Densification evolution and properties evaluation of UO2-based composites prepared by spark plasma sintering (SPS).

摘要

In current nuclear industry, uranium dioxide (UO2) is the most widely used fuel material. However, the extremely low thermal conductivity of UO2 limits both efficiency and safety of the nuclear reactor. In order to increase the thermal conductivity of UO2, the idea of incorporating high thermal conductivity material into a UO2 matrix has been proposed. A new sintering technique called spark plasma sintering (SPS) has then been reported to fabricate the UO2 composite pellets successfully. The main objective of this research is to investigate the sintering evolution in SPS of fuel pellets and to study the feasibility of novel UO 2-diamond composite fuel pellets.;Firstly, master sintering curve (MSC) theory was utilized in order to study the densification evolution of UO2 and UO2 composites in SPS. For UO2-diamond composites, the theory was proven to be not suitable. For UO2 and UO2-SiC composite, MSC was successfully applied. The apparent activation energies for sintering is determined to be 140 KJ/mol for UO2 and 420 KJ/mol for UO2-SiC composite. The ability of the derived MSCs to control and predict final density in the sintered compact was demonstrated by additional experimental runs using the isothermal heating method.;Second, the microstructure and properties of UO2-diamond composites was investigated. High density UO2-5 vol% diamond composite pellets were fabricated using the spark plasma sintering (SPS) technique at 1300°C--1600°C with a hold time of 5 minutes. The resultant density, chemical reaction, microstructure, thermal conductivity and elastic modulus of the sintered pellets were investigated.;Third, micro-Raman spectroscopy (MRS) was utilized to study the phase transformation and residual stress in diamond particles within a UO2 -diamond composite sintered by SPS. Graphitization of diamond was observed and the degree of graphitization was quantified. The relationship between Raman peak shift and stress intensity was derived.;Last, high temperature aging test was performed for UO2-diamond composite pellet. The changes of microstructure and thermal property were investigated.