Post-Shutdown Radiation Fields
After shutdown the only radiation comes from activated structure and decays quickly, allowing controlled access on human timescales.
When the plasma stops, the penetrating neutron and gamma fields from fusion stop with it. What remains is the gamma radiation from activated structures as their induced radioisotopes decay. This residual field is what sets how soon and how workers can approach the machine.
Character of the residual field
- Source is short- and medium-lived activation products in the vessel and blanket.
- Field intensity falls rapidly — hours to weeks — as short-lived isotopes decay.
- No persistent fission-product gamma sources exist — there is no fuel core.
- Access is staged: remote handling first, then hands-on after decay.
The staged approach pairs with remote maintenance: the highest-flux components near the first wall are serviced remotely, while lower-activation regions become accessible to workers after a decay period, all under the ALARA program. Radiation surveys confirm fields before any access change.
Decay heat travels with it
The same activated inventory produces the small post-shutdown decay heat, which conduction and radiation remove passively — there is no cliff requiring active cooling. See decay heat and passive cooling. The burner's low 5.44% neutron fraction makes its residual field lighter still.
In practice the post-shutdown radiological picture is dominated by short-lived activity that fades, not by a long-lived source that lingers.