Why do you claim the coal ash is intended to be "protecting against" the nuclear plant waste and not just different types of radioactive waste being buried together?
Coal ash is not classed as radioactive and it was abundant and cheap as a useless by-product of burning coal. The point is more that things classed as "low level" waste from nuclear are most often completely harmless, just regulated differently due to public understanding of the word "nuclear". As such, its disposal is heavily regulated.
Coal ash is commonly used in cement [1]. I would suspect the nuclear waste is being encased with cement to prevent its leakage with of course the punchline being that the encasement is more radioactive than its contents.
