[Jean S]

First point - the Marines would know better than to open a drum.

Would it be necessary to open the drum for a Geiger Counter to work?

[Sola Veritas]

Yes it would. Uranium only produces Alpha radiation/particles. Even if the Marines had instrumentation capable of detecting Alpha Ratiation, these detectors must be very close to the source of Alpha radiation (almost touching it). Alpha particles are totally ionized Helium nuclei that can only travel a short distant through air (not more than a few centimeters). They can be readily stopped by even a thin sheet of paper. Therefore, unless the drum was opened, Uranium inside would not be dectectable. Of course there could be residles on the surface, but this would not produce "high levels."

On the other hand, many of the radioactive isotopes found in reactor wastes are Gamma emitters. A Gamma (ray) is a photon very similar to xray. It would readily be dectected outside of the drums, and (with a sensitive enough instrument) at quite a distant. Even an old fashioned Geiger counter would be able to detect a high level from many feet away.

By the way, Geiger counters (and similar robust instruments) are what the military uses. Most standard NBC (nuclear, Bio, chem) units would not carry instruments capable of detecting Alpha particles.