| Abstract |
One type of HLW associated with the procedures of spent fuel reprocessing or conditioning as would be required in order to implement accelerator driven transmutation of waste, is the insoluble residue, which remains after the majority of the fuel, is dissolved. This material is separated as part of the head-end processing and must be suitably encapsulated within a waste-form to permit its disposal. In spite of the fact that the specific contribution of insoluble deposits, arising from SNF dissolution does not exceed 0,5-1 % of the general volume of wastes, the radionuclides, contained in them introduce a rather significant hazard and demand the most careful treatment of the material during its treatment and subsequent disposal. The main contributors to the insoluble residues are the slowly dissolving metallic fission product inclusions found in spent fuel and the certain fission products which although initially soluble precipitate during the dissolution process. The most significant elements, in mass terms are the platinum group metals (PGM) and, also molybdenum and zirconium. In turn, the formed deposits are capable of adsorbing fission products and actinides, for example, antimony, uranium and plutonium. This group of elements presents two issues one relating to the activity and heat of the relatively short-lived isotopes, in particular (sup 106)Ru/(sup 106)Rh and (sup 125)Sb and the other due to the presence of the long-lived and potentially environmentally mobile (sup 99)Tc. The main factors determining the amount and composition of insoluble residues are the temperature and degree of burnup and conditions of fuel dissolution. In this paper the results on composition and mass determination of insoluble residues (primary and secondary), derived from samples of fuel with burnup from 15 up to 54 MWd/kgU are given. Dissolution of spent fuel samples was conducted at the boiling temperature and at 80 C. The concentration of nitric acid in the final solutions varied within the limits 1,7-3,5 mole/l and concentration of uranium from 250 to 350 g/l. The mass of insoluble residues obtained during experiments was from 0,03 to 0,5% (calculated for 1 kg of UO(sub 2) in spent fuel). Secondary residue examination has shown that their amounts were from 0,01 to 0,3% of fuel mass (from (approx) 5 to 50% of whole residue mass). The results of chemical analysis of primary and secondary residues and specific (beta)- and (alpha)-activities will be presented. The main elements, defining residue composition are as follows: metals of platinum group (palladium, ruthenium, rhodium), molybdenum and zirconium. The specific (beta)-activity of the residues was 20 to (approx) 840 Ci/kg and (alpha)-activity 0,01-8 Ci/kg. Depending on dissolution conditions uranium and plutonium contents in specimens analyzed were as much as 0,2-4% and 0,1-3% respectively. These results are compared with other literature data for residue arising and the significance of the insoluble residues in the context of long-term waste behavior will be discussed. |
