Leaching of Spent Fuel and Simulated Fuel in the Presence of Environmental Materials: Integral Experiments
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ABSTRACT We have designed an apparatus which simulates the conditions of a deep geological repository in order to study the behaviour of spent fuel under leaching. A spent fuel and a Simfuel have thus been leached by synthesized clayey or granitic groundwater in sandy clay (90 % sand) or granite, in reducing conditions, at 90'C and 40 bars. The apparent leach rate for spent fuel in clayey water is 3.33 lig.m 2 .dr1 in the presence of clay and 3.37 Rg.m- 2.d-1 in the presence of granite. The apparent leach rate in granitic water is slower, being 0.37 g.g.m-Z.d Ain the presence of clay and 0.74 .tg.m-2.d in the presence of granite. For Simfuel, the apparent leach rate in clayey water is 7.4 Rg.m 2 .d-1 in the presence of clay and 1.1 .tg.m-2.d-1 in the presence of granite, which is the same order of magnitude as that for spent fuel. In granitic water, the apparent leach rate is 20 to 40 times greater than that for spent fuel. It is 14.8 J.tg.m-2 .d 1 in the presence of clay and 18.5 lag.m- .d-1 in the presence of granite. INTRODUCTION The French choice for managing the back end of the nuclear fuel cycle is reprocessing. The alternative to reprocessing, i.e. direct disposal of spent fuel, has been chosen by other countries such as the USA, Canada, Sweden, etc.. These countries have developed their own scenarios and have set up programs of study on the long-term behaviour of spent fuel, among other things. The majority of these are parameter-based studies in which fuel leaching is studied in simplified aqueous media allowing for changing parameters such as pH, redox potential, ionic force, nature of anions, etc. Other studies aim at reproducing direct disposal conditions (integral experiments), i.e. using spent fuel, synthesized groundwaters, materials of the host rocks (basalt [ 1-4 ], crushed
granite [51, clay and granite [6,7], clay and cement [8,9] or bentonite [10-12]) and/or actual container materials (steel, copper, Ticode-12) [2,3,6-9,11,12]. Simfuel leaching by granitic groundwater in the presence of a granite monolith has also been studied using this methodology [13]. It is to be noted that using groundwaters and host rock (environmental) materials significantly complicates the system being studied and makes the interpretation of the results very delicate. In general it is found that the environmental materials make for reducing conditions which favour slower alteration of the fuel and are the base for important phenomena of radionuclides sorption, reducing their concentration in solution. Although direct disposal of spent fuel is not the back end fuel cycle policy, the CEA has decided to acquire the basic scientific knowledge needed to develop its own expertise in this field. Such expertise might be applied later to fuels which were no longer economically advantageous to reprocess. A programme of specific R&D has thus been developed including studies of long-term behaviour, among which are integral experiments on the leaching of spent fuel and Simfuel. The experimental conditions chosen have been derived fr
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