The Pebble Bed Modular Reactor (PBMR) design began in 1996 as a 400 MWt annular core design with an outlet temperature of 900 °C coupled to a direct Brayton power conversion cycle. Currently, it is a 250 MWt cylindrical core design with an outlet temperature of 750 °C coupled to an indirect Rankine cycle with co-generation capability for process heat applications. The fuel is TRISO-coated UO2 particles, which were developed in Germany for the Arbeitsgemeinschaft Versuchsreaktor (AVR) and thorium high-temperature reactor (THTR) test reactors. The particles are dispersed in 6 cm diameter spherical fuel elements randomly packed in a graphite — reflected core region — 360 000 in all. The fuel elements migrate through the core at a rate of approximately 8 cm per day with a total residence time of roughly 100 days per pass and a total of three passes through the core. Reactivity is controlled using six control rods located in the reflector blocks. Eighteen shutdown channels in the reflector can accommodate absorber balls.

Several test facilities were constructed to support the original PBMR design, including a helium test facility, a heat transfer test facility and numerous component — level test facilities. Many of these tests are still valid for the current PBMR-CG design, which intended to be a co-generation plant producing both electricity and process steam. Normally deployed as a twin unit plant, the inlet feedwater and outlet steam from the secondary side of both units would be collected and shared between a turbine-generator set and a secondary heat exchanger for process steam production. A 1:1 split in steam for the turbine and process heat exchanger is expected, yielding a two-unit plant electrical output of 100 MWe. Key parameters for the PBMR-CG design are given in Figure 2.17. [15]

Figure 2.17 PBMR-CG (South Africa) — PBMR Ltd.