Flux, Fluence, and Dose Rate
Flux is how fast neutrons arrive; fluence is how many arrive in total; together they set how quickly a material can be tested to a target dose.
Rate versus total
Two quantities govern an irradiation. Flux is the rate at which neutrons cross a unit area — how intense the source is. Fluence is the accumulated total over time — flux integrated over the exposure. Damage tracks fluence, but flux determines how long it takes to reach a target dose.
Why flux is precious
A high-flux source reaches a target fluence faster, so it can test more samples to higher doses in a given time. Because fusion-spectrum sources have historically been low-flux or unavailable, achieving fusion-relevant fluences has been slow. A higher-flux domestic source shortens qualification timelines.
- Flux: intensity, sets test throughput.
- Fluence: accumulated total, sets damage.
- Higher flux shortens time to a target dpa.
- Fusion-spectrum flux has been historically limited.
Design-stage figures
Throughput sets program pace
Flux matters at the program level because it governs throughput: how many samples can be driven to a target dose in a usable time. Historically, the scarcity of high-flux fusion-spectrum sources meant that reaching service-relevant doses could take a very long time, which slowed materials programs and forced heavier reliance on modeling. A higher-flux domestic source compresses those campaigns, letting more materials be qualified to higher doses within a program's lifetime. This is why flux, and not only the total fluence eventually achieved, is a figure of real strategic interest for testing capability.
Flux and achievable fluence for the breeder's 14 MeV output are computed design attributes. What throughput the machine actually delivers for testing is a measured, FOAK-era result reported when the machine operates from ~2030.