Grammatically, it's then correct. But do native speakers say "... many UV rays" in this context, or it's better to say "... much UV radiation"?
Well, as I see it you are talking about two different uses on the term UV.
We don't say either of those. This mis-uses "rays". In the WordReference dictionary a "ray" is defined as "a narrow beam of light". If you are outdoors surrounded by bright sunshine, there is no "narrow beam". So we don't talk about "rays" in that situation.