# 2 energy groups

1. Oct 14, 2013

### sandon

Assumptions
1) a=absorption
2) f=fission
3) ∅=neutron flux
4) time independent
5) group 1, fast neutrons
6) group 2, thermal neutrons
7) All fission neutron are boring in fast group
8) All neutrons created by thermal group, thus vƩf2 exists vƩf2 does not
9) Down scattering occurs but up scattering does not, Ʃ1→2 exists, Ʃ2→1 is zero

From the prof produced 2 equations
(1) -(Ʃa1+Ʃ1→2)*∅1 + v*Ʃf2*∅2=0
(2) -Ʃa2*∅2 + Ʃ1→2*∅1=0

(1) Equation says the total neutron density of all fast neutrons(including ones scattered) is equal to the neutrons generated by thermal fission. This is correct by assumptions 7) and 8).

(2) I do not understand equation 2, why is the neutron density of the amount of slow neutrons absorbed equal to the neutron density of scattered neutrons?

2. Oct 14, 2013

### Staff: Mentor

In equilibrium (!), the number of thermal neutrons is constant. Absorption removes as many thermal neutrons as scattering produces new thermal neutrons.

I don't think "boring" is what you mean.
Is that a typo?

3. Oct 14, 2013

### sandon

sorry
7) All fission neutron are born in fast group

4. Oct 14, 2013

### sandon

8) All neutrons created by thermal group, thus vƩf2 exists vƩf1 does not