In that accident, the lack of a containment vessel surrounding the reactor meant radioactive materials from the accident were spread far and wide.

WSJ: Japan to Take 40 Years to Dismantle Fukushima Nuclear Plant

Tuesday's explosion at the No. 2 reactor in the Fukushima complex, 150 miles north of Tokyo, for the first time raised the possibility that the key containment structure of the unit, which protects the reactor vessel and keeps dangerous radioactive materials from leaking out, had been damaged.

WSJ: Japan's Nuclear Crisis Escalates

The neutrons are created in a fission reactor, and slowed to incredibly low speeds by materials known as moderators.

BBC: Neutrons could test Newton's gravity and string theory