Think about this for a minute
1 – how angry and full of a desire for vengeance will ukrainians be after this little stunt?
2 – do ukrainians have access to reall nasty radioistopes?
3 – how porous is the border between ukraine and russia generally?
4 – how difficult is it to drive a tanker truck laden with heavy ezplosives and the nastiest istopes into, say, the centre of Moscow in a few years? How about several such trucks? The panic alone would next to tear apart a city such as Moscow. 

Now imagine this material being dispersed by an ACTUAL nuclear device.  Then imagine the expelled material being a mix of radioactive cobalt, vaseium isotopes and plutonium. All these can be collected from currently active nuclear power plants operational in Ukraine. There are right now 15 such active plants, excluding the chernobyl exclusion zone. Also, Ukraine is a natural resource-rich country that has large reserves of minerals like uranium and titanium. As Europe’s top producer of uranium and with vast deposits of titanium, Ukraine ranks high in the world for its key resources.

https://iopscience.iop.org/article/10.1088/1748-0221/15/02/P02019/pdf
https://www.files.ethz.ch/isn/22204/dirtybombs.pdf

I think the Russian state just signed its own death warrant.