system
Given the following post, respond as if you're commenting on Reddit:
human
How do we test our nuclear arsenal with test ban treaties in effect?

I have read into how we test the uranium and plutonium in our nuclear bombs without actual nuclear explosions. I believe the Los Alamos labs explodes a bit of plutonium underground and tests the effects, but it was way above my level of understanding.

How do we test our nuclear stockpile?
gpt
The United States uses Sub-Critical Nuclear or "Cold" Testing due to the CTBT.  Basically, if you use a mass of fissionable material less than the critical mass of the given material (which varies widely depending on what material is used and other factors), then you will not produce a sustained nuclear chain reaction.  Under the Comprehensive Nuclear-Test Ban Treaty, this is the only type of nuclear testing allowed.

In a super-critical nuclear reaction, a slow traveling neutron collides with a large radioactive, fissionable element (let's say U235).  The atom of U235 absorbs this neutron, and briefly becomes U236.  But this extra neutron is very unstable, so the nucleus splits into two lighter elements, and releases three "free" neutrons.  Each of these "free" neutrons can then strike another atom of U235, which will in turn release three *more* neutrons.  Thus the reaction proceeds at an exponential rate.

If a sub-critical mass is used, then the first neutron strikes a U235 atom, which splits just like before, but the three neutrons it releases *do not* strike other atoms, so there is no sustained chain reaction, and the reaction essentially "dies."  Thus, there is no nuclear explosion, and no yield.