Date of Award

2018

Document Type

Open Access Thesis

Department

Mechanical Engineering

Sub-Department

College of Engineering and Computing

First Advisor

Xinyu Huang

Abstract

As a result of the Fukushima Daiichi Nuclear Disaster in 2011, a push for increasing the safety of nuclear power plants was made through research grants funded through the United States Department of Energy via the Accident Tolerant Fuels (ATF) campaign. One of the focuses of the campaign is improving the accident tolerance of nuclear fuel during a loss of coolant accident.

A promising set of being researched from the ATF is Silicon Carbide (SiC) composite cladding and Uranium Silicide (U3Si2) fuel pellets. Under high temperatures, stresses, and times the creep deformation can play an important factor in pellet to cladding contact behavior. The objective of this research was to quantify the creep behavior of U3Si2 as input data for a fuel deformation code, BISON.

A compression creep test plan was carried out. A creep test stand was designed and constructed at USC. To reduce oxidation effects, the experiments take place in a high vacuum environment (total pressures were monitored to ~1E-3mbar and oxygen partial pressures ~1E-7mbar) which is achieved by a turbomolecular and vane pump combination. A constant load is applied using a pneumatic actuator and is monitored via an inline load cell. U3Si2 is electrically conductive. Direct joule heating method was utilized to bring the sample to hightemperature. A chain ofstepdown AC transformers was implemented to obtain high amperage suitable to heat the U3Si2 sample for creep testing. A two-color pyrometer was utilized to monitor the temperature of the sample pellet. To reduce temperature fluctuation, a PID temperature controller was used. The controller uses the -pyrometer as input signal and a silicon crystal rectifier module to modulate power input to achieve a desired temperature. To accurately measure strain at high temperature, a telecentric lens and illuminator are used which captures the outline of the creep test pellets as viewed through borosilicate viewing windows. The telecentric lens and illuminator combination produced images with very low optical distortion (max theoretical distortion of 0.69um) and can measure strains in the range from 1000 to 2500 microstrain. A digital camera was used to record images at a fixed time interval as the sample deforms under the compressive stress and high temperatures. The captured images are processed in imageJ via an automatic template matching program. Using notches on the test pellets as strain markers, axial and radial deformation is calculated.

Creep tests were conducted from 850 to 1000ºC and stress states ranged from 30 to 80 MPa. Multiple stress and temperature combinations were performed on each pellet with total axial strains up to 18%. With a total running time of 4000 combined hours, 4 TB of images, a combination of 13 testing conditions and 5 U3Si2 creep test pellets, a steady state strain equation was obtained as a function of stress and temperature. At 850°C and stress of 70MPa, strain rate was found to be 4.51E-5 hr-1, at 950°C at 77MPa strain rate was found to be 2.52E-4 hr-1, and at 1000°C at 48MPa, strain rate was found to be 1.64E04 hr-1. When using the Arrhenius form of the steady state creep equation, in these temperatures and stresses, the activation energy, Q, is 168kJ/mol and the stress exponent, n, is 1.94. With a low activation energy and stress exponent it is believed that the creep regime is best described via diffusional processes. To the best of our knowledge, this is the first time that the high temperature creep properties of U3Si2 was experimentally obtained. With the data obtained from this research, the updated BISON fuel simulation code modeled a significant difference (5 GWd/MTU longer) between a case where creep deformation is taken into account and a case where no creep deformation is taken into account.

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