On April 26, 1986, the world had its first full-scale nuclear disaster. On that day, the No. 4 reactor at the Chernobyl nuclear-power plant in Ukraine exploded, sending a huge cloud of radioactive dust across Europe. Earlier this month, Ukraine's government shut down the last working reactor at the Chernobyl complex. That does not end this grim chapter in nuclear history: The impact of Chernobyl is still being felt, the lessons are still being learned.

A design flaw caused a huge power surge that blew the top off the Chernobyl reactor and released the radioactivity, estimated to be 500 times bigger than that created by the atomic bomb dropped on Hiroshima. The United Nations estimates that 9 million people were directly or indirectly affected by the accident. The then Soviet government put the death toll at 32 people, 30 of whom died within three months of the disaster. Ukraine has reported more than 4,000 radiation-related deaths among cleanup workers and the current Russian government says that more than 55,000 of the 860,000 workers who dealt with the aftermath of the accident have died, perhaps as many as 6,000 of them prematurely. More than 115,000 people were permanently evacuated in the first days of the accident.

The living suffer as well. The effects are most visible in the sharp rise in thyroid-cancer rates among children in affected areas. Youngsters in the parts of Belarus, Russia and Ukraine that received the heaviest fallout have thyroid-cancer rates at least 10 times the world average. Vast swaths of land have been left uncultivable. Scientists worry that genetic mutations in crops could continue for generations. Greenpeace estimates that the total cost of the accident could reach $300 billion.

Ukraine's president, Mr. Leonid Kuchma, promised the G7 nations five years ago that his government would shut down the complex. That has proven to be a costly promise to keep. About 2,000 of the plant's workers — nearly a fifth — will lose their jobs, and there is little chance that they will find new employment. Closing the plant will deprive the country of 5 percent of its electricity supply, putting more pressure on an already overburdened power grid just as winter sets in.

Ukraine has little choice. The country's economy is hurting, having contracted 60 percent in eight years. This year, Ukraine will register its first economic growth since it declared its independence. The International Monetary Fund froze aid to the country after charges were made that funds had been misused. The freeze was lifted this week.

Ukraine needs the money. A new shell for the damaged reactor will cost more than $750 million. Ukraine also wants help to build new reactors to compensate for the energy lost by the shutdowns. The European Bank for Reconstruction and Development approved a loan of $215 million earlier this month, on the condition that the entire complex finally be shut down.

The shutdown does not end the dangers at the complex. After the accident, reactor 4 was wrapped in a concrete "sarcophagus," or shell, that contains an estimated 170 tons of radioactive material. Experts warn that 10 percent of the shell's surface is cracked and the entire structure could collapse, which would create a new catastrophe. Although the material is being monitored, no one knows if it is stable. Most of the removal work will require robots that will have to be designed especially for work in highly radioactive environments. The entire cleanup could take another century.

Yet the events at Chernobyl must not fade from memory even after 100 years. It is fashionable to dismiss the events at Chernobyl as the product of a particular mind-set at a particular moment in human history — the Soviet mentality was notorious for its deadly combination of inefficiency and secrecy — but the truth is that the history of the nuclear industry around the world is replete with assurances that accidents could not happen, which are invariably followed by accidents, coverups, embarrassing revelations and promises of "never again."

Chernobyl is the reminder that that promise cannot be kept. And that means we must confront the reality of nuclear energy. Honesty about what is involved — the costs, the risks, the price of alternatives — is essential. Nuclear energy may be the only way for Japan to achieve energy self-sufficiency or the only means for its people to continue the lifestyles to which we are accustomed. But those policies are choices; they are not fixed in stone. We can give up on self-sufficiency or choose less consumption-oriented lives. Chernobyl obliges us to face those choices squarely.