[greenavocado]

I wonder if it would make sense to make ultra heavy spent nuclear fuel into gigantic flywheels for short-term grid energy storage

[myrmidon]

No. Because flywheels are not limited by material density, they are limited by centrifugal force.

High density is actively bad, you want to maximize strength and minimize density for flywheel designs, and this makes you much more likely to end up with low density composites (rather than high density tungsten alloys or somesuch).