codes that were verified by the critical experiments are the tools used to predict the fuel enrichment, the power distribution, reflector control worth, safety rod worth and flooding and earth burial accident configuration reactivity. The critical experiments show that the incore safety rod reactivity worth is approximately 20% greater than predicted by the neutronic design codes. All other measured parameters confirm predictions. The fuel pin (Fig. 18) development is progressing nearly as planned. The uranium nitride (UN) fuel is being fabricated into pellets in accordance with the fuel specification, which specifies 94.5 ± 1.5% of theoretical density, less than 1000 ppm oxygen, less than 3000 ppm of carbon, less than 300 ppm of iron and a nitrogen to uranium ratio of 1.00 to 1.05. The total UN pellets required for the SP-100 nuclear assembly reactor is about 60000 of which approximately 40000 have been fabricated to date. The fuel pin cladding for the GFS design is niobium 1% zirconium 0.1% carbon (NblZr-Type C) a refractory material tubing with rhenium bonded to the internal surface. The ground test reactor is being fabricated from niobium 1% zirconium (NblZr-Type A). Once the NblZr-Type A/Re bonded tubes are developed, the same process will be used to fabricate NblZr-Type C/Re bonded tubes. The NAT fuel pins will be fabricated with NblZr-Type C/Re bonded tubes. The NblZr/Re bonded tubing has been made successfully and used for some inpile fuel pin tests. However, the bonding process development is continuing until a reproducible bonded tubing product can be made with high yield.
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