Spacecraft reactor sheilding and heat radiators

[Jeremy Thomson]

I've been watching "The Expanse" and considered, if the spaceships are some sort of fusion powered drive, why aren't they covered in radiator panels to reject all the heat from the reactor/drive system? How many square meters of radiator panels would a 20MW (thermal) fission reactor require?
An idea for the overall design of a nuclear powered spacecraft is to have the habitable section in the nose, a looooong truss section (100s of metres) back to the reactor and rocket. This distance reduces the radiation the crew receives by some cubic function of the distance? Is is better to put the shielding right behind the habitable section or right in front of the reactor, or doesn't it matter?

 

[rootone]

My guess is that it would be better to have the shield close to the habitat section.
There, it has less amount of radioactivity to deal with, so is more effective at stopping what there is.

 

[glappkaeft]

I've been watching "The Expanse" and considered, if the spaceships are some sort of fusion powered drive, why aren't they covered in radiator panels to reject all the heat from the reactor/drive system?

Because the people writing the script thinks the current designs look cooler. For more realistic designs look for instance at the computer game "Children of a Dead Earth", the film 2001 or the board game "Attack Vector: Tactical".

When it comes information on realistic Science Fiction radiators, atomic rockets got you covered http://www.projectrho.com/public_html/rocket/basicdesign.php#radiators

My guess is that it would be better to have the shield close to the habitat section.
There, it has less amount of radioactivity to deal with, so is more effective at stopping what there is.

Shielding from neutrons and most other radiation don't behave that way. X meters of shielding absorbs 50% of the radiation. You want it close to the reactor so that it can be made as thick as required with the smallest diameter possible.