Editing Nuclear weapon design (section)

===Underground testing===
{{Main|Underground nuclear weapons testing}}
[[File:Nevada Test Site craters.jpg|thumb|right|Subsidence Craters at Yucca Flat, Nevada Test Site.]]
The global alarm over radioactive fallout, which began with the Castle Bravo event, eventually drove nuclear testing literally underground. The last U.S. above-ground test took place at [[Johnston Island]] on November 4, 1962. During the next three decades, until September 23, 1992, the United States conducted an average of 2.4 underground nuclear explosions per month, all but a few at the [[Nevada Test Site]] (NTS) northwest of Las Vegas.{{Citation needed|date=June 2021}}

The [[Yucca Flat]] section of the NTS is covered with subsidence craters resulting from the collapse of terrain over radioactive caverns created by nuclear explosions (see photo).

After the 1974 [[Threshold Test Ban Treaty]] (TTBT), which limited underground explosions to 150 kilotons or less, warheads like the half-megaton W88 had to be tested at less than full yield. Since the primary must be detonated at full yield in order to generate data about the implosion of the secondary, the reduction in yield had to come from the secondary. Replacing much of the lithium-6 deuteride fusion fuel with lithium-7 hydride limited the tritium available for fusion, and thus the overall yield, without changing the dynamics of the implosion. The functioning of the device could be evaluated using light pipes, other sensing devices, and analysis of trapped weapon debris. The full yield of the stockpiled weapon could be calculated by extrapolation.{{Citation needed|date=June 2021|reason=W88 was test full yield before ban}}