Activation and Inventory Codes
Activation codes evolve the nuclide inventory of materials under neutron irradiation, predicting radioactivity, decay heat, and waste classification over time.
Neutrons transmute materials
Neutrons do not only deposit energy; they transmute the atoms of structural and functional materials into new isotopes, many radioactive. An activation code takes the neutron flux and spectrum from a transport calculation and evolves the material's nuclide inventory through the chains of activation and decay reactions.
The Bateman equations
The inventory evolves according to the Bateman equations, a coupled set of ordinary differential equations describing production and loss of each nuclide by reaction and decay. Codes integrate these stiff equations over irradiation and cooling periods, using activation cross-section and decay-data libraries.
Outputs
- Specific activity of each nuclide versus cooling time
- Decay heat, important for post-shutdown cooling
- Contact dose rates for maintenance planning
- Waste classification and time to reach acceptable handling levels
Coupling to transport
Activation is downstream of neutron transport: the flux spectrum in each material region drives its transmutation. High-fidelity studies map the spectrum spatially and run activation per region, since a soft spectrum deep in a shield produces a different inventory than the hard spectrum facing the plasma.
Design relevance
Activation results guide material selection toward low-activation choices, drive the layout of maintenance and remote-handling, and inform waste management. Because fusion's activation is driven by 14 MeV neutrons, materials chosen to minimize long-lived isotopes are a recognized advantage of the fuel cycle.
For a D-T machine like the Hyperion breeder, activation modeling is part of assessing the lifecycle of components exposed to the neutron flux, always tied to the specific spectrum and irradiation history assumed.