THE June unveiling of a 130-ton (118,000-kilogram) gleaming metal sphere some 10 meters in diameter marked a much-anticipated and highly celebrated milestone for the Department of Energy's National Ignition Facility, now under construction at Lawrence Livermore. A large crowd of employees and guests, including Energy Secretary Bill Richardson, was on hand for the dedication of the giant target chamber for NIF, currently the nation's largest science construction project.|
The dedication marked the on-time completion of NIF's single largest piece of equipment. The $14.5-million vessel will serve as the working end of the largest laser in the world. The output of NIF's 192 laser beams will converge at the precise center of the chamber, where conditions of deep vacuum and temperatures far below freezing will support experiments only dreamed of for decades.
NIF's beams will enter the chamber in two-by-two arrays to illuminate 10-millimeter-long gold cylinders called hohlraums enclosing 2-millimeter capsules containing deuterium and tritium, isotopes of hydrogen. The two isotopes will fuse, thereby creating temperatures and pressures resembling those found only inside stars and in detonated nuclear weapons-but on a minute scale. By recreating these extreme conditions in a carefully instrumented laboratory setting, NIF will serve as an essential facility in DOE's Stockpile Stewardship Program to ensure the safety and reliability of the nation's nuclear arsenal.
Must Last 30 Years
The team finally agreed upon an expanded cube (6 sides) with 3 plates per side (18 plates total) to minimize welding length and overall cost. The design, looking like a giant volleyball, features 6 symmetric middle plates and 12 asymmetric outer plates. As manufactured, the 18 aluminum plates measure 2.4 by 6.9 meters and weigh about 7.5 tons each.
There was uniform agreement that the vessel should be manufactured from aluminum, specifically the aluminum alloy 5083-0. The same alloy, notes mechanical engineer Wavrik, is used in harsh marine environments such as ship superstructures.
The completed vessel was estimated to weigh some 130 tons. An outer concrete skin and final optics would add 200 tons each, for a total of nearly 530 tons. Given that estimate of final weight and the number of holes that needed to be drilled, the designers decided on a plate thickness of 11 centimeters. Although this was more than was needed theoretically, it gave the chamber the strength of a substantial structure in its own right rather than a simple vessel to contain experiments.
A prime consideration was ease of fabricating the 18 plates. "We wanted to make sure we didn't design something that would be difficult to manufacture," says Atkinson. The team chose as manufacturing contractor Pitt-Des Moines Inc., which has extensive experience fabricating vessels, from nuclear power plants to water storage tanks.
Pitt-Des Moines assembled an international team of subcontractors. Manufacturing began in the fall of 1997, when the plates were poured at the Ravenswood Aluminum Mill in Ravenswood, West Virginia. The plates were shipped to forming subcontractor Creusot-Loire Industries in France, where the plates were heated to 315∞C and then shaped in a giant press to the proper spherical geometry.
From France to Pennsylvania
Achieving Extreme Sphericity|
At regular steps along the way, the chamber was mapped with laser surveying instruments to ensure its sphericity. Wavrik notes that the American Society of Mechanical Engineers specification for spherical vessels is for the diameter to measure within 1 percent of specification, or within 10.16 centimeters for the 10-meter-diameter chamber. The Livermore-Sandia specifications called for final measurement within 0.5 percent, or 5.1 centimeters in 10 meters. In fact, the assembly crew achieved 0.25 percent, or 2.54 centimeters, across the entire diameter.
On June 5, the roof of the temporary enclosure was removed, and the target chamber was lifted out by an enormous crane, a 14-story-tall Manitowoc 4600 Ringer. Acquired from DOE's Nevada Test Site (and transported in pieces aboard 66 trucks), the crane weighs some 900 tons and has a lift capacity of 600 tons. It will remain on site for additional heavy-lifting construction jobs on NIF.
The sphere was secured to the crane with a plate lowered vertically into the top port of the sphere and then turned horizontally to support the chamber from inside (similar to installing a togglebolt). "We all held our breath," recalled Livermore Director Bruce Tarter, when the chamber was lifted out of the enclosure and then placed on a Lampson Crawler (also borrowed from the test site) as an intermediate anchor for its public dedication.
A few days later, on June 11, the world got a good look at the chamber at the dedication ceremony. The extraordinary structure became an instant magnet for employees and visitors alike. As Britain's Graham Jordan, Deputy Under Secretary for Science and Technology, Ministry of Defense, remarked, the chamber looked as if it "simply landed one night" from outer space.
The following week, the chamber was hoisted onto a massive concrete pedestal installed inside the target building. Over the next two weeks, a combination of hydraulic jacks, roller assemblies and shims, and finally anchor bolts were used to adjust the chamber for final alignment and establish its proper elevation and sphere tilt.
This fall the exterior of the chamber will be tested for leaks and then encased in 40 centimeters of concrete with 0.1 percent boron to provide shielding from the neutron and gamma rays produced by the experiments. The concrete will be applied over steel rebar tied to the chamber with welded studs.
Following application of the concrete, the chamber's exterior will be sealed with epoxy paint, and the chamber will be aligned and hung with the final optics assemblies, which will arrive next year. The chamber is expected to sag a bit from the 400 tons of the concrete shield and optics. As a result, the angle of the FOAs will be adjusted appropriately.
Although an unqualified success in its own right, the target chamber's completion serves as a striking symbol that NIF is only a few years away from history-making experiments as an essential component of DOE's Stockpile Stewardship Program.
Key Words: final optics assemblies (FOAs), hohlraum, National Ignition Facility (NIF) target chamber, Nevada Test Site, Stockpile Stewardship Program.
For further information contact Dennis Atkinson (925) 422-6984 (firstname.lastname@example.org) or Dick Wavrik (925) 422-0415 (email@example.com">).