Nuclear Weapon Hydronuclear Testing
In a hydronuclear test, fissile material is imploded, but a supercritical mass is not maintained for a long enough time to permit the device to deliver "full" nuclear yield. Depending upon the conditions of the test, nuclear energy releases may range from the unmeasurably small (milligrams or less) to kilograms or even metric tons of TNT equivalent yield.
Hydronuclear experiments, as distinguished from hydrodynamic ones, use actual fissile material assembled to form a supercritical mass in which a chain reaction be-gins. Normally, hydronuclear experiments are designed to use nuclear devices modified in one of several ways, including substituting inert material or less-fissile material for some of the HEU or plutonium in the pit, so that very little nuclear energy release occurs. Yields in experiments described as "hydronuclear" by various countries have ranged from much less than 1 kg TNT equivalent to many tons.
Subcritical Experiments are scientific experiments to obtain technical information in support of the U.S. Department of Energy, National Nuclear Security Administration's (NNSA) Stockpile Stewardship and Management Programs -- the NNSA programs are to maintain the safety and reliability of the U.S. nuclear weapons stockpile without nuclear testing. They involve chemical high explosives to generate high pressures that are applied to nuclear weapon materials, such as plutonium. The configuration and quantities of explosives and nuclear materials will be such that no nuclear explosion will take place. Thus, the experiments are consistent with the Comprehensive Test Ban Treaty. They are called "subcritical" because there will be no critical mass formed, i.e., no self-sustaining nuclear fission chain reaction will occur. Scientific data is obtained on the behavior of nuclear weapon materials by the use of complex, high speed measurement instruments.
Los Alamos National Laboratory conducted the first subcritical experiment Rebound on July 2, 1997. The purpose of the experiment was to obtain information on the response of plutonium to shock wave compression at different pressures. This information was obtained by performing fundamental shock wave experiments on plutonium. This data is a hugoniot curve, that is, a curve showing density. Rebound involved three measurements of different pressures, which were done in a single experiment room, 10 feet by 15 feet by 30 feet, located 962 feet below ground in the U1a Complex. All three of the experiments utilized high explosives for driving stainless-steel flyer plates into target assemblies generate pressure in the plutonium targets.
The second subcritical experiment Holog was conducted by Lawrence Livermore National Laboratory scientists on September 18, 1997. It was designed to yield information on the nonnuclear properties of plutonium under extreme shock conditions. The name Holog was taken from the Laboratory-developed holography technology that allows scientists to capture three-dimensional images of the particles ejected from the surface of materials shocked by high explosives. The Holog experiment was to allow scientists to answer basic questions like how plutonium reacts when shocked -- which cannot be determined today with the required precision by experimenting with substitute materials. It is anticipated that this data will be used in complex computer simulations which will help assure the safety and reliability of U.S. nuclear weapons without nuclear testing. The explosion was comparable to that of a large fire cracker or shotgun blast.
The Comprehensive Test Ban Treaty is a zero-yield ban because we determined that this was in our interest and because no threshold above zero yield would have been negotiable, for reasons that remain true. The original U.S. and British scope position, which would have allowed "hydronuclear" tests with yields up to four pounds, might have been reluctantly accepted by the non-nuclear weapon states. But it was vigorously rejected by Russia, France, and China, which preferred yield limits of 10 tons, 300 tons, or an exemption for "peaceful" nuclear explosions, respectively. Such yield thresholds would have been politically unacceptable to many non-nuclear weapon states, and the PNE exemption was rejected by almost everyone.
Subcritical tests have become largely accepted internationally for ensuring the safety and reliability of a nation's nuclear force without resorting to nuclear testing. Russia has been conducting subcritical tests involving both weapons-grade plutonium and uranium since 1995 at its Novaya Zemlya test site near the Arctic Circle.
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