If the time is 2 billion years ago, yes
If we assume an Earth-like world, where life evolved about three times as fast, then they need no technical understanding at all.
...because then natural nuclear reactors existed.
One of the reactor zones at Oklo, Gabon, Africa (image credit: US DoE)
When the Earth came to be, the isotope composition of natural Uranium was — roughly — the same as that of present day nuclear fuel, after enriching.
This then means that what was needed to get a chain reaction going was...
- A concentration of uranium ore in a compact volume
- Water, as a moderator
...and this is exactly what happened at what is present day Oklo, Gabon.
So, 2 billion years ago, for tens of thousands of years, these natural reactor zones operated in cycles. Water would flow through the ore, moderate the chain reaction, cause the zone to heat up, make water to boil off, the chain reaction stops, the zone cools down, and start over, in about 3 hour cycles.
Assuming a primitive culture existed back then, they would be able to see this happen with their own eyes.
The issue would be, that this would not be very healthy for them.
But, yes, they would be able to see that if you pile that very particular kind of dense and heavy rock together in large enough a pile, and soak it in water, then it heats up.
...and then they get sick and possibly die, without knowing why.