Award Date

5-2009

Degree Type

Dissertation

Degree Name

Doctor of Philosophy in Radiochemistry

Department

Chemistry

First Committee Member

Kenneth R. Czerwinski, Chair

Second Committee Member

Alfred P. Sattelberger

Third Committee Member

Gary S. Cerefice

Graduate Faculty Representative

Ralf Sudowe

Number of Pages

221

Abstract

Actinide mononitrides have been considered as a possible nuclear fuel for the Generation-IV nuclear reactor systems. In the process of evaluating these actinide mononitrides as nuclear fuel, it is important to study different chemical and physical characteristics of these compounds. Synthesis of the materials is thus important. Carbothermic reduction is one of the techniques that have been used to synthesize actinide mononitrides. In this method, a mixture of actinide oxide such as UO 2 and excess carbon is heat treated at temperatures greater than 1700 °C under a nitrogen atmosphere. The technique is however not promising in synthesizing the actinide mononitrides due to a number of disadvantages the technique presents. Lack of phase purity due to secondary chemical phases with carbon and oxygen, need of high temperatures such as 2200 °C, and the low density of the final product compared to theoretical density are some of the drawbacks that the researches have been encountered. Most of all this method is tiresome and difficult to handle in ordinary laboratories where the experimental setups and conditions are inadequate to synthesize actinide mononitrides up to an acceptable quality for chemical characterizations. Therefore, it is important to explore different routes that can be used to synthesize such actinide nitrides and characterize them properly.

A recent development of a low-temperature fluoride route in synthesizing UN2 at 800 °C and UN at 1100 °C, proposed a further investigation of this particular chemical route in synthesizing other nitrides of the actinide series. Thus, the possibility of making nitrides of thorium, neptunium, and mixed uranium-thorium by the above mentioned method was suggested. Mechanisms and kinetics involved in separate reactions were studied. Chemical characterizations of the as-synthesized materials were also completed using different techniques reported in Chapter 2.

Optimization of this low-temperature process to minimize the formation of secondary phases such as UO2 was also examined in a typical experimental setup by exploring the uranium system. With the thorium, however, only ThNF could be synthesized up to a temperature of 1100 °C. Addition of lithium amide (LiNH 2 ) into the reactants in synthesizing ThNx produced Th2 N3 with some ThO2 impurities. This finding is controversial and will discuss the relevant issues in the corresponding chapter. Characterization of Th2 N3 /ThO 2 samples revealed an interchangeable formation of ThO2 in Th2 N3 and vise versa suggesting a possible reason for the high susceptibility of ThNx toward oxygen as general.

Evaluation of the neptunium system revealed 6 new compounds with isomorphous crystal structures to that of uranium with similar chemical compositions. XRD powder refinements could be used in solving these crystal structures. NpN was synthesized at 900 °C and further experiments are required to check the lower temperatures for making the mononitride. However, further experiments will be necessary to optimize the heating time. Microscopic characterization of NpN x compounds was also conducted with SEM and TEM. Nanostructural studies conducted on these samples displayed high crystallographic order in their structures.

Uranium and thorium mixed system was also examined with an eye towards synthesizing uranium-thorium mixed nitrides. Less than 1 wt% thorium solubility was identified in the UN2 with XRD and microscopic studies. Further application of the technique on oxides was explored and a novel route in synthesizing (U, Th)O2 solid solutions at temperatures of 1100 °C or less, depending on the chemical composition of the oxide solid solution, was established. Moreover, this novel route itself proposes a new and easy to use low-temperature path to fabricate actinide oxide solid solutions from initial, separate oxide starting phases.

Keywords

Actinide elements; Mixed oxide fuels (Nuclear engineering); Nuclear chemistry; Nuclear fuels; Radiation; Radiochemistry

Disciplines

Chemistry | Materials Chemistry | Nuclear | Nuclear Engineering | Radiochemistry

Language

English

Comments

Signatures have been redacted for privacy and security measures.


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