| 1/00 | Reactors |
| 1/01 | Reactors General details not provided for in groups G21C 3/00-G21C 19/00 |
| 1/02 | Reactors Fast fission reactors, i.e. reactors not using a moderator |
| 1/03 | Reactors Fast fission reactors, i.e. reactors not using a moderator cooled by a coolant not essentially pressurised, e.g. pool-type reactors |
| 1/04 | Reactors Thermal reactors |
| 1/06 | Reactors Thermal reactors Heterogeneous reactors, i.e. in which fuel and moderator are separated |
| 1/07 | Reactors Thermal reactors Heterogeneous reactors, i.e. in which fuel and moderator are separated Pebble-bed reactors; Reactors with granular fuel |
| 1/08 | Reactors Thermal reactors Heterogeneous reactors, i.e. in which fuel and moderator are separated moderator being highly pressurised, e.g. boiling-water reactor, integral-superheat reactor, pressurised-water reactor (G21C 1/22 takes precedence) |
| 1/09 | Reactors Thermal reactors Heterogeneous reactors, i.e. in which fuel and moderator are separated moderator being highly pressurised, e.g. boiling-water reactor, integral-superheat reactor, pressurised-water reactor (G21C 1/22 takes precedence) Pressure regulating arrangements, i.e. pressurisers |
| 1/10 | Reactors Thermal reactors Heterogeneous reactors, i.e. in which fuel and moderator are separated moderator being highly pressurised, e.g. boiling-water reactor, integral-superheat reactor, pressurised-water reactor (G21C 1/22 takes precedence) moderator and coolant being different or separated |
| 1/12 | Reactors Thermal reactors Heterogeneous reactors, i.e. in which fuel and moderator are separated moderator being highly pressurised, e.g. boiling-water reactor, integral-superheat reactor, pressurised-water reactor (G21C 1/22 takes precedence) moderator and coolant being different or separated moderator being solid, e.g. Magnox reactor |
| 1/14 | Reactors Thermal reactors Heterogeneous reactors, i.e. in which fuel and moderator are separated moderator being substantially not pressurised, e.g. swimming-pool reactor (G21C 1/22 takes precedence) |
| 1/16 | Reactors Thermal reactors Heterogeneous reactors, i.e. in which fuel and moderator are separated moderator being substantially not pressurised, e.g. swimming-pool reactor (G21C 1/22 takes precedence) moderator and coolant being different or separated, e.g. sodium-graphite reactor |
| 1/18 | Reactors Thermal reactors Heterogeneous reactors, i.e. in which fuel and moderator are separated moderator being substantially not pressurised, e.g. swimming-pool reactor (G21C 1/22 takes precedence) moderator and coolant being different or separated, e.g. sodium-graphite reactor coolant being pressurised |
| 1/20 | Reactors Thermal reactors Heterogeneous reactors, i.e. in which fuel and moderator are separated moderator being substantially not pressurised, e.g. swimming-pool reactor (G21C 1/22 takes precedence) moderator and coolant being different or separated, e.g. sodium-graphite reactor coolant being pressurised moderator being liquid, e.g. pressure-tube reactor |
| 1/22 | Reactors Thermal reactors Heterogeneous reactors, i.e. in which fuel and moderator are separated using liquid or gaseous fuel |
| 1/24 | Reactors Thermal reactors Homogeneous reactors, i.e. in which fuel and moderator present an effectively homogeneous medium to the neutrons |
| 1/26 | Reactors Thermal reactors Homogeneous reactors, i.e. in which fuel and moderator present an effectively homogeneous medium to the neutrons Single-region reactors |
| 1/28 | Reactors Thermal reactors Homogeneous reactors, i.e. in which fuel and moderator present an effectively homogeneous medium to the neutrons Two-region reactors |
| 1/30 | Reactors Subcritical reactors |