Probably a stupid car question...

[Lissa]

... So, you have to have shielding of some kind and you have to have a given amount for it to cut down on the radiation.

I think you misunderstood me. I'm not saying that shielding is unnecessary, I'm saying that by choosing better nuclear reactions you can minimize the amount of harmful radiation emitted, and by choosing better shielding materials you can minimize the size and mass of the shielding. Water, concrete, earth, or lead are abundant and cheap, which is why for terrestrial uses or even in large vehicles (like aircraft carriers, or submarines) bulk is still not an issue.

Another point I'm trying to make is that while there have been some advances in making nuclear power smaller and safer, essentially a simple critical mass nuclear reaction is still used to generate steam power. That is a fairly archaic and inefficient way to transfer the power of the strong nuclear force into motive or electrical power. Given more attention and less political antipathy, quantum chromodynamics might have progressed much further than it has. For example, allowing for the direct conversion of the strong force into the electroweak force thereby eliminating waste altogether.

No, I'm understanding quite perfectly, what you're not understanding is how much materials are needed and what type. Radiation, depending on type and energy level, will be stopped by some amount of shielding, 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 (which is what the SNAPs do), it's stopping the various radiations that are difficult to stop (externally to humans, gammas and neutrons). We know quite a bit about radiation interaction from both Gamma Rays and Neutrons, and we know that we have to have certain materials with certain thicknesses to limit the exposure of these types of radiation. It's not as simple as you think it is to outright block radiation exposures, you have to have the right materials and those materials have to be the right thickness to limit exposure.