[Accujack]

Most of the nuclear waste that exists is from weapons production. Nuclear generating plants produce only a very small amount.

Right now, small enough amounts of waste are produced that reactors generating power actually store the stuff on site.

>but could the be modified?

Modern reactor types are specifically designed not to be proliferation risks. The only reason the older reactor types are risks is because the governments who originally built them wanted to produce weapons, so they chose the technology that allowed them to do so.

Waste could be used for a "dirty bomb" in some sense, but it wouldn't be terribly effective. "High level" is relative, and the isotopes that would make a dirty bomb truly scary aren't available except in fuel rods shortly after their removal from a reactor... at which point no one does anything to extract those isotopes anyway, they just stick the fuel in cooling ponds to decay down to lower levels of radiation.