Could exposure to radiation cause cancer 30 years later?

Yes, unfortunately. For example, patients with hodkin's lymphoma who received radiation therapy to the chest wall can have increased risk to develop breast cancer due to the radiation-years after the radiation itself. Patient who received radiation for acne or head & neck cancer- can develop thyroid cancer- many years after radiation therapy.
Yes. Yes, this is well-documented. Head and neck radiation used to be used to treat radiation and decades later can be associated with increased risks of thyroid cancer.