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. An extensive design overhaul and rebuild was required, which was completed in 2008. 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. Description 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. Under such extreme implosion forces materials tend to behave like fluids, so this mock implosion is called a hydrodynamic test, or hydrotest. Standard practice is to take a single stop-action snapshot of the weapon mockup's interior as the molten components rush inward at thousands of meters per second. X-rays that can penetrate the heavy metal in a weapon mockup are made with an electron accelerator. An electron beam moving at near the speed of light is smashed into a tungsten target. The electrons are yanked off course by the strong electrostatic pull of the positively charged nuclei in the tungsten atoms, and their sudden change in direction causes them to give off energy in the form of high-energy x-rays in a process called bremsstrahlung. Scientists already knew how to use a short burst (pulse) of high-energy electrons (rather than a continuous beam) to make a short pulse of high-energy x-rays recorded on conventional x-Ray films. The new challenge was for the accelerator to deliver a very large number of electrons in an extremely powerful pulse to generate an x-ray flash that can penetrate the mockup during the ultra-dense implosion. Specifications call for a pulse width of 60 billionths of a second. Each electron accelerator