It's a bit challenging as the submarine would need to stand still for a while and not be very deep but some people has thought about that.

Another one along these lines is antineutrinos. They're created by fission reactions, so a nuclear reactor puts out a lot of them. From what's available in the open literature, this seems like it might be getting close to being practical.

Fission reactors spew neutrinos, no need for anti-

Antineutrinos are produced in nuclear reactors when the radioactive products of nuclear fission undergo beta decay.

Source: https://physics.aps.org/articles/v10/66

Ah, right.

This publication[1] says neutrinos, but the article it references says antineutrinos[2].

That explains my misunderstanding.

1 https://www.energy.gov/science/np/articles/detecting-neutrin...

2. https://www.osti.gov/pages/biblio/1959810-evidence-antineutr...

Yes. Nato is has been testing the Watchment muon detection system that was designed to track nuclear material, specifically looking for nuclear submarines.

https://thedebrief.org/darpas-secretive-new-neutrino-detecto...