The scary part of the nuclear test programs was the U.S. and USSR high altitude nuclear test explosions, in what was virtually, Space.
Numerous nuclear test explosions were carried out at altitudes of 150 to 540kms.
The Kármán line, at 100kms out, is generally regarded as the height where Space officially starts.
High-altitude nuclear explosion - Wikipedia, the free encyclopedia
Fortunately, to some degree, the U.S. and France have been leading the way from the mid-1990's, and have built very large inertial confinement fusion facilities.
These facilities enable the detailed study of the ignition process of thermonuclear explosions.
With these technologies it is possible to conceive and test new weapons in the laboratory, without large scale explosions being needed.
But - even though the Comprehensive Test Ban Treaty has tried to prevent all nuclear tests - and failed - there is nothing to stop the development of even bigger and better nuclear weapons, without any actual test explosions.
The Doomsday Clock is still at 5 minutes to midnight, even though it has retreated a little from the 2 minutes to midnight, reached at the height of the Cold War.