The core of the NDR is a bundle of hexagonal fuel and support elements. The fuel elements are similar to those of NERVA, while the support element is the ‘tie tube' design similar to those in the Pewee. The core contains seven reinsertable safety rods to maintain subcriticality in the event of water immersion or core compaction and for permanent shutdown. The NDR core is supported radially by the NERVA lateral support design, and axially supported by the NERVA-Rover type cold and hot end support designs. The radial reflector is made of beryllium as in NERVA-Rover; it contains 12 control drums of the type used in those reactors. With the low power density, short full power lifetime, and the near-thermal spectrum, the neutron fluence and gamma dose at the specified dose plane are well below the limits specified. Therefore, the baseline NDR contains no internal shielding. The pressure vessel is made of Ti-5Al-2.5Sn. This material was selected for its high temperature capability and high specific strength. It has been used in various aerospace applications and in the Rover hydrogen turbopump. The Fuel Element The NDR fuel element is shown in Fig. 2. It is a hexagonal-shaped graphite fuel element, 1.905 cm across flats and 75 cm long. Each fuel element contains seven coolant channels that are coextruded. The fuel, in the form of UC2 kernels, is contained within TRISO type fuel beads, dispersed in the graphite matrix. All the external surfaces of the fuel elements (coolant channel and external surfaces) are coated with ZrC. At the low peak fuel temperature (1200 K) and the low fuel burnup
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