Integral reactor systems -- including the reactor vessel itself, six steam generators, five primary coolant pumps, 12 isolation valves and more -- were stored outside for up to 18 months, exposed to highly corrosive tropical sea air and weather.

CENTERFORSECURITYPOLICY: A Chernobyl reminder just off our shores

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

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

The drywell is a large steel pressure vessel that looks like a giant upside-down pear and holds the reactor and primary pumps, and the wetwell is a large toroidal vessel that looks like a donut.

FORBES: Explainer: What Caused The Incident At Fukushima-Daiichi

From Kyodo News: The containment vessel for No. 3 reactor may be damaged.

FORBES: Tracking Radioactive Drift from Fukushima Nuclear Plant

The building surrounding the vessel containing the No. 1 reactor exploded on March 12.

FORBES: New Explosion Destroys Second Fukushima Nuclear Reactor Building

In addition, the modular design could make it more difficult to inspect and maintain key components, many of which would be located within the pressure vessel at the heart of the reactor, he said.

CNN: U.S. backs project for small nuclear reactors

In late March, Tepco said readings of airborne radiation inside the containment vessel at the No. 2 reactor had surged to a deadly 79 sieverts, the highest level since the crisis began March 11 last year.

WSJ: New Leak at Japan Reactor Threatens Ocean

In 2002, Tepco admitted to the Nuclear and Industrial Safety Agency that it had falsified the results of safety tests on the containment vessel of the No. 1 reactor, which is now one of three reactors that workers are struggling to keep from overheating.

WSJ: Nuclear Issues Put Focus on Japan Regulators