The nuclear fuel doesn't really need to be shielded any better during launch than the existing RTGs used on space missions. Scaremongering notwithstanding, thos

The nuclear fuel doesn't really need to be shielded any better during launch than the existing RTGs used on space missions. Scaremongering notwithstanding, those are pretty much impossible for a mere launch accident to "blow to bits" in a way that exposes the PuO, and so would the fuel rods of a proper reactor. At least unless the reactor itself suffers a catastrophic excursion and blows itself to bits à la Chernobyl No. 4, but we've become pretty good at building reactors that don't do that.