U. S. government engineers helping with the crisis in Japan are warning of fresh threats that could persist indefinitely, and may increase as a result of the very measures being taken to keep the plant stable- a document prepared by the Nuclear Regulatory Commission.
1. Mounting stresses on containment structures as they fill with radioactive cooling water.
2. Possibility of explosions inside containment structures due to release of hydrogen and oxygen from seawater pumped into the reactors.
3. Flow of fresh water meant to cool the nuclear cores are impeded by semimolten fuel rods and salt buildup.
4. If fuel continues to heat and melt because of ineffective cooling, that could leave a radioactive mass that could stay molten for an extended period.

The document appears to rely largely on date shared with American experts by the Japanese.

The document suggests that fragments or particles of nuclear fuel from spent fuel pools were blown up to one mile from the units and a piece of highly radioactive material that fell between two reactors had to be bulldozed over to protect workers at the site.

David A. Lochbaum, who now directs the nuclear safety project at the Union of Concerned Scientists, said the welter of problems revealed in the document made a successful outcome even more uncertain.

Steps recommended by the nuclear commission include injecting nitrogen, inert gas, into containment structures in an attempt to purge them of hydrogen and oxygen, which could combine to produce explosions.
TEPCO is taking steps to inject nitrogen into one of the containment vessels.

The document also recommends adding boron to cooling water to prevent the cores from restarting the nuclear reaction, a process known as criticality. The engineers who wrote the report did not consider this likely, however.

The document, prepared for the commission's Reactor Safety Team, which is assisting the Japanese government and TEPCO, is based on numerous Japanese and American organizations (listed).

The document contains detailed assessments of each reactor.
The article contains some of these details, and is worth reading.

"The U. S. Sees Array of New Threats at Japan's Nuclear Plant," James Glanz and William Broad, New York Times, April 5, 2011.

The U. S. experts working with the Japanese update the report regularly; little substantial has been added since March 26.