Spykman et al. 2021, the only published independent forensic search for a Burevestnik-linked radionuclide signature, found no detectable trace of any candidate isotope

Spykman, Blenke, and colleagues (senior author Georg Steinhauser) ran gamma-spectrometry on air filters from two vessels near the Nyonoksa accident site and one high-volume filter in Greece, plus a dedicated search for argon-42 — a different isotope from either the Hecla–Kemp model's ⁴¹Ar or the fission products Rosgidromet reported, tied instead to a separate 2020 hypothesis (Mietelski & Povinec) that the missile used a ⁴²Ar–⁴²K radioisotope generator rather than a reactor. Their overall finding: "no traces of radionuclides were found that would provide any clues about the nature of the release." On argon-42 specifically: "we cannot confirm that the release involved radioargon from a ⁴²Ar-fueled radionuclide source."

The authors attribute the null result primarily to sample geography — their filters likely sat outside the plume's modeled path — rather than to an absence of release. Critically, this paper did not test for argon-41, krypton-85m/83m, or carbon-14, the isotopes the Hecla–Kemp model actually predicts. Its null result bears on the unrelated RTG hypothesis, not on a direct test of the Hecla–Kemp activation signature — no published independent test of that specific isotope list has been located.