Study On High Temperature Creep Property Of Uranium Dioxide Pellet

Uranium dioxide pellet is one of the crucial nuclear fuels. Conventionally, the manufacturing process of uranium dioxide pellet is high-temperature sintering (1750℃×10h) presently. The pellet sintered by this method has a large grain size and reliable capability, but this technology comsumes a large amount of energy and leads to a very high production cost. Therefore, the low-temperature sintering technology (1100℃×4h) of uranium dioxide pellet was studied at home and abroad. Compared with the high temperature singering process, low-temperature sintering process has lots of advantages such as much shorter sintering time, much lower energy consumption and production costing, easier to operate and manipulate, and besides it strengthens competitiveness of the nuclear power. Nevertheless, its grain size is so smaller which limits its use. In this paper, on purpose to explore the high temperature creep property of uranium dioxide pellet in the reactor, high-temperature creep experiment of uranium dioxide with different grain sizes was carried out to investigate whether the smaller grain size has effect on properties of uranium dioxide pellet.Comparative creep test is done for uranium dioxide with different grain sizes at high temperature.High-temperature creep property of uranium dioxide pellets with grain size of 23.8μm (high-temperature sintering) and grain size of 9.0μm (low-temperature sintering ) is tested under 20-40 MPa compression stress at 1400-1500℃. Creep curve is drawn when high temperature creep data were processed. The steady creep rate of each sample at 1400-1500℃is calculated, then the curve of compression ratio and time is fitted, similarly the logarithmic curves of steady creep rate and compression stress is also fitted, and the functional relationship bewteen stead creep rate and compression stress is deduced. The microstructure of uranium dioxide pellet after creep testing is observed by optical microscope and SEM (scanning electronic microscope), and the creep mechanism of urannium dioxide pellet is aslo discussed.Conclusions can be drawn from the test results:1. The lgε.—lgσrelationship between steady creep rate and stress of uranium dioxide pellet with different grain size are described as follows:The sample with large grain size at 1500℃: lgε1= 2.41808lgσ- 6.87601The sample with small grain size at 1500℃: lgε2=1.69804lgσ- 5.64667The sample with large grain size at 1400℃: lgε3= 2.45457lgσ- 6.99475 The sample with small grain size at 1400℃: lgε4= 1.45100lgσ- 5.482922. The steady creep rates of uranium dioxide pellet at 1400-1500℃under 10MPa compression stress were calculated. The rates of uranium dioxide pellet with grain size of 23.8μm,9.0μm are 0.3484×10-4mm /h and 1.1256×10-4 mm/h, at 1500℃, respectively. The rates of uranium dioxide pellet with grain size of 23.8μm,9.0μm are 0.2883×10-4 mm/h and 0.9294×10-4mm /h at 1400℃,respectively.There is almost no order difference of magnitude between the uranium dioxide pellets with different sizes, and this result can be referenced in the engineering design and application fields.3. Uranium dioxide pellet is re-pressure-sintered during creep process with crack formation, and the interface between two pateicles can not be seen.The amount of pores decreased and some of the pores merged into a larger crack. Nitrogen-oxygen-uranium compounds were discovered in every sample. Chemical reaction took place between nitrogen and sintered uranium dioxide pellets, and the reaction product exists in form of white substances. White these substances discovered in the sample exist in form of particle at 1400℃, but exist in form of bamboo leaf figure around the crack at 1500℃. With the pressure increasing, the white substances reduced. The amount of white substances in the sample with grain size of 9.0μm was less than that of the sample with grain size of 23.8μm.4. The results of the high-temperature experiments were compared with the theory value of Nabarro—Herring creep mode,Coble creep mode and Hamper-Dorn creep mode, and it indicates that the creep process of uranium dioxide pellet with grain size of 9.0μm is controlled by both Nabarro-Herring creep mechanism and Hamper-Dorn creep mechanism, and Hamper-Dorn creep is dominant mechanism.The creep process of uranium dioxide pellet with grain size of 23.8μm is only controlled by Hamper-Dorn creep mechanism.