15.41. The reactor core consists of stacks of hexagonal graphite prisms with holes for fuel rods, for coolant flow, and in some cases, for boron carbide control rods. As shown in Fig. 15.4, there are 10 prisms in a stacked column, arranged in an annulus. Unfueled graphite blocks serve as inner and outer radial and upper and lower axial reflectors. This reflected annular core arrangement is needed to permit passive decay heat removal while maintaining the maximum fuel temperature below 1600°C during a so — called conduction cooldown event during which there is no circulation of coolant. The core height is limited to 7.9 m to allow a maximum power rating while assuring axial power stability to xenon transients.

15.42. A set of 24 control rods located in channels in the inner ring of the annular reflector blocks are used for normal control and for emergency

shutdown. A set of six rods in the outer ring of the central reflector is inserted only during cold shutdown. A reserve shutdown capability is pro­vided by boronated graphite pellets, contained in hoppers above the core, which can be dropped into channels in selected core fuel blocks, if needed.