Thermal and X-ray diffraction analysis studies during the decomposition of ammonium uranyl nitrate
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Abstract
Two types of ammonium uranyl nitrate (NH4)2UO2(NO3)4·2H2O and NH4UO2(NO3)3, were thermally decomposed and reduced in a TG-DTA unit in nitrogen, air, and hydrogen atmospheres. Various intermediate phases produced by the thermal decomposition and reduction process were investigated by an X-ray diffraction analysis and a TG/DTA analysis. Both (NH4)2UO2(NO3)4·2H2O and NH4UO2(NO3)3 decomposed to amorphous UO3 regardless of the atmosphere used. The amorphous UO3 from (NH4)2UO2(NO3)4·2H2O was crystallized to γ-UO3 regardless of the atmosphere used without a change in weight. The amorphous UO3 obtained from decomposition of NH4UO2(NO3)3 was crystallized to α-UO3 under a nitrogen and air atmosphere, and to β-UO3 under a hydrogen atmosphere without a change in weight. Under each atmosphere, the reaction paths of (NH4)2UO2(NO3)4·2H2O and NH4UO2(NO3)3 were as follows: under a nitrogen atmosphere: (NH4)2UO2(NO3)4·2H2O → (NH4)2UO2(NO3)4·H2O → (NH4)2UO2(NO3)4 → NH4UO2(NO3)3 → A-UO3 → γ-UO3 → U3O8, NH4UO2(NO3)3 → A-UO3 → α-UO3 → U3O8; under an air atmosphere: (NH4)2UO2(NO3)4·2H2O → (NH4)2UO2(NO3)4·H2O → (NH4)2UO2(NO3)4 → NH4UO2(NO3)3 → A-UO3 → γ-UO3 → U3O8, NH4UO2(NO3)3 → A-UO3 → α-UO3 → U3O8; and under a hydrogen atmosphere: (NH4)2UO2(NO3)4·2H2O → (NH4)2UO2(NO3)4·H2O → (NH4)2UO2(NO3)4 → NH4UO2(NO3)3 → A-UO3 → γ-UO3 → α-U3O8 → UO2, NH4 UO2(NO3)3 → A-UO3 → β-UO3 → α-U3O8 → UO2.
Keywords
Ammonium uranyl nitrate Thermal decomposition Modified direct denitrationIntroduction
Ammonium uranyl nitrate (AUN) is an important intermediate product during conversion of uranyl nitrate [UO2(NO3)2] solution to UO2 powder for the fabrication of nuclear fuels, the so-called modified direct denitration (MDD) process. Many conversion processes have been developed, such as, ammonium uranyl carbonate (AUC), ammonium diuranate (ADU), and an integrated dry route (IDR). Each process has its merits and demerits. In comparison with other processes, the MDD process offers the greatest potential for cost reduction and good product quality for the production of UO2 powder.
The modified direct denitration process involves the thermal decomposition of AUN double salts, which are prepared from a mixture consisting of a UO2(NO3)2 solution and NH4NO3. The physical and chemical properties of an oxide powder depend upon its thermal treatment. Also, the sintering behavior of UO2 powder can be related to its powder characteristics and processing parameters. It has been observed that the presence of NH4NO3 in a UO2(NO3)2 feed solution prior to a thermal denitration greatly improved the sintering properties of UO2 powder [1].
Three double salts are known for the UO2(NO3)2–NH4NO3–H2O system, but there have been only a few studies done on thermal decomposition of these salts. Laboratory scale denitration tests showed that NH4UO2(NO3)3 decomposes without melting, and thus does not form a dough stage similar to that encountered during a denitration. Also, UO2 produced from NH4UO2(NO3)3 in these tests appeared to be more active than a corresponding oxide produced from UO2(NO3)2 solution. It was also reported that NH4UO2(NO3)3 decomposes without melting at 270–300 °C to give γ-UO3 powder of an average size of approximately 3 μm, with good ceramic properties for its fabrication into UO2 nuclear fuel pellets [2].
In the conversion of AUN to uranium oxides, the characteristic of the resulting powder particles depends upon the AUN preparation process and also upon the thermal decomposition procedures. ADU decomposes first and then leads to UO3. Amorphous uranium trioxide (A-UO3) is mainly formed in the absence of ammonium and nitrate ions, whereas deamination of the retained ammonia leads to β-UO3 [3]. Meanwhile, AUC decomposes at around 190 °C, giving off CO2, NH3, and H2O with the formation of an amorphous phase. In a N2, Ar, or CO2 atmosphere, the amorphous phase crystallizes to α-UO3 before decomposing to U3O8 [4]. Various reports have been published on thermal analysis studies of the reactions occurring during a decomposition of AUC and ADU. However, few studies on the thermal decomposition of AUN can be found in the literature. Therefore, the objective of this study is to investigate the reaction pathways during a thermal decomposition and reduction of AUN to achieve a better knowledge of the influence of an AUN preparation process and thermal decomposition procedures on uranium oxides under a nitrogen, air, or hydrogen atmosphere.
Experimental
The UO2(NO3)2·6H2O and NH4NO3 solutions were prepared using various mole ratios of NH4 +/U. (NH4)2UO2(NO3)4·2H2O was resulted when the pH of the reaction solution was 2.58 and the mole ratio of NH4 +/U was 2.14. And NH4UO2NO3 was resulted when the pH of the reaction solution was 2.01 and the mole ratio of NH4 +/U was 1.07. The volume of both UO2(NO3)2·6H2O and NH4NO3 solutions were 50 mL. The mixed reaction of these two solutions was performed in a heating mantle where the temperature could be automatically controlled. The reaction temperature was maintained at 90 °C. After the reaction had progressed to a point which a precipitate was generated, the heating was stopped and the precipitated solid was filtered. The precipitate remaining on the filter paper was then left at room temperature to dry. As a result, a primary sample was prepared. The primary sample contained unreacted UO2(NO3)2·6H2O and NH4NO3 as impurities. To eliminate these impurities, the primary sample was recrystallized. The recrystallization was performed by dissolving the primary sample in distilled water at 40 °C, and then cooling it down to room temperature. The crystals acquired through the recrystallization were referred to as the secondary sample. The pH of the mixed solution was controlled by adding UO3 or concentrated nitric acid to the mixed solution. Next the synthesis of AUN was carried out. When the UO3 was added to that, it was dissolved at a temperature of 30 °C. To eliminate any undissolved UO3 from the reaction solution, it was filtered, and the filtrate was used as the reaction solution for AUN synthesis. The characterization of synthesized AUN was reported in previous studies [5].
To analyze the thermal decomposition and reduction pathways of each AUN, the respective thermal decomposition and reduction temperature must be determined beforehand to identify any intermediate phase produced from each reaction stage. For this purpose, a thermogravimetric (TG)/differential thermal analysis (DTA) experiment was carried out in various atmospherics, which were 100% nitrogen, air, and hydrogen gas. The flow rate for each gas was 50 mL/min, the heating rate was 5 °C/min, and the temperature was varied from room temperature to 800 °C. About 8 mg of sample was used in each run. Each reaction stage was identified on the basis of DTA results, and the temperature for the intermediate phase was determined. Samples used to acquire the intermediate phase were obtained by heating each AUN sample in the thermal analyzer up to the temperature as determined from the DTA results. About 40 mg of AUN was thermally decomposed and reduced in the thermal analyzer to produce the samples for use in the analyzer in order to identify the intermediate phases. The intermediate reaction phases were determined and identified by TG analysis and X-ray diffraction. The characteristic analyzes of the intermediate phases and sample preparations were performed by a TG-DTA unit (TA Instrument, Simultaneous SDT 2960). The X-ray diffraction analysis was performed at room temperature at a scan speed of 0.4°/min and by varying the value of 2θ from 10° to 70°. The target was Cu, and one slit was used for the divergence and scattering (Rigaku Max/3D).
Results and discussion
DTA analysis
DTA curves for (NH4)2UO2(NO3)4·2H2O in a N2, b Air, and c H2 atmosphere
The thermal decomposition under the air atmosphere showed a different trend. NH4UO2(NO3)3 was thermally decomposed to UO3 at around 275 °C, which was about 7 °C higher than the corresponding temperature under the nitrogen atmosphere. An exothermic reaction was observed at a temperature slightly higher than 400 °C. However an endothermic reaction took place at around 599 °C. It is assumed that such a difference in the reaction temperature between an air and a nitrogen atmosphere was due to the different activation energy that was dependent on the atmospheric under which the decomposition was carried out. Also, it has been reported that if the composition of a compound is different or an impurity is present, the peak of the DTA curve moves toward a higher temperature [4].
Under a hydrogen atmosphere, the thermal decomposition was followed by a reduction reaction. First, the dehydration of (NH4)2UO2(NO3)4·2H2O occurred, and the thermal decomposition of NH4NO3 then took place, which had the same trend as that under nitrogen and air atmospheres. However, both the temperature at which the thermal decomposition took place and the maximum peak temperature shifted toward a lower temperature. Like the exothermic peak under the nitrogen and air atmospheres, the exothermic peak appeared at around 400 °C. However, the subsequent reactions were different from those under the nitrogen and air atmospheres due to the reduction by hydrogen, which caused the appearance of two different exothermic peaks. An exothermic reaction took place at 440 °C, then shortly after another exothermic reaction started at around 590 °C. Halldahl and Nygren [4] also observed a similar phenomenon, the DTA curves for decomposition of (NH4)2UO2(NO3)4·2H2O were dependent upon the atmospheric gas used during the decomposition. Halldahl also reported that the maximum peak temperature when AUC was thermally decomposed into UO3 under a nitrogen, air, and hydrogen atmosphere was 195, 198, and 185 °C, respectively. The thermal decomposition temperature became lower in the following order: hydrogen < nitrogen < air. This result is similar to the result observed in the current study, where the maximum peak temperature appeared when (NH4)2UO2(NO3)4·2H2O was decomposed into UO3.
DTA curves for NH4UO2(NO3)3 in a N2, b air, and c H2 atmosphere
The endothermic peak resulting from the thermal decomposition of NH4NO3, which was the first reaction under a nitrogen atmosphere, appeared between 266 and 305 °C, and a weak exothermic reaction then took place at around 400 °C followed by an endothermic reaction at 578 °C. Also, in air, an endothermic reaction was observed at 277 °C, an exothermic reaction around 400 °C, and two endothermic reactions at around 598 °C. Under a hydrogen atmosphere, one endothermic reaction was observed at 270 °C, and three exothermic reactions were then observed at around 400, 441, and 590 °C.
Preparation of samples for analysis of intermediate phases
Thermal treatment conditions for the preparation of intermediates decomposed from (NH4)2UO2(NO3)4·2H2O and NH4UO2(NO3)3
| Atmosphere | Final temperature of thermal treatment for intermediates of (NH4)2UO2(NO3)4·2H2O and NH4UO2(NO3)3 (°C) | |||
|---|---|---|---|---|
| N2 | 390 | 480 | 800 | |
| Air | 390 | 480 | 800 | |
| H2 | 390 | 430 | 480 | 600 |
X-ray diffraction analysis of intermediate phases obtained from AUN
X-ray diffraction patterns of intermediates produced from (NH4)2UO2(NO3)4·2H2O in N2 atmosphere
X-ray diffraction patterns of intermediates from (NH4)2UO2(NO3)4·2H2O in air atmosphere
X-ray diffraction patterns of intermediates produced from (NH4)2UO2(NO3)4·2H2O in H2 atmosphere
X-ray diffraction patterns of intermediates produced from (NH4)UO2(NO3)3 in N2 atmosphere
X-ray diffraction patterns of intermediates produced from (NH4)UO2(NO3)3 in air atmosphere
X-ray diffraction patterns of intermediates produced from (NH4)UO2(NO3)3 in H2 atmosphere
Comparison of the decomposition mechanisms of AU, AUC and AUN with previous results
| Researchers | Decomposition mechanism of AU | Atmosphere |
|---|---|---|
| Woolfrey [8] | \( \begin{aligned} {\text{AU III}}/{\text{IV}} \to {\text{AU II}} \to {\text{AU I}} \to \beta {\text{-UO}}_{ 3} \to \\ {\text{A-UO}}_{ 3} \to {\text{U}}_{ 3} {\text{O}}_{ 8} /{\text{UO}}_{ 2. 9} \to {\text{U}}_{ 4} {\text{O}}_{ 9} \to {\text{UO}}_{ 2+ x} \\ \end{aligned} \) | H2 |
| Kim [12] | \( \begin{aligned} {\text{AU II}}/{\text{III}} \to {\text{AU II}}/{\text{I}} \to {\text{AU I}} \to {\text{A-UO}}_{ 3} \to \beta{\text{-UO}}_{ 3} \to \\ \alpha{\text{-U}}_{ 3} {\text{O}}_{ 8} \to {\text{U}}_{ 4} {\text{O}}_{ 9} \to {\text{UO}}_{ 2} \\ \end{aligned} \) | H2 |
| \( {\text{AUC}} \to {\text{A-UO}}_{ 3} \to \alpha {\text{-U}}_{ 3} {\text{O}}_{ 8} \) | Air | |
| \( {\text{AUC}} \to {\text{A-UO}}_{ 3} \to \alpha {\text{-UO}}_{ 3} \to \alpha {\text{-U}}_{ 3} {\text{O}}_{ 8} \) | N2 | |
| \( {\text{AUC}} \to {\text{A-UO}}_{ 3} \to \alpha {\text{-UO}}_{ 3} \to \alpha {\text{-U}}_{ 3} {\text{O}}_{ 8} \to {\text{UO}}_{ 2} \) | H2 | |
| Sato [13] | \( {\text{AU}} \to {\text{UO}}_{ 3} {\text{NH}}_{ 3} \to {\text{A-UO}}_{ 3} \to \beta {\text{-UO}}_{ 3} \to {\text{U}}_{ 3} {\text{O}}_{ 8} \) | Air |
| Price [15] | \( {\text{AU}} \to {\text{UO}}_{ 3} x{\text{NH}}_{ 3} \to {\text{U}}_{ 3} {\text{O}}_{ 8} \to {\text{UO}}_{ 2} \) | H2 |
| Current study | \( \begin{aligned} & \left( {{\text{NH}}_{ 4} } \right)_{ 2} {\text{UO}}_{ 2} \left( {{\text{NO}}_{ 3} } \right)_{ 4} \cdot 2 {\text{H}}_{ 2} {\text{O}} \to \left( {{\text{NH}}_{ 4} } \right)_{ 2} {\text{UO}}_{ 2} \left( {{\text{NO}}_{ 3} } \right)_{ 4} \cdot {\text{H}}_{ 2} {\text{O}}\\ & \to \left( {{\text{NH}}_{ 4} } \right)_{ 2} {\text{UO}}_{ 2} \left( {{\text{NO}}_{ 3} } \right)_{ 4} \to {\text{NH}}_{ 4} {\text{UO}}_{ 2} \left( {{\text{NO}}_{ 3} } \right)_{ 3} \to {\text{A-UO}}_{ 3}\\ & \to \gamma {\text{-UO}}_{ 3} \to {\text{U}}_{ 3} {\text{O}}_{ 8}\\ & {\text{NH}}_{ 4} {\text{UO}}_{ 2} \left( {{\text{NO}}_{ 3} } \right)_{ 3} \to {\text{A-UO}}_{ 3} \to \alpha {\text{-UO}}_{ 3} \to {\text{U}}_{ 3} {\text{O}}_{ 8} \\ \end{aligned} \) | N2 |
| \( \begin{aligned} & \left( {{\text{NH}}_{ 4} } \right)_{ 2} {\text{UO}}_{ 2} \left( {{\text{NO}}_{ 3} } \right)_{ 4} \cdot 2 {\text{H}}_{ 2} {\text{O}} \to \left( {{\text{NH}}_{ 4} } \right)_{ 2} {\text{UO}}_{ 2} \left( {{\text{NO}}_{ 3} } \right)_{ 4} \cdot {\text{H}}_{ 2} {\text{O}}\\ & \to \left( {{\text{NH}}_{ 4} } \right)_{ 2} {\text{UO}}_{ 2} \left( {{\text{NO}}_{ 3} } \right)_{ 4} \to {\text{NH}}_{ 4} {\text{UO}}_{ 2} \left( {{\text{NO}}_{ 3} } \right)_{ 3} \to {\text{A-UO}}_{ 3} \\ & \to \gamma {\text{-UO}}_{ 3} \to {\text{U}}_{ 3} {\text{O}}_{ 8}\\ & {\text{NH}}_{ 4} {\text{UO}}_{ 2} \left( {{\text{NO}}_{ 3} } \right)_{ 3} \to {\text{A-UO}}_{ 3} \to \alpha {\text{-UO}}_{ 3} \to {\text{U}}_{ 3} {\text{O}}_{ 8} \\ \end{aligned} \) | Air | |
| \( \begin{aligned} & \left( {{\text{NH}}_{ 4} } \right)_{ 2} {\text{UO}}_{ 2} \left( {{\text{NO}}_{ 3} } \right)_{ 4} \cdot 2 {\text{H}}_{ 2} {\text{O}} \to \left( {{\text{NH}}_{ 4} } \right)_{ 2} {\text{UO}}_{ 2} \left( {{\text{NO}}_{ 3} } \right)_{ 4} \cdot {\text{H}}_{ 2} {\text{O}} \\ & \to \left( {{\text{NH}}_{ 4} } \right)_{ 2} {\text{UO}}_{ 2} \left( {{\text{NO}}_{ 3} } \right)_{ 4} \to {\text{NH}}_{ 4} {\text{UO}}_{ 2} \left( {{\text{NO}}_{ 3} } \right)_{ 3} \to {\text{A-UO}}_{ 3}\\ & \to \gamma {\text{-UO}}_{ 3} \to \alpha {\text{-U}}_{ 3} {\text{O}}_{ 8} \to {\text{UO}}_{ 2} \\& {\text{NH}}_{ 4} {\text{UO}}_{ 2} \left( {{\text{NO}}_{ 3} } \right)_{ 3} \to {\text{A-UO}}_{ 3} \to \beta {\text{-UO}}_{ 3} \to \alpha {\text{-U}}_{ 3} {\text{O}}_{ 8} \to {\text{UO}}_{ 2} \\ \end{aligned} \) | H2 |
TG curves of (NH4)2UO2(NO3)4·2H2O in a N2, b air, and c H2 atmosphere
TG curves of NH4UO2(NO3)3 in a N2, b air, and c H2 atmosphere
Conclusions
An intermediate phase, amorphous UO3 was produced when (NH4)2UO2(NO3)4 and NH4UO2(NO3)3 were thermally decomposed under air, nitrogen, and hydrogen atmosphere, regardless of the atmosphere used. γ-UO3 was produced as the intermediate product irrespective of the atmospheric gas used during the decomposition of (NH4)2UO2(NO3)4·2H2O. However, in the case of NH4UO2(NO3)3, when decomposed under the nitrogen and air atmospheres, α-UO3 was produced, whereas β-UO3 was produced under a hydrogen atmosphere.
- Under the nitrogen atmosphere
-
\( \begin{aligned} \left( {{\text{NH}}_{ 4} } \right)_{ 2} {\text{UO}}_{ 2} \left( {{\text{NO}}_{ 3} } \right)_{ 4} \cdot 2 {\text{H}}_{ 2} {\text{O}} \to \left( {{\text{NH}}_{ 4} } \right)_{ 2} {\text{UO}}_{ 2} \left( {{\text{NO}}_{ 3} } \right)_{ 4} \cdot {\text{H}}_{ 2} {\text{O}} \to \left( {{\text{NH}}_{ 4} } \right)_{ 2} {\text{UO}}_{ 2} \left( {{\text{NO}}_{ 3} } \right)_{ 4} \to \\ {\text{NH}}_{ 4} {\text{UO}}_{ 2} \left( {{\text{NO}}_{ 3} } \right)_{ 3} \to {\text{A-UO}}_{ 3} \to \gamma{\text{-UO}}_{ 3} \to{\text{U}}_{ 3} {\text{O}}_{ 8} \\ \end{aligned} \)
-
\( {\text{NH}}_{ 4} {\text{UO}}_{ 2} \left( {{\text{NO}}_{ 3} } \right)_{ 3} \to {\text{A-UO}}_{ 3} \to \alpha {\text{-UO}}_{ 3} \to {\text{U}}_{ 3} {\text{O}}_{ 8} \)
-
- Under the air atmosphere
-
\( \begin{aligned} \left( {{\text{NH}}_{ 4} } \right)_{ 2} {\text{UO}}_{ 2} \left( {{\text{NO}}_{ 3} } \right)_{ 4} \cdot 2 {\text{H}}_{ 2} {\text{O}} \to \left( {{\text{NH}}_{ 4} } \right)_{ 2} {\text{UO}}_{ 2} \left( {{\text{NO}}_{ 3} } \right)_{ 4} \cdot {\text{H}}_{ 2} {\text{O}} \to \left( {{\text{NH}}_{ 4} } \right)_{ 2} {\text{UO}}_{ 2} \left( {{\text{NO}}_{ 3} } \right)_{ 4} \to \\ {\text{NH}}_{ 4} {\text{UO}}_{ 2} \left( {{\text{NO}}_{ 3} } \right)_{ 3} \to {\text{A-UO}}_{ 3} \to \gamma {\text{-UO}}_{ 3} \to {\text{U}}_{ 3} {\text{O}}_{ 8} \\ \end{aligned} \)
-
\( {\text{NH}}_{ 4} {\text{UO}}_{ 2} \left( {{\text{NO}}_{ 3} } \right)_{ 3} \to {\text{A-UO}}_{ 3} \to \alpha {\text{-UO}}_{ 3} \to {\text{U}}_{ 3} {\text{O}}_{ 8} \)
-
- Under the hydrogen atmosphere
-
\( \begin{aligned} \left( {{\text{NH}}_{ 4} } \right)_{ 2} {\text{UO}}_{ 2} \left( {{\text{NO}}_{ 3} } \right)_{ 4} \cdot 2 {\text{H}}_{ 2} {\text{O}} \to \left( {{\text{NH}}_{ 4} } \right)_{ 2} {\text{UO}}_{ 2} \left( {{\text{NO}}_{ 3} } \right)_{ 4} \cdot {\text{H}}_{ 2} {\text{O}} \to \left( {{\text{NH}}_{ 4} } \right)_{ 2} {\text{UO}}_{ 2} \left( {{\text{NO}}_{ 3} } \right)_{ 4} \to \\ {\text{NH}}_{ 4} {\text{UO}}_{ 2} \left( {{\text{NO}}_{ 3} } \right)_{ 3} \to {\text{A-UO}}_{ 3} \to \gamma {\text{-UO}}_{ 3} \to \alpha {\text{-U}}_{ 3} {\text{O}}_{ 8} \to {\text{UO}}_{ 2} \\ \end{aligned} \)
-
\( {\text{NH}}_{ 4} {\text{UO}}_{ 2} \left( {{\text{NO}}_{ 3} } \right)_{ 3} \to {\text{A-UO}}_{ 3} \to \beta {\text{-UO}}_{ 3} \to \alpha {\text{-U}}_{ 3} {\text{O}}_{ 8} \to {\text{UO}}_{ 2} \)
-
Notes
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