diff --git a/300s School/NUCE 2100 - Fundamentals of Nuclear Engineering/2024-09-24 Week 5.md b/300s School/NUCE 2100 - Fundamentals of Nuclear Engineering/2024-09-24 Week 5.md
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 # 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.
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+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.
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