The answer, it turns out is “probably” or “we’re pretty sure they do.” or “Almost for sure, most of them should.”
That’s right. We’re not entirely sure, and as time goes on we’re becoming less sure. That’s because we don’t test them and haven’t done so for two decades.
How it was and how we got here:
A nuclear weapon is a very complex piece of engineering and physics. There are many parts that have to work properly for the weapon to actually detonate. The core must implode in a manner that results in the correct final geometry. It must undergo fission before it is blown apart, sometimes requiring additional neutrons be provided by a pulsed neutron generator or by boosting with a small amount of fusion. In hydrogen bombs, energy from the primary must be channeled into the secondary and produce fusion. The time tolerances involved are less than nanoseconds.
For this reason, nuclear weapon designs were initially always tested at full scale, in prototype devices that would then become production weapons. The first tests were conducted in the atmosphere. Hundreds of such tests, some of multiple megatons were conducted by the United States and Soviet Union in the 1940’s, 1950’s and 1960’s. These tests had multiple purposes. In addition to validating the viability of the weapons designs, they were used to better understand the physics involved, with data collected to help guide future weapons design. Tests were also used to determine the effects of weapons on structures, aiding in the design of nuclear-resistant structures, communications systems and weapons platforms.
In 1963 the United States and Soviet Union signed the Partial Test Ban Treaty. The treaty ended the testing of nuclear devices in the atmosphere, underwater or outer space by the signing parties. After 1963, all US and Soviet tests would take place underground, in shafts designed to completely contain the explosions and prevent any fallout from entering the atmosphere. For the most part, this was successful, although there were occasional minor leaks and at least one major breach of containment due to an unmapped fissure in 1970. France and China continued to conduct atmospheric testing, having not been party to the 1963 treaty. The last atmospheric nuclear test was conducted by China in 1980. Since that time, all tests have been underground.
By the late 1960’s, the superpowers had generally ended the practice of testing nuclear weapons at their full yield. Having acquired a much better understanding of the physics and engineering behind nuclear weapons, it was no longer considered necessary to test the secondary stages of nuclear weapons at their full yield. Testing the fission primaries, with either no secondary component, or a greatly reduced secondary yield provided ample data on the reliability of the weapon design.
The only exception to this was the rare circumstance where a new type of weapon was developed, with a vastly different design than previous weapons. The 1971 Cannikin test was one example of a high yield weapon tested underground. At five megatons, the exceptional yield of the test device required extreme measures be taken to contain the blast. The test was conducted at the bottom of a 1.8 kilometer deep shaft, drilled through solid rock on a remote island off the coast of Alaska. The weapon tested was the W71, a highly unique warhead designed for the Spartan anti-ballistic missile system. The new warhead was designed to produce an extremely high x-ray and neutron flux and to operate in the extreme environment of outer space, possibly being subjected to radiation from other nuclear explosions. Given these special design criteria, it was determined that a full scale test of the system was necessary.
In 1974, the US and Soviet Union signed the Threshold Test Ban Treaty, limiting nuclear tests to a maximum of 150 kilotons. By the time the treaty was signed, it was no longer necessary to test weapons at their full design yield, so the treaty was largely symbolic. Since larger tests require more complex and extensive containment measures, and because they were no longer necessary, both countries had generally abandoned large tests by that time. Although other nuclear powers were not party to the treaty, by the 1970’s, full yield weapons testing was no longer necessary for established nuclear powers.
The United States and Soviet Union continued to conduct nuclear tests, mostly with yields of a few kilotons, throughout the 1980’s. France, China and the UK also conducted nuclear tests through the 1980s and into the early 1990’s.