The Small-Fuel-Inventory Principle

At any instant only grams of fuel are inside the chamber, so the maximum energy the reaction can release is intrinsically small.

The single most consequential safety number for a fusion device is how much fuel is present in the reacting region at once. In Kronos machines that answer is grams. Fuel is supplied continuously and burned continuously; the chamber is never loaded with the fuel for future operation. This caps the energy the reaction can release before it must be re-supplied.

Why so little

Fusion requires low density and extreme temperature, the opposite of a dense fuel load. The breeder (Hyperion) runs a dilute plasma at a plasma current of 9.86 MA and fusion power of 88.7 MW; the burner confines an even thinner plasma between magnetic plugs. Both hold only a tiny mass in the burn region at any moment, replenished by fueling systems that can be shut off instantly.

What the small inventory buys

• A bounded worst case: there is simply not enough fuel present to power a large event.
• Instant shutdown by fuel cutoff — no need to ‘unload’ a core.
• A small tritium inventory in the fuel cycle, contained and monitored separately.

The tritium in the fuel loop and plant is larger than the grams in the plasma, and it is the inventory that safety analysis tracks most closely because tritium is mobile and radioactive. It is held in engineered containment with permeation barriers and continuous monitoring — see tritium inventory and tritium handling.

The principle is general: with grams reacting and no stored reservoir, the reaction's ceiling is set by physics, not by how well an operator keeps it in check.