USA’s subcritical experiments will contain the ingredients of a nuclear bomb, but fizzle out without any thermonuclear blast. The reasons for this test is to help the nuclear bomb scientists to understand how the basic physical properties of plutonium change under intense pressure. This is useful information both for improving old warheads and for developing new nuclear devices.

By feeding the data gathered during the test into computer models, weapons scientists will be able to check on the reliability and safety of the ageing stockpile of warheads without detonating the device itself.

There is very limited information about how USA will use the results from these tests in the development of the next generation nuclear devices. It is clear, however, that the tests will help to show how plutonium deforms under blast conditions; information crucial to understanding exactly how separate masses of the metal contract into the critical mass necessary for a nuclear explosion when a warhead`s detonator is fired.

The first US subcritical bomb test was conducted earlier this summer. The test on Thursday will be carried out in order to investigate how the plutonium surface fractures during the blast. The subcritical tests were designed by the Los Alamos National Laboratory, and are carried out underground just like real nuclear bomb tests. The Nevada test site near Las Vegas has been rocked by almost a thousand nuclear tests in the past fifty years.

Subcritical tests do not violate the provisions of the Comprehensive Test Ban Treaty, and are not accompanied by radioactive emissions. Nils Bohmer, a nuclear physicist at the Bellona Foundation, says that this shows the weakness of the new Test Ban Treaty. –Superpowers like USA can continue to develop nuclear devices, while other countries which does not have this tecnology and corresponding big research programs are banned from continuing. This can make it it easier for small contries to argue not to sign the test ban treaty, or to withdraw existing signatures, says Bohmer. The Comprehensive Test Ban Treaty has been signed by more than 140 nations, but has not yet been put into force.

A dozen anti-nuclear organisations in USA has brought a lawsuit against the US Department of Energy (DOE), demanding that the subcritical tests be stopped. Thomas Cochran, a physicist with the Washington DC based environmental group Natural Defence Council, says he is worried that the ongoing subcritical experiments will fuel the arms race. –The tests allow American scientists to design new, more powerful weapons. This possibility will make other countries nervous, he says.