The Mark 1 Pebble Bed FHR (Mk1 PB-FHR) pre-conceptual design focused on questions related to coupling FHRs to air Brayton power conversion. The Mk1 PB-FHR uses a modified GE 7FB gas turbine in a conventional combined cycle configuration. This selection determined the reactor thermal power, 236 MWth, as well as the other key design parameters listed below.
| Mk1 Reactor Parameters | |
| Thermal power | 236 MWth |
| Core inlet temperature | 600°C |
| Core bulk-average outlet temperature | 700°C |
| Primary coolant mass flow rate (100% power) | 976 kg/sec |
| Primary coolant volumetric flow rate (100% power) | 0.54 m3/sec |
| Mk1 Power Conversion Parameters | |
| Gas Turbine (GT) model number | GE 7FB |
| Nominal ambient temperature | 15°C |
| Elevation | Sea level |
| Compression ratio | 18.52 |
| Compressor outlet pressure | 18.58 bar |
| Compressor outlet temperature | 418.7°C |
| Compressor outlet mass flow | 418.5 kg/sec |
| Coiled tube air heater outlet temperature | 670°C |
| Base-load net electrical power output | 100 MWe |
| Base-load thermal efficiency | 42.5 % |
| Co-firing turbine inlet temperature | 1065°C |
| Co-firing net electrical power output | 241.8 MWe |
| Co-firing efficiency (gas-to-peak-power)† | 66.4 % |
† The co-firing efficiency is the ratio of the increased power produced (total minus base load) during peaking, to the energy input from combustion of natural gas, and represents the efficiency with which the natural gas combustion energy is converted into electricity.