Based on the wikipedia article, it looks like a passive decay heater attached to a bunch of sterling engines. Pretty much a souped up RTG that trades more energy output for having moving parts. Neat.
[...] the core of the reactor will be of solid, cast uranium-235 surrounded by a beryllium oxide reflector. This reflector focuses neutron emissions and returns their energy back into the core to minimize nuclear gamma radiation [...].
Nuclear reaction control is provided by a single rod of boron carbide which is a neutron moderator that is initially fully inserted, [...]. Once the moderator is extracted the nuclear chain reaction will start but can not be stopped completely, although the depth of insertion provides a mechanism to adjust the heat output [...].
Passive heat pipes filled with liquid sodium then transfer the reactor core heat to one or more Stirling engines, which converts heat into rotary motion that drives a conventional electric generator.
It is self-regulating (load following) to some extent so it doesn't have many moving parts. It has two main control mechanisms: a neutron absorbing control rod and a Beryllium neutron reflector.
Lots of details here: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/201400...
it's probably more complex than that. Is high speed natural decay of an artificially created or enriched heavy element natural or artificial? I'm not sure how that gets classified.
To put another way: If you increase the size of a sphere of fissionable material, its rate of "natural" decay stays the same, but more of these decays lead to neutrons triggering additional fission reactions. So doubling the size of the sphere leads to >>2x fission events due to neutrons having more fissile material to hit on the way out.
A large fission reactor is just a scaled up version of this with control rods to adsorb extra neutrons to control secondary "induced" fission.
[0] https://en.wikipedia.org/wiki/Natural_nuclear_fission_reacto... - your terminology immediately made me think of this