Nuclear power was beginning to look like a panacea — a way to lessen our dependence on oil, make our energy supply more self-sufficient and significantly mitigate global warming, all at the same time. Now it looks more like a bargain with the devil.

I wish this were not so. In recent years, some of the nation’s most respected environmentalists — including Stewart Brand, founder of the Whole Earth Catalog — have come to champion nuclear power. But as Japanese engineers struggle frantically to keep calamity from escalating into catastrophe, we cannot ignore the fact that nuclear fission is an inherently and uniquely toxic technology.

The cascading sequence of system failures, partial meltdowns and hydrogen explosions at the Fukushima Daiichi nuclear power plant was touched off by a once-in-a-lifetime event: the most powerful earthquake in Japan’s recorded history, which triggered a tsunami of unimaginable destructive force. It is also true that the Fukushima reactors are of an older design, and that it is possible to engineer nuclear plants that would never suffer similar breakdowns.

But it is also true that there is no such thing as a fail-safe system. Stuff happens.

The Earth is alive with tectonic movement, volcanism, violent weather. We try our best to predict these phenomena, but our best calculations are probabilistic and thus imprecise. We have computers that are as close to infallible as we can imagine, but the data they produce must ultimately be interpreted by human intelligence. When a crisis does occur, experts must make quick decisions under enormous pressure; usually they’re right, sometimes they’re wrong.

The problem with nuclear fission is that the stakes are unimaginably high. We can engineer nuclear power plants so that the chance of a Chernobyl-style disaster is almost nil. But we can’t eliminate it completely — nor can we envision every other kind of potential disaster. And where fission reactors are concerned, the worst-case scenario is so dreadful as to be unthinkable.

Engineers at the Fukushima plant are struggling to avert a wholesale release of deadly radiation, which is the inherent risk of any fission reactor. In the Chernobyl incident, a cloud of radioactive smoke and steam spread contamination across hundreds of square miles; even after 25 years, a 20-mile radius around the ruined plant remains off-limits and uninhabitable. Studies have estimated that the release of radioactivity from Chernobyl has caused at least 6,000 excess cases of thyroid cancer, and scientists expect more cancers to develop in the years to come.

It seems unlikely that the Fukushima crisis will turn into another Chernobyl, if only because there is a good chance that prevailing winds would blow any radioactive cloud out to sea. Japanese authorities seem to be making all the right decisions. Yet even in a nation with safety standards and technological acumen that are second to none, look at what they’re up against — and how little margin for error they have to work with.

At first, the focus was on the Unit 1 reactor and the struggle to keep the nuclear fuel rods immersed in water — which is necessary, at all times, to avoid a full meltdown and a catastrophic release of radiation. Pumping sea water into the reactor vessel seemed to stabilize the situation, despite a hydrogen explosion — indicating a partial meltdown — that blew the roof off the reactor’s outer containment building.

But then, attention shifted to Unit 3, which may have had a worse partial meltdown; it, too, experienced a hydrogen explosion. Officials said they believed they were stabilizing that reactor but acknowledged that it was hard to be sure. Meanwhile, what could be the most crucial failure of all was happening in Unit 2, whose fuel rods were fully exposed. Scientists had no immediate way of knowing how much of that reactor’s fuel had melted — or what the consequences might be.

The best-case scenario is that Japanese engineers will eventually get the plant under control. Then, I suppose, it will be possible to conclude that ultimately the system worked. As President Obama and Congress move forward with a new generation of nuclear plants, designs will be vetted and perhaps altered. We will be confident that we have taken the lessons of Fukushima into account.

And we will be fooling ourselves, because the one inescapable lesson of Fukushima is that improbable does not mean impossible. Unlikely failures can combine to bring any nuclear fission reactor to the brink of disaster. It can happen here.

Eugene Robinson’s e-mail address is eugenerobinson(at)washpost.com.