There are two conflicting official explanations of what caused the accident.
The first, published in August 1986, places the blame for the accident squarely on the shoulders of the plant's operators.
The second, released in 1991, attributed the accident to design flaws in the Pressure Tube Graphite Moderated Boiling Water Reactor (RBMK), particularly the design of the control rods. Both sides of the inquiry were lobbied on multiple fronts, including reactor designers, Chernobyl staff and the government.
Expanded:
Environmental impact of Chernobyl:
In the aftermath of the accident, the quarantine zone was turned into a haven for some of the wildlife.
While animals also suffered from radiation exposure, the damage to them was so mild compared to that to humans that the accident was a blessing to them. Animals in the quarantine zone, such as rats, have adapted to the radiation and live about the same life span as rats in areas not affected by radiation.
The following animals have reappeared or been introduced in the quarantine zone: bobcats, owls, great egrets, swans, a suspected bear, European bison, Mongolian wildebeests, badgers, beavers, boars, deer, elk, foxes, rabbits, otters, raccoons, wolves, waterfowl, gray and blue titmice, black grouse, black storks, cranes, and white-tailed eagles.
The Chernobyl Forum report came out in September 2005, when the United Nations, the International Atomic Energy Agency, the World Health Organization, the United Nations Development Program, the governments of Ukraine and Belarus, and other U.N. groups, worked together to complete an overall report on the nuclear accident.
The report put the death toll*** at 4,000, with the World Health Organization including 47 disaster relief workers who died of radiation and nine children who died of thyroid cancer. The UN released the WHO results in April 2006, and perhaps another 5,000 or so victims died in areas of radioactive fallout (including Ukraine, Belarus, and Russia, among others). So the total comes to about 9,000 victims.
References:
Baidu Encyclopedia - Chernobyl Accident