 Pu Activity(Bq)238 Pu Activity(Bq)Mean 2.55E+04 1.58E+04 1.53E+04Q1 1.56E+04 8.73E+03 9.59E+03Median 2.52E+04 1.57E+04 1.52E+04Q3 4.08E+04 2.66E+04 2.31E+04Minimum 2.20E+03 2.68E+03 5.01E+03Maximum 6.34E+05 3.09E+05 8.81E+04One particle has been found on Drigg beach with a combination of processed actinidematerial (i.e. 4.48E+03 Bq of 241 Am) and activation products (i.e. 8.65E+03 Bq of 60 Co). Thisfind was recovered in October 2007 and no further finds of this type have been recoveredsince this date. There are no site processes that combine these two radionuclides so this findwas probably formed within the effluent system from a combination of waste streams.Six alpha-rich stones have been found on the Sellafield beach with processed actinideactivity (i.e. 241 Am); two of which also had fission product activity (i.e. mainly 137 Cs but onewith a small amount of 154 Eu). The most probable source of these finds is the absorption ofactivity from the authorised discharges from the Sellafield pipeline. The stones may havebeen in close proximity to a hole in the pipeline over an extended period of time and thenreleased during the pipeline removal project(s).A beta-rich find sub-group assessment report is currently in draft awaiting further data fromSerco. Up to the 31/03/2011 428 beta-rich finds have been categorised as stones and 270 asparticles, with 27 of these particles also categorised as excess beta-rich. The radionuclideactivity is almost completely attributed to 137 Cs with low levels of other radionuclides presentin a small number of finds. As the data is more limited than that available for alpha-rich findsit was not possible to use the approach taken for the source identification of alpha-rich findsfor the beta rich finds. The age of a mixed fission product source can be estimated if both134 Cs and 137 Cs isotopes are present and the fuel source is known. In this case, the 134 Csactivity estimates are almost all at the limit of detection and the fuel source is not known. Thebest age estimate that can be achieved is therefore based on a default Magnox referencefuel type and LOD 134 Cs results, which gives a find age of greater than 16 years old. As thisage is greater than 16 years it can not be directly compared with the 35 years estimated foralpha-rich finds. The limiting factor in the age calculation comes from the short 134 Cs half lifeof just 2 years compared to the 30 year half life for 137 Cs. Table 3.10 shows how quickly the134 Cs half life becomes critical with the Caesium isotope ratio becomes very large after 15years.© Nuclear Decommissioning Authority 2011. 48