Yes, radiation from uranium is real even if you don't realize it, because it causes measurable impact on our world. The definition of real I'm working with is an attempt to synthesize the demands of empiricist and rationalist epistemologies. While an objective world exists, it only matters insofar as we have access to it. Radiation is real because even if you don't know about it, it will kill you. Its existence affects what happens to us, and therefore it is a thing we can conceive of verifying experimentally.
String theory on the other hand operates at energy levels so small that it's completely inaccessible to us. A collider would have to be the size of the galaxy to probe the energy levels we're discussing. At the energy levels that govern on our lives and environment, quantum field theory doesn't require string theory to explain what's going on, and we would require godlike powers to approach testing it. This makes it very unreal. Not entirely unreal, and still important for understanding the corner cases, but it is about as ephemeral as it can get.
Something must not just be in principle falsifiable, but the means of falsifying it must be something we could possibly carry out. The more readily available those means, the more real a thing becomes.
Where did you learn all of these things anyways, and which meaning of the word real are you working with?