Quick crash course: there's two major kinds of radiation, actual particles (alpha and beta radiation, which are helium nuclei and electrons, respectively) and h

Quick crash course: there's two major kinds of radiation, actual particles (alpha and beta radiation, which are helium nuclei and electrons, respectively) and high-energy electromagnetic waves (gamma radiation: like x-rays on steroids). Radioactive isotopes (such as those coming from Fukushima) tend to emit both, as they decay into non-radioactive elements. So once an atom emits radiation, it's done radiating (unless it's decayed into another radioactive particle, but eventually it'll hit something stable and never radiate ever again). So the isotopes arrive at some rate, and they decay into non-radioactive elements over time. (That's what the "8-day half-life" in the article means: after 8 days, only half of it will be left.) At some point, the Japanese engineers will get everything cleaned up, no more isotopes will arrive, and all isotopes will eventually decay. Even if the Japanese take their sweet time getting things cleaned up, the isotopes are still decaying over time, so they won't accumulate unless the Japanese truly screw up and start releasing increasing amounts of isotopes.