EFFECTS OF CELLULOSIC DEGRADATION PRODUCT CONCENTRATION ON ACTINIDE SORPTION ON TUFFS FROM THE BORROWDALE VOLCANIC GROUP, SELLAFIELD, CUMBRIA

The Nirex Safety Assessment Research Programme includes an investigati on into the effects of cellulosic degradation products on the sorption of radioelements onto geological materials. Previous batch sorption s tudies have shown that the presence of high concentrations of both aut hentic cellulosic degradation products (produced by alkaline degradati on of wood/tissue) and the well-characterised simulant, gluconate, can cause marked reductions in actinide sorption. This work has now been extended to cover a range of concentrations of both authentic cellulos ic degradation products and their simulants, gluconate and iso-sacchar inate. Geological samples were from the proposed Nirex underground rad ioactive waste disposal site at SellafieId, Cumbria. The nuclides stud ied were thorium and plutonium. In the presence of gluconate or iso-sa ccharinate, at concentrations above 10(-4) M, the present work has con firmed the trends shown by earlier experiments, with a significant red uction in actinide sorption (R(D) values are often reduced by over an order of magnitude). However, in the presence of lower concentrations of organic materials, sorption was reduced at most by only a small mar gin (R(D) values reduced by less than a factor of two), and in some ca ses the results suggested a slight increase (R(D) values increased by up to a factor of four).