Neutron Radiation Safety
14 MeV fusion neutrons are shielded during operation, and they vanish the instant the plasma stops — there is no residual neutron field.
D–T fusion in the breeder (Hyperion) releases energetic 14 MeV neutrons, and these are the principal penetrating radiation during operation. They are managed the same way all neutron radiation is: shielding, distance, and access control. The important safety feature is that the neutron field exists only while the plasma burns — stop the plasma and the neutrons cease immediately.
Operational shielding
- Neutron shielding: hydrogen-rich and high-mass materials in the blanket and biological shield.
- Access control: no personnel access to the vault during operation.
- Interlocks: the machine cannot run with access barriers open.
- Streaming control: penetrations designed to prevent neutron streaming paths.
Because the neutron source is the plasma itself, it is not a stored source that persists after shutdown — unlike a sealed radioactive source, it turns off completely. What remains after shutdown is activation of structures, a separate and smaller concern addressed by activation safety and remote maintenance.
The burner difference
The burner (Aegis / MetroVolt) is a D–³He machine with only a 5.44% neutron fraction, so it produces far fewer neutrons and at lower average energy than the breeder. Its shielding requirements are correspondingly lighter, and its activation is much reduced. See burner plug safety.
Neutron safety during operation is a standard shielding-and-interlock problem; the fusion-specific advantage is that the source switches off with the plasma.