WHAT'S GOING ON UNDERGROUND?

Subcritical Testing

At first, the Nevada Test Site looks like an alien landscape, a vast desert peppered with various sizes of craters. These craters are reminders of the underground nuclear testing that the United States conducted at this site until our country stopped all nuclear testing in 1992.

High Pressures using Conventional Explosives

Deep underground—nearly 1,000 feet down, it's a different story and important experiments are still conducted. It is here that tunnels and chambers make up what is called the U1a complex, which scientists use to conduct subcritical experiments. These experiments test the basic properties of plutonium driven to high pressures using conventional explosives. These experiments do not generate sustained nuclear chain reactions and thus do not produce nuclear explosions—that is what is meant by subcritical.

To conduct one type of subcritical test, scientists prepare an experimental package containing a very small amount of plutonium that has aged for a certain amount of time. Some distance away is a thin stainless-steel flyer plate behind which are some high explosives. These materials are then sealed permanently in a chamber at the end of a tunnel about 1,000 feet below the earth's surface.

Once triggered, the explosives slam the plate into the package, sending shock waves through the plutonium sample. When the waves emerge from the backside of the sample, light is generated in a special material that is transmittedalong fiber-optic cables to a series of recording stations.

Developing the Computer Model

The data collected by these stations help us better understand how plutonium's aging affects the performance of nuclear weapons and other weapons materials. Such knowledge will improve the accuracy of computer simulations, which in turn will help the Laboratory evaluate weapons reliability and safety without nuclear testing.

NEVADA TEST SITE

Covering 1,350 square miles, an area larger than Rhode Island, the Nevada Test Site is one of the largest outdoor laboratories in the United States.

From its first aboveground test on January 27, 1951, to the last underground test on September 23, 1992, the site served as the primary place the United States conducted nuclear testing. Now the test site is the place where Los Alamos conducts occasional subcritical experiments - nuclear experiments that do not produce self-sustaining nuclear reactions.