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United States - Subcritical Nuclear Testing

During the 1958–1961 moratorium, the U.S. had conducted hydronuclear experiments in which nuclear devices were tested in a weapons configuration but in a subcritical state by reducing the amount of fissile material in the device. The degree of criticality of devices was gradually increased up toward criticality, but the devices were not allowed to produce any significant nuclear yield. The maximum yield in any of the U.S. experiments was 0.4 lb of TNT equivalent.

LLNL and LANL scientists disagreed on the utility of hydronuclears for the modern U.S. nuclear stockpile. LANL believed that hydronuclears were much more useful than did LLNL. LLNL based its assessment on the nature of modern U.S. designs, which were a lot more sophisticated than those that existed during the 1958–1961 moratorium.

Between 1997 and 2014, the United States had completed 27 sub-critical tests. These subcrits, as they’re called, are underground experiments at NNSS that are typically conducted inside steel confinement vessels. Subcrits are intended to help scientists study—without a full-scale nuclear weapon test—what, for example, are the negative effects aging plutonium pits have on the performance (the yield) of weapons in the U.S. nuclear stockpile. The pit assembly doesn’t have enough plutonium or high explosives to reach a critical mass.

The U1a complex lies some 300 meters beneath the dry desert sands of the Nevada National Security Site (NNSS), formerly known as the Nevada Test Site. It is a dense warren of sealable experimental chambers arranged in clusters, which connect to each other by several main tunnels, themselves connected to the surface by three vertical shafts. The tunnels are relatively spacious, with high ceilings and concrete floors; the experiment alcoves would likely feel roomy were they not crammed full of scientific instruments and equipment. The complex is where Los Alamos, Lawrence Livermore, and Sandia national laboratories collaborate with the National Nuclear Security Administration (NNSA), NSTec, and each other to conduct subcritical tests.

The underground complex consists of several main tunnels (called drifts), each about one-quarter of a kilometer long, and a series of small experimental alcoves branching off from them. The alcoves are also called zero rooms, from the “ground zero” parlance of the nuclear test era. The downhole environment is surprisingly comfortable, with well-lit rooms, concrete floors, tall ceilings, and lunchrooms.

Both Livermore and Los Alamos have designated testing areas in the complex. Los Alamos scientists conduct experiments about every 15 months, while Livermore conducted its tests every six weeks, thanks to the use of expendable steel vessels that confine debris from the experiment.

The complex’s main vertical shaft is equipped with a mechanical hoist to transport workers and equipment. The shaft was originally mined in 1968 for an underground test that was later canceled. In 1988, the shaft was reopened and a 445-meter horizontal tunnel was mined south of the shaft for a low-yield Los Alamos nuclear test. The test, called Ledoux, was conducted in 1990, two years before President Bush announced a nuclear test moratorium that remains in effect. A second vertical shaft about 305 meters away, constructed in 1991–92, provides cross ventilation, utility access, and emergency egress.

In 1996, Lawrence Livermore started mining its first downhole experimental area, called the 101 drift, using the same mining techniques as those for subway construction. The drift and three small experimental alcoves were completed about 10 months later. The mined areas were stabilized with 5-meter-long steel rods drilled into the tunnel walls, secured with epoxy cement, and sprayed with a slurry of fibercrete, material similar to concrete. The Holog, Bagpipe, and Clarinet test series were all conducted in their assigned alcoves, which afterwards were permanently sealed.

The first Livermore subcritical test, consisting of two experiments, was conducted in September 1997. It was named Holog, after the major holographic diagnostic technique that was used to examine ejecta. Because Holog was the first experiment, scientists paid special attention to understanding the physics of the explosions and the effects on the containment barrier to the alcove. One year later, Livermore conducted its second subcritical test. Code-named Bagpipe, this set of four experiments was designed to investigate both ejecta production and spall at different pressure regimes.

The February 1999 Clarinet test consisted of three experiments that were evaluated by a larger array of diagnostic packages than were used previously. Diagnostic techniques included Fabry–Perot velocimetry, holography, pins, and radiography. Holog, Bagpipe, and Clarinet each were fired as clusters of experiments in dedicated alcoves. It took about one year to complete mining activities for the separate alcoves and to set up each experiment cluster, with much of the diagnostic equipment installed in the same room.

Beginning in September 1999, the Oboe test series inaugurated the new 102 minicomplex, where expendable steel vessels that completely confine the experiments were first used. By significantly reducing the need for new alcoves, the vessels save mining costs of about $20 million and double the usable lifetime of the 102 minicomplex from two years to four. Scientists expect to stage up to 12 Oboe experiments in the first alcove of this minicomplex before they start running out of room from the accumulation of entombed vessels. Then, they planned to fire the first Piano shot.

Two recent tests, Castor and Pollux, comprised the Gemini experimental series, which was intended to get data on plutonium hydrodynamics as far into the implosion process as possible. Castor, the shakedown experiment, imploded a surrogate material. Pollux, fired on December 5, 2012, was the real deal, imploding a modified plutonium shell. Both experiments fielded test devices that were scaled-down versions of a primary. The Pollux subcritical test was the 27th in a series of exercises completed to examine how plutonium responds to a conventional explosive detonation. The last such trial, "Barolo B," took place on Feb. 2, 2011.

The weapons community wanted to look at the symmetry of the shell as it imploded, and so desired simultaneous velocity measurements from 100 or more places on the shell. Pollux was notable in that the test device was a scaled-down version of a weapon component. It also fielded a new diagnostic: multiplexed photonic Doppler velocimetry (MPDV). Developed through a partnership between Los Alamos and National Security Technologies, LLC (NSTec), the MPDV system gathered so much high-quality data that scientists are already gaining new insight into plutonium’s behavior under extreme conditions.

After a lapse of 5 years, while the United States was pressing North Korea to abandon its nuclear program, it began carrying out new subcritical nuclear tests. The US Department of Energy ’s National Nuclear Security Administration announced in its annual report that starting in fiscal 2020, the United States plans to conduct two subcritical nuclear tests each year. NNSA requested $773.1 million for the Assessment Science program in FY2021, an increase of 30% over the $594.8 million allocated to comparable programs in FY2020. This includes $152.8 million for Hydrodynamic and Subcritical Experiment Execution Support, an increase of 31% over the FY2020 funding of $116.2 million. It funded not only the science and engineering programs, but also large experimental facilities, like the Enhanced Capabilities for Subcritical Experiments (ECSE) program.