The NDR is designed to operate at temperatures substantially lower then those of NERVA or Rover (about 1/2 of the temperatures for the propulsion reactors). As a consequence, conduction losses from the fuel elements to the support elements are much smaller. This permits a smaller coolant flow cross-sectional area in the tie elements and the accommodation of more ZrH in the core than was possible in the Pewee. Redundant, Diverse, Engineered Safety Features The NDR is provided with redundant and diverse (independent and different) engineered nuclear safety features as the principal means of reactor protection. Seven in-core, reinsertable rods are used for subcriticality, permanent shutdown, as well as the independent and diverse means for reactor shutdown (in addition to the radial control drums). Decay heat removal is carried out through hydrogen flow using two separate and independent sources of hydrogen and through two separate and independent heat flow paths. The principle source of hydrogen is the normal inventory for core cooling. This flows first through the radial reflector and then through the core. The emergency source of hydrogen is held in an auxiliary tank that is filled prior to reactor startup. In the event of loss of primary hydrogen flow or loss of pressure in the reactor, a check valve opens passively and automatically and hydrogen from the auxiliary tank flows first through the tie tubes and then through the core. Hydrogen flow is induced by blowdown through the initially pressurized tank. Normally, reactor control and shutdown are carried out using the control drums, which consist of two separate and independent sets of six. Each set is ganged together and controlled separately and independently. In addition, the in-core safety rods can be used, as the diverse method, to shut down the reactor. Redundant, Inherent Passive Nuclear Safety Capabilities In addition to the engineered safety features, the baseline NDR has a number of unique passive nuclear safety features that are inherent to the reactor. They provide additional redundancy and a fail-safe reactor. In the baseline NDR, the hydrogen coolant contributes significant reactivity to the core. As a consequence, a loss-of-flow or loss-of-coolant accident would cause failsafe, self shutdown of the reactor. In the event of an accident in which the core heats up inadvertently, hydrogen loss from the ZrH moderator will take place. Hydrogen depletion will increase rapidly with increasing temperature. The resultant loss of reactivity will cause the reactor to be permanently shut down. The relatively large reactor mass, the low core power density, and the low full power operational duration result in sufficiently low decay heat generation and decay energy that the baseline NDR has the capability of absorbing the decay heat without exceeding temperatures that would cause excessive hydrogen loss from the moderator and loss of strength in the structural components. Alternatively, the NDR can be designed such that the decay heat can be used to drive off the hydrogen in the moderator (by not providing any active cooling) to effect passive, permanent, irreversible shut down. The fissile fuel is proliferation resistant from two important standpoints: the low inventory in a highly dispersed form discourages its diversion, and the recovery of fuel from TRISO beads is a difficult and costly process.
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