Four factor formula

The four- factor formula describes how the multiplication factor of an infinitely extended homogeneous thermal nuclear reactor results from four parameters.

Formula

Importance

→ Main article: multiplication factor

Is the multiplication factor of the above-mentioned reactor:

• , The reactor is assumed to be infinitely great, that is, occurring in fact seepage or leakage losses ( losses neutrons through the surface of the reactor to the outside ) can be neglected. The effective multiplication factor of a finite homogeneous reactor is therefore always less than.
• , The reactor is assumed to be a homogeneous mixture of all of its materials. A real reactor is almost always made ​​up of heterogeneous nuclear fuel, structural material and moderator / coolant. This can decrease or increase depending on the structure compared to the homogeneous case.

Explanation of the four factors

This number is less than the neutron yield per division because there is absorption in the fuel which do not lead to cleavage. is the cross section for fission, the cross section for absorption.

For 235U

The fast split factor takes into account that even fast neutrons can cause fissions.

Resonance passage probability

Also, thermal neutrons can be absorbed by the Moderator or other non- fissile material (parasitic absorption). is the macroscopic cross section for the absorption of thermal neutrons in the fuel, one for absorption somewhere in the material.