Images of the damaged Japanese nuclear reactor and tsunami wreckage.

From a distance, Japanese reactors are similar to American ones, on which they are based, said Donald R. Olander, a professor of nuclear engineering at the University of California at Berkeley.

Japan was reported on Nov. 24 to have refused a Russian offer to sell low-grade uranium from dismantled nuclear weapons for use in Japanese nuclear reactors.

The problems began to emerge late last year when inspection documents accompanying a shipment of nuclear fuel pellets bound for Japanese reactors were found to have been falsified.

In April all Japanese nuclear reactors were closed.

The United Nations predicted that the initial radiation plume from the stricken Japanese reactors would reach the United States by 18 March.

The detector, called Kamland - short for Kamioka Liquid-scintillator Anti-Neutrino Detector - was built in 1997 to detect antineutrinos, the antimatter opposites of neutrinos, emitted by Japanese nuclear reactors.

Does the sea water being sprayed on the damaged Japanese nuclear reactors become toxic and does it re-enter the ocean and endanger the aquatic life?

The Bush Administration is reviewing the security plans for the first sea shipments, which it has the right to approve or disapprove because it supplied the original uranium fuel used in the Japanese reactors.

British government inspectors reported in February that for months, employees who were supposed to be performing a final measurement of fuel pellets bound for Japanese reactors were instead copying numbers from previous shipments.