Nuclear power reactors include the part where the fission happens, the core, and around that,  neutron and gamma-ray shields. These shields, or you could call them mufflers, conceivably could be as light as 200 tonnes, IIRC, but then two operators leaning on the outside of the shield might get more radiation from the core, through the shield, than from each other's radiopotassium.

A belt-and-braces approach is almost always preferred: a thick enough shield that two operators standing on the same side irradiate each other 100 times more than the reactor irradiates either one, and then another six feet of concrete for good measure.

So you can have a reactor with a small, lightweight core but you cannot have a small, lightweight reactor.

How shall the car gain nuclear cachet?

What a concept.  A much better plan would be to keep the reactors in a safe, already contaminated area and export the electricity.

Than haul the fuel and waste all over the place.  This is the problem we now face with conventional nukes.  Divide each 1200 mw plant into thousands of 200 kw plants?  Doesn't seem prudent.

Who would be responsible when they need decommisioning?  or if terrorists decide to blow one up?  Or a tsunami tumbles one around and breaks it open?  

http://amazngdrx.blogharbor.com/blog John Schneider, Northern Wisconsin

We are accustomed to think that nuclear accident = armageddon but it seems quite likely that smaller nuclear units could easily constitute much smaller and more manageable risk than both large nuclear installations and the non-nuclear alternatives currently being considered for low-emission baseload power: e.g. "clean" coal with CO2 sequestration.

Sure, we all want renewables like solar and wind to do the job but they are not without their own environmental and performance problems. If there is to be an environmentally-acceptable nuclear future, this could well be what it looks like.

The true meaning of life is to plant trees, under whose shade you do not expect to sit.