Was It Really a Meltdown? Western Scientists Not Sure

Nearly two weeks have passed since the Soviet Union’s nuclear power plant at Chernobyl began spewing deadly radioactive materials into the atmosphere, and Western scientists still are uncertain whether the reactor actually experienced a meltdown, one of the most dreaded of all nuclear accidents.

Some have suggested that, until more is known about the incident, the accident be described as a “burnup” rather than a meltdown.

A meltdown occurs when the coolant is somehow prevented from reaching the fuel, which continues to produce heat through radioactive decay--to the point that the reactor itself melts. It can occur if the radioactive core cannot be cooled even after the plant is completely shut down.

U.S. Accident Cited

Nuclear critics have long argued that, in a major meltdown, the reactor’s core would melt right through its concrete floor into the ground and continue down until it reached ground water. At that point, high-pressure steam would open passages through the soil to the atmosphere outside the containment structure, according to the most popular scenario among the critics.

Nuclear proponents cite the 1979 accident at Three Mile Island near Harrisburg, Pa., in arguing that at least a partial meltdown can be contained because most of the radioactive materials resulting from that accident were contained within the reactor’s housing.

A meltdown is such a serious problem because some of the fission processes that make reactors work cannot simply be turned off with the flip of a switch.

Even if the chain reaction that produces the heat to run the turbines has been stopped, “you will still be generating decay heat” that must be removed from the reactor core, said David Okrent, professor of nuclear engineering at UCLA and a member of the U.S. Nuclear Regulatory Commission’s Advisory Committee on Reactor Safeguards.

“Normally, (decay heat) accounts for about 7% of your power being generated by fission products,” Okrent said.

In a 1,000-megawatt reactor, like the Chernobyl plant in the Soviet Union, that amounts to a substantial amount of heat. If the plant’s cooling system has been damaged, and that heat cannot be removed, the fuel itself could heat up to the point that it would melt through the bottom of the reactor--the so-called China Syndrome.

The term China Syndrome, which became the title of a popular movie in the 1970s, refers to an uncontrolled nuclear reaction melting through its shell and eroding into the Earth. HOW A MELTDOWN OCCURS 1. Meltdown begins when fuel rods in the reactor overheat. Extremely high heat develops. The reactor’s uranium core goes into an uncontrolled reaction and the core melts.

2. The mass of radioactive molten metal burns through protective devices of containment structure and enters the earth.

3. Heat hits the water table and steam develops.

4. Steam rises to the surface carrying radiation cloud.