The safest approach to nuclear testing by far is the underground method, although "safe" is a relative term.
Underground testing provides the possibility of containment, but containing a nuclear blast is no simple task. The smallest nuclear bomb imaginable will break through 20 meters (65 feet) of earth as if it were a tissue paper.
A 1-kiloton-yield bomb needs to be at least 90 meters (300 feet) underground in order for its explosion to be fully contained. For comparison, the Castle Bravo accident involved a 15 megaton yield. And these depths are just estimates; it's unlikely to know exactly how a new nuclear technology is going to react until you test it. Even under the most tightly controlled conditions, underground nuclear tests can break through into the atmosphere, which is a worst-case scenario because an underground nuclear explosion irradiates tons of soil that then rains down on everything in the surrounding area. Ground contact can be the most damaging aspect of a nuclear explosion, so if an underground nuclear detonation does break through the surface, you're looking at fairly serious fallout.
The final nuclear-testing method falls under the "Are you kidding? What were they thinking?" category: detonating a nuclear bomb in outer space. Both the United States and Russia performed these high-altitude tests during the Cold War, sending up the devices by way of rockets, for the purpose of testing the efficacy of the weapons in decommissioning enemy satellites.
While radioactive fallout on Earth was not a problem (the radiation gets deflected by Earth's atmosphere), they stopped performing these tests when several things became apparent:
- Nuclear blasts can't tell which satellites are yours and which are the enemy's.
- The deflection of radiation in the Earth's atmosphere resulted in a powerful electromagnetic pulse that wiped out electrical systems in major cities on Earth.
- The blasts left bands of radiation in space the posed risks to any future manned spaceflights.
Besides the most far-reaching effects of nuclear testing, there are also significant dangers to those involved in carrying out the test. More than 4,000 workers at a former French testing facility have filed suits against the government alleging that radiation exposure has compromised their health. Many of those workers have been diagnosed with serious cancers. France conducted nuclear tests until 1996, long after most other countries has stopped.
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