Defense in Depth

Independent barriers and layered systems mean no single failure can produce a significant release.

Defense in depth is the practice of stacking independent barriers and provisions so that safety does not rest on any one of them. Kronos applies it around a hazard that is already small — grams of fuel, no chain reaction — which makes each layer's job easier and the overall margin large.

The layers

• Physics layer: self-limiting burn and no chain reaction — the reaction ends on any upset.
• Fuel-cycle containment: tritium held in sealed, permeation-controlled systems with monitoring.
• Vacuum vessel: the primary structural boundary around the plasma.
• Building confinement: ventilation and detritiation to capture any release.
• Administrative: procedures, monitoring, and right-sized emergency provisions.

The barriers are chosen to be independent so that a common cause is unlikely to defeat several at once. A tritium leak from a process line, for example, is caught by secondary containment and building detritiation before it can reach the public — and none of that is required to stop the fusion reaction, which has already ended.

Layered against a bounded source term

Because the maximum credible accident releases only a limited amount of tritium and no large fission-product inventory, the layers do not need to defend against a catastrophic source. Defense in depth here is margin on top of an already-small hazard, not a desperate last line against a large one.

See the confinement barriers and permeation control for the barrier detail.