The reason I added this comment at all, was I felt the headline title was a little misleading. We have evidence, of a naturally occurring fission reaction on Earth, a long long time ago.
100kW can raise the temperature of 1L of water from 20C to 100C in 3.34 seconds. It's enough power to brew about 300 shots of espresso in 30 seconds. That seems like a lot to me.
1 kW is a lot if you put it into a small enough space. Or even 1 W, as my background in electronics design has shown me on occasion.
Of course, the title calls it a fission reactor and a 100 kW one would not be much. Would charge an electric car pretty nicely, though. Or make some mean espresso.