I think they used to have the calutrons there that would take the partially-enriched material from the K-25 plant (the Oak Ridge GDP) and fully enrich it to weapons-grade material. When plutonium became the material of choice for weapons the calutrons were used for stable isotope enrichment, but those have been retired in favor of "cheaper foreign suppliers" (sound familiar?). So the Y-12 plant is essentially a decommissioned facility.
Yet another example of how this country has sold out it's technological soul.
If you ever had to deal with the traffic in and out of Y-12 during shift change, you'd say otherwise. Y-12 is still a busy facility, involved in the manufacture, decommission, and refurbishment of nuclear weapons. It's still a very active facility.
K-25 - the enrichment facility in the next valley over - that's been decommissioned, and is going through clean up and refurb for re-use.