The claim that Israel tested a small tactical atomic bomb in the Syrian city of Tartus is a serious allegation, but it's crucial to approach such information with skepticism and verify it through multiple, credible sources. Here's an analysis based on available information:

Immediate Concerns: 

Source Credibility: The mention of a "Swiss nuclear researcher" without specific naming or institutional affiliation raises immediate questions about credibility. Unnamed or vaguely identified sources can often be used to spread misinformation or unverified claims.

Context and Corroboration:

Recent Events in Tartus: According to various reports, there have been significant Israeli airstrikes in the Tartus region, targeting military sites, particularly after the fall of the Assad regime. These strikes are noted for their intensity, with some sources suggesting they caused explosions large enough to be recorded on seismic sensors as minor earthquakes (around 3.0 magnitude). However, these reports do not confirm the use of nuclear weapons but rather conventional munitions causing significant secondary explosions due to stored munitions.

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Radiation Levels: There are mentions of increased radiation levels in nearby areas like Turkey and Cyprus following these strikes, which could suggest nuclear activity, but these reports are not definitive proof of a nuclear blast. Increased radiation could result from various non-nuclear sources, including damaged conventional munitions or natural background radiation spikes.

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Nuclear Weapon Use: 

Physical Evidence: The description of a 200-foot-wide swath of mountain disappearing and trees vaporizing over a kilometer radius would be consistent with a nuclear explosion's effects. However, no credible, independent sources or satellite imagery have confirmed such an event in Tartus. 

International Reaction: A nuclear attack would typically provoke immediate international outcry and investigation, especially considering the involvement of nuclear weapons in a conflict. There's no evidence of such a global response in this scenario.

Social Media Claims: 

Posts on social media platforms like X have echoed similar claims, describing radiation spikes and seismic activity, but these should be treated with caution due to the potential for misinformation.

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In summary, while there were significant military actions in Tartus, the claim of a nuclear test needs substantial evidence to be considered credible. Without confirmed reports from reliable sources, satellite imagery, or international acknowledgment of such an event, it remains an unverified claim. The use of nuclear weapons would typically result in unmistakable signs and international repercussions, neither of which have been observed or confirmed in this case. Always look for corroboration from multiple, reputable news outlets or official statements when assessing such grave allegations.