[jamest (OP)]

Laurie with this: if we can put small nuclear reactors on submarines and the Mars rovers, why can?t those small nuclear reactors be used along with sustainables to power our towns and cities?

[alancalverd]

It's been done, particularly in Siberia, but it only works for very small, isolated towns - about the population of a submarine. And the Mars rover doesn't have a reactor, just a thermoelectric generator powered by the unregulated decay of plutonium.   

The object of grid transmission, whether of electricity, gas, aviation fuel, or water, is to provide local resilience through global diversity. If you are going to use, say, offshore wind, you need a long, high power, primary connector anyway, so you may as well add  a large nuke or coalfired power station with all the benefits of scale (whether you are generating 1 kW or 1GW at a station, you still need two controllers and a policeman at the gate, so why not generate a gigawatt?) and concentration of hazard.   

We can already see the problems of inadequate provision for diversity, when wind farmers in Scotland are paid to shut down and gas burners in England are fired up, because there is insufficient transmission capacity to move peak supply to the point of peak demand. Even assuming you can define "local" (where does a farm halfway between cities A and B get its supply from?), each local generator needs to be able to supply local peak demand, which means it will be working at 20% capacity most of the time.

Radiation shielding of small reactors is an economic problem which has also prevented their use in aviation. Essentially, if you put a nuke in the tail of a ship, you only need to shield one bulkhead, since the reactor is mostly surrounded by fish when it's in full steam, and you can run the ship on batteries if you are close to other folk. Problem with an airplane is that you need maximum power and minimum weight to take off, with the passengers staring at the  naked reactors hanging from the wings. The economics comes down to a surface to volume ratio question. If you need a meter thickness of concrete to shield the core, then the bigger the core, the smaller the ratio of shielding material to power output, so build big wherever you can. 

Which leads me to a neat idea for a sci-fi plot. If you put a small, naked reactor in space, it could be a thousand miles from the point of demand. So we could have a nuclear propulsion unit towing a "space glider" full of passengers......

[Petrochemicals]

Maybe, but as they use a greater proportion of U235 than conventional plants the fuel costs will increase as fuel becomes scarce.nuclear plants at present are not particularly cheap.

[paul cotter]

I think there is still plenty of uranium ore available as it is a common element on earth. Thorium is even more common and a lot of it goes to waste in separating lanthanide elements from monazite.

[alancalverd]

Even more reason to "go big" - thorium reactors need a larger startup investment than uranium-fuelled nukes so are more likely to be developed on the basis of national strategy than private capital.

Time was (1950s) that the UK was very good at strategic infrastructure, but governmental incompetence has since grown exponentially, to the point that we can't even build 100 miles of railway track or separate sh1t from rivers any more, so I don't see this happening here in my lifetime.

[Petrochemicals]

I think there is still plenty of uranium ore available as it is a common element on earth. Thorium is even more common and a lot of it goes to waste in separating lanthanide elements from monazite.

Maybe, but the is plenty of oil in Canada, but it's so much cheaper and easier to go to Saudi arabia and pick at the ground slightly. The more you rely on U235 the pricier it gets.

[Bored chemist]

If you build a reactor that fits in a shipping container(which is possible) then you need to be very sure that nobody can put it on a truck, drive it into a city and dynamite it.

Good luck with that

[paul cotter]

In reply to Petro: yes, there is plenty of oil in the Canadian tar sand deposits but it is heavy dirty oil that takes a lot of processing to turn it into any fuel other than bunker oil. It no way compares favourably with Brent light crude.