Measurement of Gap and Grain-Boundary Inventories of 129 I Used in Candu Fuels
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MEASUREMENT OF GAP AND GRAIN-BOUNDARY INVENTORIES OF 1291 IN USED CANDU FUELS S. Stroes-Gascoyne, D.L. Moir, M. Kolar, R.J. Porth, J.L. McConnell and A.H. Kerr, AECL Research, Whiteshell Laboratories, Pinava, Manitoba, Canada ABSTRACT Combined gap and grain-boundary inventories of 1291 in 14 used CANDU fuel elements were measured by crushing and simultaneously leaching fuel segments for 4 h in a solution containing KI carrier. From analogy with previous work a near one-to-one correlation was anticipated between the amount of stable Xe and the amount of 1291 in the combined gap and grainboundary regions of the fuel. However, the results showed that such a correlation was only apparent for low linear power rating (LLPR) fuels with an average linear power rating of 44 kU/m), the 1291 values were considerably smaller than expected. The combined gap and grain-boundary inventories of 1291 in the 14 fuels tested varied from 1.8 to 11.0Z, with an average value of 3.6 + 2.4Z which suggests that the average value of 8.1 + 1Z used in safety assessment calculations overestimates the instant release fraction for 1291. Segments of used CANDU fuels were leached for 92 d (samples taken at 5, 28 and 92 d) to determine the kinetics of 1291 release. Results could be fitted tentatively to half-order reaction kinetics, implying that 1291 release is a diffusion-controlled process for LLPR fuels, and also for HLPR fuels, once the gap inventory has been leached. However, more data are needed over longer leaching periods to gain more understanding of the processes that control grain-boundary release of 1291 from used CANDU fuel. INTRODUCTION The concept
fuel
bundles,
of disposing of
contained
intact
used CANDU (CANada
within corrosion-resistant
Deuterium Uranium)
containers
deep in
a
granitic geological formation, is being studied in the Canadian Nuclear Fuel Waste Management Program (CNFVMP)[1]. Geological disposal will eventually result in contact between groundwater and the used fuel upon failure of the disposal containers, which have a minimum design lifetime of 500 years. Considerable research, therefore, has been carried out to understand the mechanisms of radionuclide release from used CANDU fuel upon contact with solutions typical of those found at depths of 500 to 1000 m in granitic rock of the Canadian Shield [2-9]. From these studies it has been established that on contact of used CANDU fuel with an aqueous solution, radionuclide release occurs via three overlapping mechanisms, which operate on very different time scales [10-12]. These mechanisms are (i) A rapid release of fission products such as Cs and I from the gap region of the fuel, which occurs on a time scale of hours or days to a few months. (The "gap" is a collective term for fuel cracks, the fuel-sheath gap and the interconnected, open porosity in the fuel). (ii) A slower release of fission products such as Cs, I, Sr, and Tc from grain boundaries in the fuel, which is thought to occur on a time scale of tens to several hundreds of years or more. (iii) A ve
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