Control In the prismatic designs, control rods are inserted in holes cut in the graphite blocks that make up the core. The VHTR will be controlled like current PBMR designs if it utilizes a pebble bed core, the control rods will be inserted in the surrounding graphite reflector. Control can also be attained by adding pebbles containing neutron absorbers. Materials challenges The high-temperature, high-neutron dose, and, if using a molten salt coolant, the corrosive environment, of the VHTR require materials that exceed the limitations of current nuclear reactors. In a study of Generation IV reactors in general (of which there are numerous designs, including the VHTR), Murty and Charit suggest that materials that have high dimensional stability, either with or without stress, maintain their tensile strength, ductility, creep resistance, etc. after aging, and are corrosion resistant are primary candidates for use in VHTRs. Some materials suggested include nickel-base superalloys, silicon carbide, specific grades of graphite, high-chromium steels, and refractory alloys. Further research is being conducted at US national laboratories as to which specific issues must be addressed in the Generation IV VHTR prior to construction. Safety features and other benefits The design takes advantage of the inherent safety characteristics of a helium-cooled, graphite-moderated core with specific design optimizations. The graphite has large thermal inertia and the helium coolant is single phase, inert, and has no reactivity effects. The core is composed of graphite, has a high heat capacity and structural stability even at high temperatures. The fuel is coated uranium-oxycarbide which permits high burn-up (approaching 200 GWd/t) and retains fission products. The high average core-exit temperature of the VHTR (1,000 °C) permits emissions-free production of high grade process heat. Reactor is designed for 60 years of service. See also List of nuclear reactors Nuclear reactor physics Time-dependent neutronics and temperatures References External links Idaho National Lab VHTR Fact Sheet (from the year 2002) Generation IV International Forum VHTR website Pebble Bed Advanced High Temperature Reactor (PB-AHTR) IAEA HTGR Knowledge Base ORNL NGNP page INL Thermal-Hydraulic Analyses of the LS-VHTR IFNEC slides from 2014 about Areva's SC-HTGR: The Office of Nuclear Energy reports to the IAEA in April 2014: Nuclear power reactor types Graphite moderated reactors