Some Evidence of Radiolysis in a Uranium ore Body - Quantification and Interpretation
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SOME EVIDENCE OF RADIOLYSIS IN A URANIUM ORE BODY - QUANTIFICATION AND INTERPRETATION
JINSONG LLU AND IVARS NERETNIEKS Department of Chemical Engineering and Technology Royal Institute of Technology S-10O 44 Stockholm SWEDEN ABSTRACT Locally oxidising conditions in the near-field of the Cigar Lake uranium deposit was observed. Ongoing processes of water radiolysis has also been predicted previously by our mass transport model. In the ore there was an enhanced concentration of helium, hydrogen and sulphate. Sulphate is the only oxidising species of substantial amount in the groundwater samples, and is possibly indirectly produced by water radiolysis and oxidation of sulphides. The ongoing oxidant production rate has been calculated by the mass transport model. In this paper, the issue of water radiolysis is addressed from a more fundamental angle of approach. The maximum oxidant production rate is calculated based on the assumptions of geometric dispersion of the ore constituents, the estimate of the total radiation energy, the fraction of energy deposited into the pore water, and the G-values of water. The results show that only a few percent of the total radiation energy is deposited into the pore water to cause water radiolysis. If the recombination factor projected by other researchers are accounted for, the oxidant production rate thus calculated agrees with the present-day ongoing oxidant production rate predicted by the mass transport model. INTRODUCTION In the concept of final disposal of nuclear waste by isolation in geosphere, water radiolysis is an important issue for the long-time effective isolation of the various hazardous radionuclides. Radiolytically generated oxidants may change the redox condition of the nearfield of the disposal repository. A recent natural analogue study showed evidence of locally oxidising conditions in the near-field of the Cigar Lake uranium deposit [ 1]. The oxidants may oxidise either the uranium ore, or more likely, the other reducing minerals in the near-field of the deposit, because it is evident that the ore has been preserved during the geological time. Mass transport modelling [2] also predicted ongoing radiolysis in the same ore. In the ore, there was an enhanced concentration of helium, hydrogen and sulphate. Sulphate is observed to be the only oxidising species of substantial amount in the groundwater samples, and is possibly indirectly produced by water radiolysis. Negligible concentrations of hexavalent uranium, oxygen and ferric iron are present. In this paper, the radiolysis in the ore is interpreted and quantified on a more fundamental basis. The maximum possible oxidant production rate is calculated based on the assumptions of geometric dispersion of the ore constituents, the estimate of the total radiation energy from nuclear reaction, the fraction of energy deposited into the pore water from knowledge of energy transfer in the various ore constituents, and the G-values of water. The energy source for water radiolysis in the ore is primarily the energy of al
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