Over the first 43 years of the Price-Anderson Act to 2000, the secondary insurance was not required. A total of $151 million was paid to cover claims (including legal expenses), all from primary insurance, including $71 million for Three Mile Island. Additionally, the Department of Energy paid about $65 million to cover claims under liability for its own nuclear operations in the same period.
The handout is mostly theoretical in the sense that, if something did happen it would become a significant handout, but since the nuclear industry in the US hasn't screwed up in any catastrophic* way since it's inception, the theoretical handout hasn't been required. The actual handout of 151 million dollars over 43 years (about 3.5 million per year) is tiny by government standards.
* Catastrophic in this case meaning like Chernobyl, not TMI. TMI was the biggest single payout under this act, weighing in at almost half the total.
"No, no, I don't mind being called the smartest man in the world. I just wish it wasn't this one." -- Adrian Veidt/Ozymandias, WATCHMEN