This article from SpaceDaily and press release from Los Alamos National Laboratory describes the technical details of the reactor being considered for the first unmanned mission under Project Prometheus, the Jupiter Icy Moons Orbiter (JIMO).
"Los Alamos and NASA-Marshall researchers, working with colleagues from NASA's Jet Propulsion Laboratory and Sandia National Laboratories, have built successively more powerful nuclear electric propulsion reactor components, including a 30-kilowatt reactor core, one-third of a 100-kilowatt system (core plus heat exchanger) and a single module suitable for a 500-kilowatt reactor core. Extensive non-nuclear testing of these and other components continues."
+ Los Alamos Hopes To Lead New Era Of Nuclear Space Tranportion With Jovian Mission (SpaceDaily)
+ Reactor research to power journey to Jupiter's moons (Press Release)