Probably a stupid car question...

[Lissa]

... the higher the energy of the incident radiation, the more shielding required. It's not simply getting better reactions or smaller masses or using different methods to gather energy from standard radioactive decay ...

I agree with Lissa on this point. One can say "choose a better nuclear reaction", but nature really doesn't offer one. You can say "choose better shielding materials" but even the best possible still take pretty good thicknesses to work.

The Catch-22 is that if you use low energy reactions, you don't need as much shielding but then the pile has to be much larger to get a sustained reaction. If you use high energy reactions, you can make the pile smaller, but you then need more shielding. Optimizing just for weight, you still end up with something more suitable for railroad engines than cars or trucks. And that optimization involves using some nasty materials and insufficient containment for purely mechanical accidents.

On a (slightly) unrelated note, just how dense does the shielding have to be to impart Bremsstrahlung from Nitrogen-16 decay? I mean, just how energetic is that beta particle event?

Alpha and Beta decay is a non-issue outside the human body. Your own skin will stop the most energetic betas. The problem with Alphas and Betas is when they happen inside the human body. Externally, they're not a threat, internally they're very much one.