MODULE 4

Section 1: 

Prompt Neutrons

Section 2: 

Delayed Neutrons

Photoneutrons

Section 4: 

(n,2n) and (n,3n) reactions

Section 5: 

Exercises

Photoneutron Production

Prompt and delayed neutrons are produced as a result of fission, while photoneutrons are a characteristic of D₂O reactors or Be reflector:

(4-4)

These reactions are referred to as (g,n), since one gamma photon interacts with a nucleus and expels a neutron.

It is necessary to have g with energy levels in excess of 2.23 MeV in order for this reaction to be possible, since the energy of the photon must be larger than the binding energy of the deuterium, which is 2.23 MeV.

When the reactor has been operating for a certain period of time, the fuel accumulates a stock of fission products, whose decay via g  provides energy of over 2.23 MeV.

This source of photoneutrons will continue to exist, even when the reactor is out of service, since the  decaying fission products can still produce photoneutrons in the D₂O present in the reactor core.

This is a peculiarity of the D₂O reactor or Be reflector (see Exercise 3).

Another condition is the need to have high-energy g.

In the first startup stage, the photoneutrons are not yet present, since we are dealing with a new core.

In subsequent startups, they will be of vital importance.

(Exercise 2).

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