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The United States is planning to test underground detonation of "a small amount of plutonium" at the Nevada Test Site to test the effectiveness of, and modify, existing nuclear weapons.
Nuclear testing treaties and status:
Limited Test Ban Treaty (LTBT) – 1963 (in effect)
Bans all nuclear weapon testing except underground
Threshold Test Ban Treaty (TTBT) – 1974 (in effect)
Bans underground weapon testing above 150 kt
Peaceful Nuclear Explosions Treaty (PNET) – 1976 (in effect)
Limits underground peaceful (excavation, etc) nuclear explosions to 150 kt
Comprehensive Nuclear-Test-Ban Treaty (CTBT) – 1996 (not ratified by US)
Bans all nuclear explosions
* note - US is not bound by this treaty but has observed a voluntary moratorium since 1992
So this looks like it will be the first nuclear explosion in the US since 1992.