Let's park the issue that natural fission reactor happened too early in the history of the planet to have intelligent life forms around.
The way natural nuclear fission reactor works is roughly through the following cycle:
- enough density of active Uranium isotope is accumulated in one place
- from time to time enough water collects around the place, slowing down neutron
- the slowed down neutrons trigger chain reaction
- chain reaction boils the water away
- lacking water to moderate the reaction, the reaction stops
- go back to 2.
Boiling water reaches the outside, if possible, under the form of hot steam. This is also what happens with geysers, plus the radioactivity.
As far as I know, geysers have not been used for energy supply by primitive civilization, for a quite obvious reason: hot and high pressure steam is nasty to handle!
If you don't have access to decent metals, it's hardly possible to harvest the energy contained into a stream of steam. While a wooden wheel works fine into a river to power a mill, it would quickly fail under the conditions provided by a geyser. Same goes for cloth: you can build a windmill with it, but hot steam is another story.