"A small reactor could give you 50 MW of power. Get there and back again in 30 days or so. Why settle for anything other than a simple solution?"

Well, subs have this size or larger and power plants have much bigger nuclear power plants. But neither of these are very concerned about the mass of the power plant.

For space nuclear power plants they usually in the Kw range.

Subs:
"Reactor sizes range up to 550 MW in the larger submarines and surface ships. The French Rubis-class submarines have a 48 MW reactor which needs no refueling for 30 years."
http://en.wikipedia.org/wiki/Nuclear_marine_propulsion

"The plant's design is similar to that of a Generation III+ “light water” reactor, but the size is unusual. “The whole thing is 65 ft. long,” explains Jose Reyes, head of the nuclear engineering department at Oregon State and a co-founder of NuScale Power.
...
It has to be built and serviced on-site, but NuScale's units could be manufactured at the factory, then shipped on a rail car or heavy truck to any location and returned for refueling.
Each 45-megawatt electrical unit would generate enough power for about 45,000 homes."
http://www.popularmechanics.com/science/earth/4273386.html

So, would reasonable to say a 50 MW nuclear reactor would
weigh about 20 or more tons?
Another:

"The aim is to create a sealed reactor that can be delivered to a site, left to generate power for up to 30 years, and retrieved when its fuel is spent.
...
A version producing 100 megawatts would be 15 metres tall, three metres in diameter and weigh 500 tonnes. A 10-megawatt version is likely to weigh less than 200 tonnes."
http://www.newscientist.com/article/dn6344-us-plans-portable-nuclear-power-plants.html

Obviously these things aren't what is needed. 5 years of fuel life is probably more than you need- but that's probably not a significant issue in terms of mass.

"Cleveland (OH) - Researchers at NASA's Glenn Research Center are looking at a sub-surface fission-based nuclear reactor to power the first permanent Lunar outpost. A relatively small 40 Kilowatt reactor - enough to power about 8-10 homes on Earth - would prove a constant, predictable power source for the remote base. "
http://www.tgdaily.com/content/view/39533/135/

Anyways, I like to see something in the 10 to 100 ton range that's going to produce 10 to 100 MW in space.
You got something in mind that is going to do this?