Before describing the physics specific to hybrid reactors, it is appropriate to review the basics of nuclear reactor theory. We first recall some elements of neutron physics which apply to both critical and subcritical systems.

 

3.1 Interaction of neutrons with nuclei

 

A — 1 A + 1

 

A — 1 A + 1

 

cos(e)

 

1

 

(3.1)

 

1

 

‘ The absorption cross-section <ra = ac + <rF.

If the scattering in the centre of mass is isotropic it follows that all final energies between E0 and E0{[{A — 1)/(A + l)]2} are equiprobable. Since, in this latter case, the neutron energy loss is proportional to its initial energy, it is convenient to measure energies in terms of lethargy u = ln(E0/E) where E0 is some arbitrary initial energy (usually the average energy of fission neutrons), and E is the actual neutron energy. We define

It is convenient to define the average lethargy gain, or, equivalently, average logarithmic energy loss per collision

f$E° E0 dE, $

C = ln = 1 +

e0 E E0(1 — $) 1 — $

which expressed as function of the mass A yields

For large A, C — 2/(A +1).