ANALYSIS OF WATER ABSORPTION ONTO CERIA AND THORIA THIN FILMS BY DIREST MASS AND CONTACT ANGLE MEASUREMENTS

摘要

Plutonium oxide (PuO_2) is one of the most highlyradioactive products of the nuclear fuel cycle and itsstorage poses particular challenges due to the hightemperatures produced by its decay and the production ofgases from, inter alia, surface adsorbed water entrainedwith the PuO_2 during the packaging process. Its highradiotoxicity necessitates the use of actinide oxides insimilar oxidation states, such as CeO_2 and ThO_2, asmodel systems to allow the comprehensive study of itsinteraction with water under storage conditions.We have developed a method which enables directgravimetric measurement of the adsorption of microgramamounts of water onto CeO_2 and ThO_2 thin films, alsowith masses in the microgram range. Additionally sessilecontact angles of water droplets deposited onto the samefilms have been measured to provide a correlatingmeasure of the affinity of the oxide surface with water.Porous CeO_2 and ThO_2 films were deposited from asurfactant based precursor solution onto thin GaPO_4crystal and glass substrates. The absorption of water ontothe CeO_2 or ThO_2 coating at different relative humiditieswas then studied in a closed reactor using crystal nanobalancegravimetry, wherein changes in crystal resonantfrequency due to absorbed mass are directly and linearlyrelated to mass changes occurring at the crystal surface.Using this method, we have determined the enthalpy ofabsorption of water onto CeO_2 to be 49.7 kJmol~(–1) andonto ThO_2 to be 54.6 kJmol~(–1) at 75°C, 11 and 15 kJmol-1greater than the enthalpy of evaporation. Sessile contactangle measurements on the same films provide values forCeO_2 and ThO_2 of 56° and 27° respectively – indicatingthat water shows a greater affinity for the ThO_2 surfacethan CeO_2, an observation consistent with the hierarchyof water adsorption enthalpies derived hereThese enthalpies are within the range predicted forthe reversible absorption of water onto PuO_2, confirmingthis method allows for the investigation of waterabsorption onto plutonia using microgram samples. Thesignificantly higher enthalpy of water absorption forthoria over ceria, and the correspondingly smaller waterdroplet contact angles indicates a high variation in waterabsorbingproperties of commonly used plutoniaanalogues, further emphasizing the need for studies onactive PuO_2 samples.