you wouldnt have to expose them to x-ray. i was thinking about getting some uranium ore a while back, crushing it up, and using it as substrate...
while the purified stuff is illegal to possess, you can get the ore as long as you order small quantities of it. if used as a substrate, it would expose the developing eggs of the marmorkrebs to constant low levels of radiation, which would probably have the highest chances of causing some random mutations in the developing young. in the ore form, it doesnt actually produce a lot of radiation, and being in water, it would pose very little risk to those working with it. water is an excellent modifier.
another source would be old smoke detectors, since they use a radioactive metal in them. its quite a bit more radiation than you would find in natural uranium ore, and i have no idea how likely the metal would be to dissolve in water. i think it would be more toxic than wild uranium ore.
another option would be the old coleman lamps. there is a white cloth looking cover that , when exposed to a flame, glows bright white. again though, its much more radioactive than uranium ore, and i have no idea how likely it is to dissolve.
finally, there is the option of UV germicidal bulbs. they produce a wavelength that is known to damage DNA, and are often used for studies where random mutations in-vivo are preferred to those produced in a PRC. i dont yet know if it would work for shrimp, but i know from experience it works on the eggs of houseflies.