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

"The spacecraft are able to stay in orbit for months by using a solar array to generate power."

http://news.yahoo.com/secretive-x-37b-military-space-plane-could-land-132030466.html

Why are the ground crew wearing protective gear if it just solar power? Seems like a radioactive power source with that type of protection.

Thoughts?