Gas reactors have been operating successfully in the UK over many years. High — temperature gas reactors has been operated in the UK, US and Germany and new smaller plants are in operation in China and Japan. There is a revived interest in HTRs and in particular the South African Pebble Bed Modular Reactor (PBMR) (Clegg, 2002; http:// www. bnfl. com/website. nsf/researchmenu. htm; Hittner, 2002). R&D activities can, there­fore, build on considerable previous experience.

The very-high-temperature reactor (VHTR) system is another one of the thermal reactor types under consideration in the Gen IV-A programme (Sinco, 2003). It would operate at very high core outlet temperatures, > 1000°C and have very high efficiency compared with current generation plant. It could be used for electricity generation or hydrogen production using water cracking technology (Overview of Generation IV Roadmap). It could also be used in the process heat and chemical industries. A timescale of 2020-2030 is envisaged for commercial deployment of these systems.