The UN fuel must be capable of operating at 1400 K. peak surface temperature at the beginning of life and approximately 1450 K at the end of life with a peak burnup of 6 atom percent. The inpile accelerated irradiation of UN fuel as of this date has achieved 5.7% burnup at 1500 K fuel surface temperature with a fuel density of 85% of theoretical and 2.7% burnup at 1500 K with a fuel density of 95% of theoretical. The fuel fission gas release and swelling of the fuel in the inpile tests have been slightly less than the analytical codes have predicted. The inpile fuel pins are being tested at 1250 K, 1300 K and 1500 K surface temperature with three different pin diameters, 0.635 cm, 0.762 cm and 0.889 cm so that the resulting data can be used to verify the fuel behaviour analysis codes over a range of temperatures and diameters for future space reactor designs. The results to date of the fuel pin analyses and the test data have been used to specify the fuel density, the X/U ratio, the liner material, the liner configuration and the fuel to clad gap dimensions. The completion of SP-100 fuel pin lifetime tests will verify and calibrate the fuel pin behavior design analysis codes, establish the UN fuel and bonded NblZr-Type C/Re fabrication processes and establish the fuel pin fabrication and acceptance test procedures. The GFS reactor design specifies NblZr- Type C as the refractory material for all the reactor components that are in contact with the lithium coolant. The nuclear assembly ground test reactor will be fabricated with all NblZr-Type A components. The only difference between NblZr-Type C and NblZr-Type A is that the Type C composition has 0.1% carbon, which results in larger allowable creep stress. The NblZr-Type A allowable creep stress for 1% and 2% strain in seven years is being determined from previous long-term creep data, from uniaxial creep tests at ORNL, NASA-LeRC and W-AESD and biaxial creep tests at WHC. From this data an interim creep data base and a creep equation for NblZr-Type A are available for
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