No abstract

Uranyl nitrate is a key species in the nuclear fuel cycle. However, this species is known to exist in different states of hydration, including the hexahydrate ([UO2(NO3)2(H2O)6] often called UNH), the trihydrate [UO2(NO3)2(H2O)3 or UNT], and in very dry environments the dihydrate form [UO2(NO3)2(H2O)2]. Their relative stabilities depend on both water vapor pressure and temperature. In the 1950s and 1960s, the different phases were studied by infrared transmission spectroscopy but were limited both by instrumental resolution and by the ability to prepare the samples for transmission. We have revisited this problem using time-resolved reflectance spectroscopy, which requires no sample preparation and allows dynamic analysis while the sample is exposed to a flow of N2 gas. Samples of known hydration state were prepared and confirmed via X-ray diffraction patterns of known species. In reflectance mode the hexahydrate UO2(NO3)2(H2O)6 has a distinct uranyl asymmetric stretch band at 949.0 cm(-1) that shifts to shorter wavelengths and broadens as the sample desiccates and recrystallizes to the trihydrate, first as a shoulder growing in on the blue edge but ultimately results in a doublet band with reflectance peaks at 966 and 957 cm(-1). The data are consistent with transformation from UNH to UNT as UNT has two inequivalent UO2(2+) sites. The dehydration of UO2(NO3)2(H2O)6 to UO2(NO3)2(H2O)3 is both a structural and morphological change that has the lustrous lime green UO2(NO3)2(H2O)6 crystals changing to the matte greenish yellow of the trihydrate solid. The phase transformation and crystal structures were confirmed by density functional theory calculations and optical microscopy methods, both of which showed a transformation with two distinct sites for the uranyl cation in the trihydrate, with only one in the hexahydrate.

Executive SummaryThe UO 3 -water system is complex and has not been fully characterized, even though these species are common throughout the nuclear fuel cycle. As an example, most production processes for UO 3 result in a mixture of up to six or more different polymorphic phases, and small differences in these conditions will affect phase genesis that ultimately result in measureable changes to the end product. As a result, this polymorphic feature of the UO 3 -water system may be useful as a means for determining process history. This research effort attempts to better characterize the UO 3 -water system with a variety of optical techniques for the purpose of developing some predictive capability for estimating process history in polymorphic phases of unknown origin. Three commercially relevant production methods for the production of UO 3 were explored. Previously unreported low temperature routes to β-and γ-UO 3 were discovered. Raman and fluorescence spectroscopic libraries were established for pure and mixed polymorphic forms of UO 3 in addition to the common hydrolysis products of UO 3 . An advantage of the sensitivity of optical fluorescence microscopy over x-ray diffraction has been demonstrated. Preliminary aging studies of the α and γ forms of UO 3 have been conducted. In addition, development of a 3-D phase field model used to predict phase genesis of the system was initiated. Thermodynamic and structural constants that will feed the model have been gathered from the literature for most of the UO 3 polymorphic phases.v

A liquid semiconductor-based radioisotope micropower source has been pioneerly developed. The semiconductor property of selenium was utilized along with a 166 MBq radioactive source of S35 as elemental sulfur. Using a liquid semiconductor-based Schottky diode, electrical power was distinctively generated from the radioactive source. Energetic beta radiations in the liquid semiconductor can produce numerous electron hole pairs and create a potential drop. The measured power from the microbattery is 16.2 nW with an open-circuit voltage of 899 mV and a short-circuit of 107.4 nA.