A new GAO report (Government Accountability Office) has been issued that perhaps waxes philosophical about subcritical nuclear tests. A sidebar titled 'Principles of Zeus' notes that a stimulus to an assembly 'can initiate' a chain reaction 'based on the properties of the assembly, such as the geometry (e.g., sphere or flat sheet) and the density,' which means Zeus was a wise dude. In all seriousness, GAO says that subcrits are getting less subcritical: 'The Zeus system simulates that process by bombarding the subcritical assembly with many additional neutrons to increase the number of fissions and gamma rays.'

Scaled subcritical test 'Pollux'

The Global Conversation about United States sub-critical nuclear experiments

Primer on Transparency on NNSA SNEs

The Department of Energy (DOE) conducted two subcritical nuclear experiments (SNE) in late 2010 and early 2011. This information was only learned following the discovery of a PDF file released by the DOE in June 2011, a record--in lag time of disclosure--that was shattered by President Biden's new record on subcritical nuclear test notification delays that stands at 209 days. Subcritical nuclear tests aren't zero-yield since they comprise a tiny nuclear explosive yield.

Scaled subcritical testing started under the Obama administration involves greater amounts of fissile fuel and thus higher nuclear explosive releases. A 2019 scaled subcritical test named Ediza breached its steel experimental vessel and leaked radiation in an alcove. (If you can actually define a nuclear test, Congrats—it's alot to take in. It's been going on since the 1930s.)