WSJ: Two Other Reactors Suffer Serious Damage

TOKYO—Substantial damage to the fuel cores at two additional reactors of Japan's Fukushima Daiichi nuclear complex has taken place, operator Tokyo Electric Power Co. said Sunday, further complicating the already daunting task of bringing them to a safe shutdown while avoiding the release of high levels of radioactivity. The revelation followed an acknowledgment on Thursday that a similar meltdown of the core took place at unit No. 1. <...>

Tepco separately released its analysis on the timeline of the meltdown at unit No. 1. According to the analysis, the reactor core, or the nuclear fuel, was exposed to the air within five hours after the plant was struck by the earthquake. The temperature inside the core reached 2,800 degrees Celsius in six hours, causing the fuel pellets to melt away rapidly.

Within 16 hours, the reactor core melted, dropped to the bottom of the pressure vessel and created a hole there. By then, an operation to pump water into the reactor was under way. This prevented the worst-case scenario, in which the overheating fuel would melt its way through the vessels and discharge large volumes of radiation outside.

The nuclear industry lacks a technical definition for a full meltdown, but the term is generally understood to mean that radioactive fuel has breached containment measures, resulting in a massive release of fuel. http://online.wsj.com/article/SB10001424052748703509104576325110776621604.html