NASA is currently developing prototypes of small nuclear power plants for use in robotic and crewed missions. While this work is early and low priority, it is on the critical path to space exploration and settlement.
The reactors will use uranium as a heat source and Stirling cycle mechanics. This will avoid the issue of plutonium being so rare and expensive to make, and the complex mechanics of a conventional reactor.
The smaller project is called Kilopower. It would be 6.5 feet tall (1.9 meters) and operate at 1 kilowatt. The project will cost $15 million. The technology readiness level is currently 2 to 3, and the goal is to push it to 5. The system will be tested in a vacuum environment. Interestingly, the smaller plant will burn almost no fuel (0.1 percent) in the first 15 years of operations. While this has implications for very long duration missions and power demands, it has a more practical near-term benefit. As nuclear fuel burns, it tends to swell and release fission gases. This can cause issues with the reactor long before the fuel is expended. In this case, it's a non-issue. Because the fuel doesn't swell, it can be wrapped in a cladding of material that won't rupture if the fuel rod swells during operation. This dramatically improves the safety of the reactor, while reducing the cost and time of development.
The larger project is called the Technology Demonstration Unit (TDU). This will push the power level from 10 to 100 kilowatts. This would be a more conventional (heavy water cooled) nuclear power plant. The ultimate goal of these projects is to have a reactor that can be tested at the International Space Station.