X-37B plane can stay in space for years, power source?

The way you're self-educating is very similar to the strategy used by those who wish to imply that there is something being deliberately withheld. You might want to change your form of autodidacticism in order to draw a larger distinction between the two.

For example, consider the two statements "Seems like a radioactive power source with that type of protection." and "Odd, no?" They imply that you know enough to make a reasonably confident statement about 1) the different types of clothing used for different hazmat situations and what's appropriate for space operations, and 2) that you understand enough about space power systems to judge what's odd.

You received many responses pointing out that these are standard chemical hazmat suits for this sort of task, with decades of examples and pointers to multiple space agencies, and not similar to what's used for space-based radionuclear power sources.

That should have been a pointer that you don't have enough knowledge to be able to say if something is "odd", as regards spacecraft. Your default assumption should be that nothing is odd, and that it's your lack of understanding which is the source of confusion. Resolve that first before looking outside for the point of confusion. Eg, "Thanks for the comments. I don't know much about spacecraft. Could you also help me understand why ... ?"

You could even have done so in your original question. There was no need to add "Seems like a radioactive power source with that type of protection."

While on the other hand, those who believe (perhaps correctly, perhaps falsely) that they've uncovered something secret will use that sort of phrasing to imply that something is secret, even though they don't have the ability to outright say that something is secretive without taking the large chance that doing so will reveal their lack of actual knowledge on the topic.

Ok, you should definitely look in to RTGs then:

http://en.m.wikipedia.org/wiki/Radioisotope_thermoelectric_g...

I'll reiterate that hazmat suits are essential when working around rocket propellant, because the oxidizers involved will cause any organic material to ignite on contact. Literally, a pair of cotton gloves would turn in to a ball of flames on contact. That is all that is needed to explain their presence.

The government would have no reason to hide the presence of a nuclear power device, so the power systems described as powered by solar cells + LiIon batteries is entirely plausible. Preferred, in fact, because you needn't address the concerns of radioactive material in a tightly confined space. Arguably, it makes the engineering easier (marginally).