Just how dangerous is the situation at Japan's crippled Fukushima Daiichi nuclear plant? Very, according to U.S. Senator Ron Wyden, a senior member of the Senate's energy committee who toured the plant earlier this month.

Another big earthquake or tsunami could send Fukushima Daiichi's fragile reactor buildings tumbling down, resulting in "an even greater release of radiation than the initial accident," Mr. Wyden warned in a Monday letter to Japanese Ambassador to the U.S. Ichiro Fujisaki.

In...