The Maximum Credible Accident
Even the bounding beyond-design accident releases only a fraction of a small tritium inventory, with limited off-site consequence.
Every safety case must confront its worst case. For a fusion plant the maximum credible accident is not a core melt or a criticality excursion — those cannot occur — but a release of part of the site's tritium inventory following a combination of failures. Because that inventory is small and tritium disperses and clears quickly, the bounding consequence is limited.
Constructing the bound
- Start from the total site tritium inventory — the physical ceiling on any release.
- Assume a severe, low-probability combination of barrier failures.
- Credit only conservative dispersion and no favorable weather.
- Compute off-site dose — dominated by tritium as HTO.
The analysis is tractable because the source term is dominated by a single, well-understood radionuclide with a short biological half-life and no persistent environmental accumulation. There is no volatile fission-product inventory, no actinides, and no long-term land contamination pathway of the kind that drives large fission evacuation zones.
The consequence
The result is a bounded off-site dose that supports right-sized, largely on-site emergency planning rather than large evacuation zones. This is the payoff of the small inventory and the absence of a chain reaction. See the source term.
A worst case set by grams of a fast-clearing isotope is a fundamentally different problem from a worst case set by a reactor's fuel core.