In the first test of its kind, an experimental nuclear reactor was deliberately melted down in the Idaho desert yesterday to see how much radioactivity is released in such an accident.

Though the test was a complete success, it almost surely destroyed the $200 million test reactor, which will have to be cut up and buried in the Idaho desert where the test was run. The only part of the reactor that can be saved is the dome-shaped building that serves as the reactor containment structure and the undamaged fuel rods that can be reprocessed for their unburned fuel.

The partial meltdown at the Loss of Fluid Test (LOFT) reactor in the Snake River Basin 55 miles west of Idaho Falls took 4 1/2 minutes and produced temperatures of more than 4,400 degrees Fahrenheit at the center of the uranium core.

The test was conducted by the Department of Energy and the Organization for Economic Cooperation and Development to imitate as nearly as possible the conditions that brought on the partial meltdown in 1979 of the uranium fuel core of the Three Mile Island nuclear plant in Middletown, Pa.

"It appears that the test was a complete success," a spokesman for the Energy Department's Idaho National Engineering Laboratory said when the test ended just before 5 p.m. EDT. "We were aiming for meltdown temperatures of 3,300 degrees and we achieved better than 4,400 degrees."

The test was run to see how a partial meltdown triggers nuclear fission, and to track the release, movement and disposition of radioactive fission products inside the reactor containment building. In the months ahead, the radioactivity of every part of the inside of the vessel will be measured and analyzed to see how much escaped from the fuel rods and how far away from the fuel rods it escaped.

Burning uranium fuel is usually covered by tons of cool water circulated over and around the fuel. The fuel is also covered with a zirconium cladding that prevents fission products that build up inside the fuel from escaping into the environment.

The test run yesterday purposely exposed eight raised fuel rods at the center of an assembly of 100 fuel rods. This was done by forcing the cooling water down until the eight fuel rods were exposed for 4 1/2 minutes. The high water level was then restored.

In the 4 1/2 minutes, the temperature of the exposed rods rose from less than 120 degrees to 4,400, high enough to melt the zirconium cladding around the fuel rods and the fuel rods themselves.

Though it will be weeks before engineers have any idea what happened after the meltdown, they fully expect fission products such as strontium 90, iodine 131 and cesium 137 to be splattered throughout the inside of the reactor vessel. CAPTION: Picture, The $200 million government test facility used in loss-of-coolant experiment. Dept. of Energy photo