What actually caused the accident at Chernobyl?

As is usually the case in any accident, a number of things combined to cause this one at Chernobyl. Unlike power reactors operating in the U.S. and other nations, the Chernobyl RBMK reactor (which is a graphite rather than a light water system) has a built-in instability that occurs at low power, which is how the reactor was operating at the time of the accident. If some of the cooling water in this reactor converts to steam, the RBMK increases in power. This in turn causes more steam to form, which causes _another_ increase in power. (In Western light water reactors, the power decreases.)

The power increase feature of the RBMK caused a rupture in the cooling system and a large steam explosion occurred. This caused the cooling system to fail and the outer covering (or cladding) of the fuel elements to increase in temperature. The cladding was hot enough to react with the steam, causing hydrogen to form. The hydrogen then caused a second explosion. The release of this energy set the graphite core on fire.

In spite of its dangerous features, the RBMK -- unlike other reactors -- had no actual containment structure to prevent release of contamination. Such a design could not be licensed by the Nuclear Regulatory Commission in this country, nor in most countries of the world. Studies done since the Chernobyl accident have shown that its releases would have been successfully contained by a U.S. type reactor. As a matter of fact, a test of a 37-foot tall scale model of a nuclear plant containment building was made at Sandia National Laboratories in New Mexico in 1987. The test showed that the type of light water containment used at U.S. nuclear plants could withstand more than three times the pressure it was designed for without rupturing or fragmenting.

A second factor in the Chernobyl accident involved a safety experiment being conducted. It required that the reactor be run in a very unusual manner. Because of a series of operational problems, the operators found themselves running the reactor far outside its safety limits. In their efforts to finish the experiment anyway, the operators --in spite of running the reactor under unfamiliar conditions-- turned off seven of the safety systems in the reactor and its control systems. Any one of these seven automatic controls could have prevented the accident had it been on.

All this reflects important differences between Western and Soviet operators and their training. Unlike the Soviets, U.S. reactor operators take continued training in classroom situations and on reactor simulators. Further, operators in Western countries are strictly bound by what are called "technical specifications" which forbid operation of the reactor outside of preset safety limits.

How are U.S. reactors different from the reactor at Chernobyl?

There are many differences, which include not only physical differences but philosophical ones as well. The key differences have already been noted in the previous answer. These physical differences were made worse by the totally different attitude toward safety between the two countries. The U.S. is cautious to the extreme by comparison. It took seven years to restart Three Mile Island Unit 1 following the accident in the Three Mile Island Unit 2 reactor in 1979, the results of which were much less severe than in the Soviet Union. The Soviets restarted their other reactors (of the identical design) at Chernobyl in a matter of a few weeks.

What would be the effect of a Chernobyl-type accident if it occurred in the U.S.?

Because of major differences in technology, a Chernobyl-type accident cannot occur in a light water reactor such as those used in the U.S. A reactor similar to the Chernobyl design simply could not be licensed in the U.S. either now or before the accident.