Investigation of water adsorption on metal oxide surfaces under conditions representative of PuO 2 storage containers.
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Investigation of water adsorption on metal oxide surfaces under conditions representative of PuO2 storage containers. Patrick Murphy1, Colin Boxall1 and Robin Taylor2 Engineering Department, Lancaster University, Lancaster, LA1 4YR, UK 2 Central Laboratory, B170, National Nuclear Laboratories, Sellafield, Seascale, Cumbria, CA20 1PG, UK
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ABSTRACT We have developed a QCM (Quartz Crystal Microbalance) based method for direct gravimetric determination of water adsorption on PuO2 surrogate surfaces, especially CeO2, under conditions representative of those in a typical PuO2 storage can. In this application, the method of transduction of the QCM relies upon the linear relationship between the resonant frequency of piezoelectrically active quartz crystals and the mass adsorbed on the crystal surface. The spurious effect of high temperatures on the resonant frequency of coated QCM crystals has been compensated for by modeling the temperature dependence of the frequency response of the surrogate coated-QCM crystal in the absence of water. Preliminary results indicate that water is readily adsorbed from the vapor phase into porous metal oxide structures by capillary condensation, an observation that may have important ramifications for water uptake within the packed powder beds that may obtain in PuO2 storage cans. INTRODUCTION Standardised packaging and storage of plutonium oxide powders involves sealing the materials in welded, stainless steel containers. Pressurization of these containers arises from decomposition of adsorbed water contained in and on the surface of hygroscopic PuO2 [1]. In an attempt to remove water from the surfaces of these oxides, and thus minimize pressurization, PuO2 samples are calcined at temperatures as high as 700°C but water often readsorbs onto the plutonia powders during packaging [2]. The potential of PuO2 to generate a water vapor-derived pressure in a storage can headspace is directly related to its capacity for H2O adsorption. This adsorption must be fully understood in order to reliably underpin the PuO2 interim storage safety case. Water adsorption on PuO2 has previously been investigated by measuring headspace pressure, as a function of temperature within a closed system containing a fixed quantity of PuO2 in the presence of varying amounts deliberately added water [1]. This involves making a number of assumptions relating to the PVT behaviour of the headspace of the closed system, usually based on the behaviour of an ideal gas, in order to estimate the mass of water adsorbed at the PuO2 surface. Assuming ideality at high temperatures and pressures is problematic and, at best, only gives an indirect measurement of water adsorption on PuO2. The Quartz Crystal Microbalance (QCM) measures mass per unit area by measuring the change in frequency of a quartz crystal resonator. In our system we have coated QCM crystals with a PuO2 surrogate (CeO2) so as to measure directly the mass of water adsorbing at the surrogate surface, thus avoiding many of the issues associated with indirect measur
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