As heroic workers and soldiers strive to save stricken Japan from a new horror--radioactive fallout--some truths known for 40 years bear repeating.

An earthquake-and-tsunami zone crowded with 127 million people is an unwise place for 54 reactors. The 1960s design of five Fukushima-I reactors has the smallest safety margin and probably can't contain 90% of meltdowns. The U.S. has 6 identical and 17 very similar plants.

Every currently operating light-water reactor, if deprived of power and cooling water, can melt down. Fukushima had eight-hour battery reserves, but fuel has melted in three reactors. Most U.S. reactors get in trouble after four hours. Some have had shorter blackouts. Much longer ones could happen.

Overheated fuel risks hydrogen or steam explosions that damage equipment and contaminate the whole site--so clustering many reactors together (to save money) can make failure at one reactor cascade to the rest.

Nuclear power is uniquely unforgiving: as Swedish Nobel physicist Hannes Alfvén said, "No acts of God can be permitted." Fallible people have created its half-century history of a few calamities, a steady stream of worrying incidents, and many near-misses. America has been lucky so far. Had Three Mile Island's containment dome not been built double-strength because it was under an airport landing path, it may not have withstood the 1979 accident's hydrogen explosion. In 2002, Ohio's Davis-Besse reactor was luckily caught just before its massive pressure-vessel lid rusted through.

Regulators haven't resolved these or other key safety issues, such as terrorist threats to reactors, lest they disrupt a powerful industry. U.S. regulation is not clearly better than Japanese regulation, nor more transparent: industry-friendly rules bar the American public from meaningful participation. Many presidents' nuclear boosterism also discourages inquiry and dissent.