Delayed Neutrons & Reactor Period

Only about 0.2–0.7% of the neutrons from fission are emitted late — seconds after the fission event, when a fission-product fragment itself decays. This tiny delayed fraction β is what makes a nuclear reactor controllable at all: without it, a chain reaction at k slightly above 1 would double on the prompt-neutron generation time, microseconds to milliseconds, far faster than any control rod can move. This simulator solves the one-delayed-group point-kinetics equations in real time — set a step reactivity in dollars (ρ/β), swap the delayed fraction between a U-235-like and Pu-239-like fuel, and watch the population make its fast "prompt jump" and then ride the slow precursor-decay curve, right up to the moment reactivity crosses 1$ and the reactor goes prompt critical.