The threat of terrorism is massively overblown across the board, not just with respect to nuclear waste. The means to make conventional explosives are widely available, yet outside of a handful of conflict zones it is rarely done. Americans can buy gunpowder at walmart, yet how many pipebomb attacks have you heard of? A handful, but not many. Not many considering the country has hundreds of millions of people with ready access to the means to make them.
The incidence rate of people inclined to terrorism is vanishingly low. And when you start talking about plots that require numerous people to cooperate, rather than a single lonewolf nut, there is even less to worry about. The more people a plot requires, the less likely it is to actually happen. Coordinated attacks against nuclear power plants or waste disposal facilities are fantasy.
The threat of terrorism was huge and salient in 2002, and a lot of NRC regulations were updated at that time. It was less obvious in 2010. Here in 2021 it's hardly a threat at all. If we see another 9/11-scale attack this decade you can bet that will change.
