A confidential Nuclear Regulatory Commission document obtained by the New York Times details serious threats still facing workers at the severely damaged Fukushima nuclear plant, as well as previously undisclosed details about what has happened at the plant so far. Ordinarily, fuel in a nuclear plant cools to the point that it is easy to manage within several days after the reactor is shut down. But difficulty circulating water and damage to the fuel rods suggest keeping them cool could be a challenge for months.

From the Times story:

Among the new threats that were cited in the assessment, dated March 26, are the mounting stresses placed on the containment structures as they fill with radioactive cooling water, making them more vulnerable to rupture in one of the aftershocks rattling the site after the earthquake and tsunami of March 11. The document also cites the possibility of explosions inside the containment structures due to the release of hydrogen and oxygen from seawater pumped into the reactors, and offers new details on how semimolten fuel rods and salt buildup are impeding the flow of fresh water meant to cool the nuclear cores …

Among other problems, the document raises new questions about whether pouring water on nuclear fuel in the absence of functioning cooling systems can be sustained indefinitely. Experts have said the Japanese need to continue to keep the fuel cool for many months until the plant can be stabilized, but there is growing awareness that the risks of pumping water on the fuel present a whole new category of challenges that the nuclear industry is only beginning to comprehend.