Direct mass analysis of water absorption onto ceria thin films
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Direct mass analysis of water absorption onto ceria thin films 1
D. Laventine, R. Wilbraham, 1 C. Boxall,1 R. Taylor,2 and R. Orr. 2 1 2
University of Lancaster, Department of Engineering, Lancaster, LA1 4YR, UK National Nuclear Laboratory , Central laboratory, B170, Sellafield, CA20 1PG, UK
ABSTRACT Plutonium oxide (PuO2) is one of the most highly radioactive components of nuclear fuel waste streams and its storage poses particular challenges due to the high temperatures produced by its decay and the production of gases (particularly H2 and steam). Its high radiotoxicity necessitates the use of analogues, such as CeO2, to allow the comprehensive study of its interaction with water under storage conditions. We have developed a method which enables direct gravimetric measurement of water adsorption onto CeO2 thin films with masses in the microgram region. Porous CeO2 films were fabricated from a surfactant based precursor solution. The absorption of water onto the CeO2 coating at different relative humidities was studied in a closed reactor. Quartz Crystal Microbalance (QCM) gravimetry was used as a signal transducer, as changes in crystal resonant frequency due to absorbed mass are directly and linearly related to mass changes occurring at the crystal surface. Using this method, we have determined the enthalpy of absorption of water onto CeO2 to be 49.7 kJmol–1 at 75°C, 11 kJmol-1 greater than the enthalpy of evaporation. This enthalpy is within the range predicted for the absorption of water onto PuO2, indicating this method allows for investigation of water absorption using microgram samples. INTRODUCTION Plutonium oxide (PuO2) is one of the most highly radioactive components of nuclear fuel waste streams. This poses particular challenges in storing large amounts for reuse as mixed oxide fuel for the next generation of fast reactors or for eventual deposition into long-term geological storage, which has been proposed at a number of sites worldwide. Interim storage is typically within a series of nested steel canisters under a partially inert atmosphere. These canisters develop high internal temperatures due to the PuO2 decay heat and have been observed to become pressurised due to the evolution of hydrogen gas and steam. A greater understanding of PuO2 interaction with water is therefore imperative to allow the design of safe long-term storage facilities [1-3]. Ceria (CeO2) is widely employed as a model substrate for radioactive metal oxides such as plutonium oxide, due to the metals similar ionic radii and the oxides similar fluorite structure [4]. Ceria polycrystals exhibit a significant absorption of water vapour at temperatures up to 600°C [5, 6], and previous studies have indicated this to be true of plutonium oxide. The absorption of water results in numerous physio- and chemi-absorbed layers, but the subsequent fate of this water under elevated temperatures and pressures and in the presence of highly radioactive material is unknown. Water adsorption on PuO2 has previously been investigated by measuring h
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