If they were nuclear-capable, we'd probably know about them. Even long after the concrete had crumbled into dust and the steel had rusted into minerals, the isotope makeup of the spent fuel would be unmistakable. See: http://en.wikipedia.org/wiki/Natural_nuclear_fission_reactor for a really interesting story. In Oklo, the reactor was critical 1.7 billion years ago and it's still easily recognizable.
It's easily recognizable as a nuclear reactor. But how do you distinguish a natural reactor from an artificial one?
Furthermore, if we assume an intelligent species, wouldn't they either
1) Try to bury and hide leftover stuff, to the point where it's no longer easily found and/or recognizable?
2) Draw more energy out of it than we currently do, to the point where all that would be left would be Fe or Pb?
Also, I forgot to mention that of course, my intelligent dinosaurs would have to be space faring. And that's where a stronger objection appears (IMO): on the surface of the moon, stuff tends to remain unaltered for a long time.
In isolation it's hard to distinguish. But if you find 30-50 reactors all over the globe that were all critical at the same time and all went silent at the same time, I think you'd have to think very hard about how that happened.
As for 1) current proposals (which continually fail to be implemented) are actually to concentrate the stuff and put it in geologically stable locations (c.f. Yucca Mountain). That would seem to be easier to find.
And 2) isn't really plausible. You can't select out and "convert" isotopes like that. You get the fission products you get, and your choice to get "more" out of them is really about how long you are willing to wait for them to decay.
For (1), we choose well hidden geological location, e.g. stuff that is about a mile deep. My guess is we don't have such an extensive knowledge of geologically stable locations one mile below surface.
For (2), I'd venture to say that this is based on what we know today. But then, nuclear reaction or coherent light or traveling to the moon were not exactly "plausible" in 1900 either.
What we know from physics is that there's still a lot of residual energy that could theoretically be extracted, to the point where these isotopes are naturally radioactive.
So to me, that means we are likely to one day figure out how to harness that residual energy.
It depends on where they bury it. On a scale of millions of years, the landscape can be changed radically. Continents get eroded, tectonic plates collide and get subducted. On mere thousands of years, sea levels rise and fall, rivers change course etc.
For instance, the ancient cities of Ur and Uruk used to be close to the shore of the Red Sea, whereas nowadays the sites are about 200km away (silt deposits built up over millenia). The Aegean sea used to extend into a large gulf into Western Anatolia around the time of Homer's Troy.
Everything humans built won't last more than a few thousands of years and modern buildings just a few decades. Even satellites in orbit will decay and burn up in a similar time.
It would be difficult for any visitor coming to an abandoned Earth a million later to discover anything about our presence.
The Earth's crust recycles itself every so often, so given a large enough timeframe, most of the existing surface will be gone, nuclear accidents or no. I suppose a ocean spills could last longer.
