Size distribution of radionuclides in nuclear fuel reprocessing liquids after mixing with seawater

Discharges from the Sellafield nuclear site are a major source of artificially produced radionuclides detected in the North Sea. The transport, distribution, and biological uptake of radionuclides in the marine environment depends, however, on the physico-chemical forms of radionuclides in the discharged effluent and on the transformation processes which occur after entry into the coastal waters. Information on these processes is needed to understand the transport and long-term distribution of the radionuclides. In the present work, waters from two separate waste tanks at the nuclear fuel reprocessing site at Sellafield were mixed with coastal waters. The diluted radioactive material was fractionated with respect to particle size by using cross flow ultrafiltration. The size distribution patterns of α-emitters, (Pu-isotopes), β-emitters, (⁹⁰Sr, ⁹⁹Tc), and γ-emitters, (¹³⁷Cs, ⁹⁵Nb, ⁹⁵Zr, ¹²⁵Sb), were determined over 48 h. Except for ¹³⁷Cs, the radionuclides from the two sources exhibit different size distribution patterns. The association of ⁹⁰Sr and ^239,240Pu with colloids is particularly evident. ¹³⁷Cs is present in a presumably colloidal form, with a mol. wt. < 10 kDa. The distribution patterns do not appear to be significantly influenced by transformation processes taking place in the mixing zone during 48 h.