Not a nuclear reactor, not a potential nuclear bomb, this can power satellites and space vehicles in the future. Satellites often use solar power, but if its a mission to the moon or some other position where deploying panels might not be possible, you need something that lasts a long time and produces a lot of power.
And I thought this was interesting. They are using Cobalt 60, not Plutonium.
“Cobalt-60 has a half-life of five years (compared to plutonium-238’s 90 years), which is enough for the cislunar missions that the DOD and NASA are looking at, Morrison says. He says that EmberCore should be able to provide 10 times as much power as a plutonium-238 system, providing over 1 million kilowatt-hours of energy using just a few pounds of fuel. “This is a technology that is in many ways commercially viable and potentially more scalable than plutonium-238,” he says.”