I would suggest that the real rationale behind both Nigeria and SAs' desire for nuclear power is to ultimately acquire nuclear armaments capability - like Iran
Although details of the agreement remain sketchy, it sounds suspiciously similar to the one concluded between South Africa and Rosatom last year (for a nuclear plant that most analysts suggest South Africa doesn’t need; other energy sources, such as renewables, are likely to be cheaper and easier to develop). According to South African investigative journalism outfit amaBhungane, which published the details of the South African agreement last year, the terms of the deal are highly favourable to Russia, and include the creation of a favourable tax regime for Rosatom and a de facto veto clause excluding other bidders from working with South Africa without Russia’s consent.
On the surface, at least, this deal is less about cost-effective power generation and more about global geopolitics, or perhaps even corruption at some level; it is not unreasonable to suspect the Nigerian deal is similarly compromised.