Fluoride-cooled high-temperature reactors (FHRs) combine liquid fluoride salt coolants (like MSRs), pool type cores and vessel configurations as with many SFR designs, and coated particle fuels as with HTR. Fluoride salts offer better heat transport characteristics compared to helium, enabling power densities 4 to 8 times higher and power levels over 4000 MWft . AHTRs and PB-AHTRs both use coated particle fuel embedded within a graphitic matrix cooled by liquid fluoride salt (Forsberg et al., 2008). The higher flux level in AHTR and PB-AHTR systems means more frequent reflector graphite replacement, which can be avoided by the use of inner and outer pebble blankets to reduce the radiation damage to the fixed reflector graphite (Renault et al., 2009). Two examples of FHR systems are:

1 The 1200 MWe advanced high-temperature reactor (AHTR), which employs prismatic fuel elements

2 The 410MWe pebble-bed advanced high-temperature reactor (PB-AHTR)

A commercial-scale PB-AHTR has been reported operating at ~900 MWth (Bardet et al., 2008). The characteristics of the fuel cycle are more or less the same as the VHTR, while intermediate heat transport, power conversion and balance of plant are similar to an MSR.