The physics of a fast reactor depends on the materials that compose the core and on its size and shape but not strongly on the details of its structure, as explained in section 1.1.2. In broad terms the composition is determined as follows. The only material in the core that is essential to the physics is the fuel (unlike a thermal reactor in which the moder­ator is also essential), but the demands of heat transfer usually require that about 50% of the core volume is occupied by coolant. Structural material takes up another 20% or so leaving about 30% for fuel.

For a sodium-cooled power reactor in which the heat rating of the fuel is maximised the various demands of heat transfer and heat transport, which are discussed in Chapter 3, limit the height of the core to about 1 m and the average power density to about 500 MW m-3. The diameter of the core is then determined by the required power output, and criticality is adjusted by changing the proportion of fissile material in the fuel. For gas-cooled or lead-cooled reactors the power density is lower, possible about 200 MW m-3 or less, and the optimum core height is greater, possibly 1.5 m for gas coolant or 2 m for lead.

Most fast reactors that have been built so far, apart from experi­mental or test facilities, have been cooled with sodium and designed primarily to produce power.