Pull the neutron back, release — the shot flies exactly where you aim. Each round deals a fuel mix, a limited neutron supply and an energy goal: every fission banks 200 MeV (its fragments settle in as moderators — and those fission products eat slow neutrons); capture transmutes a nucleus in place (watch the β⁻ decay chain) and the bred isotope stays in play. The reaction map under the meter charts every in-play isotope’s odds across the whole energy axis — in-flight neutrons ride it as white markers. Chain reactions are live (up to four neutrons, and they bounce off each other), hard walls bank shots, and a neutron kept too long β⁻-decays. Clear the goal — or split every heavy nucleus — to advance; “Free play” (top right) opens the sandbox: place fuel from the palette and drag the chamber around.

moderator — light nuclei shed neutron energy each bounce; fission fragments settle in as labeled moderators that can swallow slow neutrons

Round 1 — a forgiving core: three ²³⁵U seeds. Go thermal.