Dual-Axis Radiographic Hydrodynamic Test Facility

The Dual Axis Radiographic Hydrodynamic Test Facility (DARHT) is a facility at Los Alamos National Laboratory which is part of the Department of Energy's stockpile stewardship program. It uses two large X ray machines to record three dimensional interior images of materials. In most experiments, materials undergo hydrodynamic shock to simulate the implosion process in nuclear bombs and/or the effects of severe hydrodynamic stress. The tests are described as "full scale mockups of the events that trigger the nuclear detonation". The powerful pulsed X ray beams allow for an ultra fast motion picture to be constructed showing the details of the process being studied in three dimensions. The tests are compared with computer simulations to help improve the accuracy of the computer codes. Such testing falls under the category of sub critical testing.

Planning for DARHT began in the early 1980s. The origin of the electrical breakdown turned out to be unexpectedly high electric fields between the high voltage plate and the oil insulated magnetic cores and at sites where metal, high voltage insulator, and vacuum meet inside the cells. After much analysis, the design error was tracked to faulty equipment used when doing voltage calibrations. The project was initially expected to cost $30 million in 1988, but costs ultimately rose to $350 million by 2008 when the facility became completely operational.

During a weapon's crucial triggering phase, explosive charges that surround the nuclear fuel are detonated at multiple points. The result is a shock wave that moves inward (implosion) at supersonic speeds, compressing the fuel to higher and higher density. Implosion ends when the fuel reaches a supercritical density, the density at which nuclear reactions in the fuel build up an uncontainable amount of energy, which is then released in a massive explosion. To make the mockup non nuclear, a heavy metal surrogate (such as depleted uranium or lead) stands in for the nuclear fuel, but all other components can be exact replicas. Sub critical masses of plutonium may also be used. used to image x rays on the second axis. These cameras utilizes the world's largest LSO crystal array to convert the x rays into visible light which are then imaged with the world's highest speed CCDs (developed jointly by MIT and Los Alamos) at more than two million frames per second. These scintillation cameras are further augmented by a large anti scatter (“Bucky”) grid to improve image contrast. The unique combination of diagnostics solves technical surveillance problems that have persisted since the Manhattan Project enabling the United States to assert higher confidence in its nuclear stockpile performance and safety margins without the need for nuclear testing.