[TexasGator]

“But why can’t you go really small?”

As in eight grams? Because you have to have a critical mass and eight grams ain’t gonna get it done.

Doubly for the MSR since the fuel is dispersed in the cooling medium the total amount is going to be multiple of a critical mass.

See the below. Note the thick concrete walls around the entire plant.

http://en.wikipedia.org/wiki/Molten-Salt_Reactor_Experiment

[frithguild]

I have seen the MSRE depictions many times. The MSRE operated at a 650c deg peak for 1.5 years. Just because they used that much concrete in an experiment does not mean that you need that much shielding material. So please.

Ok 8 grams is ridiculous - I'll bite. Why is the mass of the fuel a factor for criticality? I thought the primary factor in achieving criticality was the proximity of the fuel materials, not the volume.