The core is where fission happens. It is small, about 1.2 meters across and 1 meter tall, and it makes about 30 megawatts of heat. These parts hold the fuel, reflect neutrons back in, set the power, and stop the reaction when needed.
The core is the compact region where fission takes place. Fuel assemblies sit in a tight hexagonal lattice, and the reflector surrounds them.
| Thermal power | ≈ 30 MWt |
|---|---|
| Active height | ≈ 1.0 m |
| Diameter | ≈ 1.2 m |
| Refueling | Whole-core swap every 5–10 years |
Each assembly is a hexagonal steel duct that holds a bundle of fuel pins. Sodium flows upward through the duct and carries the heat away.
| Count | 61 |
|---|---|
| Duct material | HT-9 ferritic steel |
| Lattice pitch | ≈ 135 mm |
| Pins per assembly | 91, wire-wrapped |
Fuel pins are sealed steel tubes filled with uranium fuel pellets. A wire wrapped around each pin keeps the pins apart and lets sodium flow between them.
| Fuel form | U-Zr metal or UO₂, HALEU (< 20% U-235) |
|---|---|
| Cladding | HT-9 steel |
| Pin diameter | ≈ 9 mm |
| Peak cladding temp. | ≈ 650 °C |
The reflector ring surrounds the core and bounces escaping neutrons back in. It lets a small core sustain a chain reaction with less fuel.
| Material | Beryllium oxide or graphite in steel cans |
|---|---|
| Radial thickness | ≈ 280 mm |
| Height | ≈ 1.1 m |
| Also houses | 12 control drums |
Control drums are rotating cylinders inside the reflector. One face is a neutron absorber: turned toward the core it lowers power, turned away it raises power.
| Count | 12 |
|---|---|
| Absorber | Boron carbide (B₄C) sector, 120° |
| Diameter | ≈ 220 mm |
| Drive | Stepper motor, 0–180° rotation |
Shutdown rods drop into the core to stop the chain reaction. Gravity inserts them when the holding magnets lose power, so the system needs no operator action.
| Count | 3 |
|---|---|
| Absorber | Boron carbide (B₄C) |
| Insertion time | < 2 s |
| Hold | Electromagnetic latch, fail-safe |