Lecture 6 Flashcards
Under steady state conditions, neutron conservation requires that
(neutrons leaking) + (neutrons absorbed) = (source neutrons emitted) + (fission neutrons produced)
The neutron diffusion equation is
Fick’s approximation is
relating the current to the flux so that
D =
Diffusion equation for uniform, non-multiplying system
diffusion area =
L2=D/Σa
Differential equation for source-free, plane geometry
general solution of
Boundary conditions for a source-free example in semi-infinite plane geometry
For a uniform source in plane geometry, φh =
For a uniform source in plane geometry, φp =
boundary conditions for uniform source in plane geometry
cosh(x)=
sinh(x) =
The vacuum boundary condition
The vacuum boundary condition 2/3λ is the ______, at which point the flux is _____.
extrapolation distance, zero
If the solution is symmetric about x=0 such that the _________ is zero, then we may solve the problem only for _____, and bc becomes
net number of neutrons crossing the plane, x>0, dφ(0)/dx=0
For a surface source
s’’
albedo eqn
α = (1-2D/L)/(1+2DL) or
Flux for surface source
Diffusion theory requires continuity of ______, ______, and _____ at interface conditions.
flux, normal component of current,
If there is a localized source along the interface, the diffusion equation becomes
For a point source, sp =
Neutron diffusion eqn in sperical geometry
Find current equation for point source
Jr =
For a point source, φ(r) =
Find φh for spherical geometry with uniform source
Find φp for spherical geometry with uniform source
Boundary and interface conditions for a uniform sphere in non-multiplying medium
Diffusion theory is valid for situations where neutrons ______ with a _____ distribution. And if the neutron distibution is changing _____ in space.
travel in all directions, uniform, gradually
Diffusion theory leads to significant errors for saptial distributions of neutrons near _____ and at ______.
boundaries, materials with sharply different properties
In reactor lattices consisting of fuel, coolant, moderators, and other materials, we use diffusion theory to examine the _____ distribution of neutrons only after taking appropriate _____ over the lattice cells.
global, averages
Diffusion length, L, in terms of r
Diffusion length in terms of λ and c
The uncollided flux drops off as _____ while total flux drops of as ____
1/r2, 1/r
root mean square idstance between birth and first collision
For the diffusion equation to give reasonable accuracy, ____ of the neutron populatin at a point can remain uncollided
only a small fraction
material buckling (B2) equation
Geometric buckling equation
B2=
In a reactor, the flux level becomes _____ when it is critical
infinite
For a fixed source, the flux level __________ with increasing k∞
increases
laplacian in cartesian coordinates
Laplacian in cylindrical coordinates
Laplacian in spherical coordinates