A minute of internet research suggests that specific $100m figure is from a 45-year-old law[1]. I don't know why every government law and regulation that references specific monetary values like this aren't pegged to inflation. That equivalent value today is almost $400m.
EDIT: Actually the number might come from a 29-year-old amendment[2]. It is disappointing how hard it is to track these things down.
[1] - https://www.nrc.gov/about-nrc/regulatory/rulemaking/flexibil...
[2] - https://www.congress.gov/bill/104th-congress/senate-bill/942