Deep Dive: Neutronics & Activation
How a low-neutron design transforms the shielding, material lifetime, and waste picture — with a high-fidelity OpenMC calculation planned to fix the numbers.
- Neutron fraction
- 7.70–10.20% (operating → banking; retired hot-ion evaluation 5.25–7.05%)
- Wall damage
- ≈22 dpa over 30 FPY at 0.74 dpa/FPY (≤ ~36 dpa qualification)
- Shield
- Compact center-stack shield; explicit fluence budget
- High-fidelity
- OpenMC (S63, planned)
Neutronics — how a reactor's neutrons deposit energy, damage materials, and activate structures — sets the shield thickness, the component lifetimes, and the waste profile. For Kronos it is unusually gentle, because being low-neutron changes every one of those quantities.
The neutron fraction runs 7.70–10.20% along the fuel schedule (operating baseline to ³He-banking leg; the retired hot-ion evaluation carried 5.25–7.05%) — roughly an order of magnitude below D–T. That means wall loading of 0.074–0.089 MW/m², a first wall that accumulates only ~22 dpa over 30 full-power-years (0.74 dpa/FPY, within its ~36-dpa qualification, so it lasts the plant life), and far less activation and waste. It also lets the compact center-stack shield fit in the tight inboard space.
The design carries an explicit neutron-fluence budget for the shielded center stack, and a high-fidelity OpenMC neutronics calculation (S63) is planned to fix the true shield performance and lifetimes — currently based on the reduced-order fluence ledger.