We give a schematic determination of critical sizes and masses of two model homogeneous reactors: a lead cooled fast neutron reactor and a heavy-water moderated thermal neutron reactor.

Fast reactor

In the one-group diffusion formalism, the critical size of a spherical homo­geneous reactor is given by equation (3.38)

2T2 — L

ki = 1 + — дГ (3-78)

with

^

c _ xa _ 3sasT which leads to the minimum size of the sphere,

(3.80)

The physical characteristics of the medium components are given in table 3.3. The relative atomic concentrations are also given in table 3.3. The relative concentration of 232Th and 233U are in the proportion required for regeneration:

The number of lead atoms is taken as set to that of the fuel atoms. Table 3.4 gives the macroscopic cross-sections.

With a value of v = 2.53 one gets

ki= VP = 1.17

Ea

and

and

The mass of the fuel is around 7.2 metric tons.