Recently as of today, May 1st 2018, NASA completed a series of tests on small portable nuclear reactors called the Kilopower. The Kilopower exceed the expectations of its creators by producing more thermal power than was expected and maintained a steady state when placed in extreme conditions.

For more information visit the MSN Article: Nasa Completes Full Power Tests of Small Portable Nuclear Reactors.

A video is available here provided by the Independent summarizing the components of the reactor.