This is Stephen Baxter's Manifold: Space. But it is from 2000, not 70s-80s.
The only light came from the flames of rush torches, flickering in that downwards breeze, and Malenfant’s impressions built up slowly.
He made out a large heap of ore, crushed to powder, contained within a rough open chamber hollowed out of the stone. Maybe that ore was the yellow-cake de Bonneville had talked about. Long spears of what appeared to be charcoal – like scorched tree-trunks – stuck out of the heap from all sides and above. Water was carried in channels in the walls and pipes of clay, and poured into the heart of the heap. He guessed the heap contained a hundred tonnes of yellow-cake; there were at least forty charred trunks protruding from it.
The chamber was full of people.
There were a lot of tall Uprights, many squat habilines, and some Waganda: men, women and children who limped doggedly through the darkness, intense heat and live steam, serving the heap as if it were some ugly god. They hauled at the charcoal trunks, drawing them from the yellow-cake, or thrusting them deeper inside. Or else they hauled simple wheelbarrows of the yellow-cake powder to and from the heap, continually replenishing it. Their illness was obvious, even from here. Peering down from far above, it was like looking over some grotesque ant-hill, alive with motion.
The heap was intensely hot – Malenfant could feel its heat burning his face – and the water emerged from the base of the heap as steam, which roared away through a further series of pipes. There was a lot of leakage, though, and live steam wreathed the heap’s ugly contours.
The principle was obvious. The heap was an energy source. The steam produced by the heap must, by means of simple pumps and other hydraulic devices, power the various gadgets he’d witnessed: Mtesa’s ascending throne, the fountains. Maybe the water which passed through the system was itself pumped up from some deeper water table by the motive power of the steam.
There had to be a lot of surplus energy, though.