The TOPAZ reactor (Figure 2.5) incorporates a zirconium hydride moderator between the fuel elements to reduce critical mass requirements. The presence of a zirconium hydride moderator limits coolant temperatures to about 900 K. Because the thermal neutron cross-section of natural tungsten is too high to permit its use in a moderated reactor, tungsten-184 would have to be used for emitter fabrication. Except for this substitution, the required thermionic fuel element design is virtually identical to the thermionic fuel element for the all-thermionic fuel element concept. In the driver fuel concept (Figure 2.6), auxiliary non-thermionic fuel elements are used to achieve criticality. The concept uses SNAP-reactor-type fuel elements (zirconium hydride with 15 w/o uranium loading) as the driver fuel elements. Nuclear heat produced in the driver fuel is not utilized for electrical power; consequently this approach is less efficient than the other two thermionic fuel element concepts. The use of SNAP fuel also limits coolant temperatures to about 900 K.
RkJQdWJsaXNoZXIy MTU5NjU0Mg==