You'd think control over the materials in these small plants would be much better than for the radioactive materials already widespread and easy to obtain, like cesium 137 calibration sources used in healthcare [1]. Apparently one of these would allow a dirty bomb to contaminate 40 city blocks [2]. So I reckon there are easier existing sources for radioactive materials if one wanted to cause mass panic, I don't think spent fuel rods are going to end up in the municipal waste.

1 https://www.perkinelmer.com/au/product/cesium-137-calibratio...

2 https://www.wired.com/2011/10/ff-radioactivecargo/

Dirty bombs are already easy to make: there is radioactive material in every town for medical purpose.