---
title: Week 5
allDay: false
startTime: 18:00
endTime: 21:00
date: 2024-09-24
completed: null
---
# Module 6.1 - Neutron Balance
Talks about production rate of neutrons, absorption rate of neutron, and leakage rate. Gets more specific about how we define things.
# Module 6.2 - Diffusion Theory
Flux $\phi$ as a function of space is a hard thing to calcualte.
**The Diffusion Approximation**
Since neutrons have an equal probability of traveling in any direction, then it is reasonable to assume that neutrons will "diffuse" from regions of high density (flux) to regions of low density
Scalar flux: $\phi$ but minus the little top thing 
It's a good approximation when in large, highly scattering materials.
It's a bad approximation when:
1. Near leakage boundaries
2. Near strong absorbers or sources
3. In a vacuum
4. When scattering is strongly asnisotropic.
# Module 6.3 - Neutron Diffusion - Non-Multiplying Media
s(x) is generation
Basically it's just solving differential equations. 
# Module 6.4 - Multiplying Media
Add fission
Modes can happen.
Interestingly, the higher order modes of neutron diffusion are subcritical. They fade away over time
