If we consider the entire cycle from construction and fuel mining+refinement up to the storage of fission products and decomissioning of a power plant, it's very questionable if fission power has ever been economically viable.

Kristin Schrader-Frechette's 2009 paper Climate Change, Nuclear Economics, and Conflicts of Interest (DISCLAIMER: Contact the author or use unpaywall services at your own discretion) analyses 30 studies on the economic viability of fission power plants and finds that all papers that were funded by pro-nuclear agencies or nuclear-friendly governments trimmed costs in one or, more often, many ways.

Quote:

One of the interesting facts about these 30 prominent nuclear-cost studies is that a majority of them appear to trim nuclear-cost data in at least three ways. Subsequent paragraphs argue that a majority of these studies ignore taxpayer subsidies that reduce nuclear costs, ignore reactor-construction times and interest costs associated with nuclear-plant-construction capital, and inflate reactor capacity (or load-factor) and lifetime data.