JAEA-Data/Code 2007-004 - Welcome to Research Group for ...

P ijk , g = ViΣ
tr,
m,
g ( ∑V
jΣ
tr,
m,
g ) (homogeneous limit) (7.5-7)
independent of k, we have
j
D h g = ∑V
3∑V
Σ ) .
i
i
( i tr,
m,
g
i
, (7.5-8)
Contrary, if a neutron born in any region i is certain to suffer its first collision in this region (the
assumption that each region is large compared to the mean free path), then
and
Pijk
, = δ
(7.5-9)
g
ij
D ϕ ) .
(7.5-10)
t , g
( Vi
ϕ i,
g
i
i
= ∑Vi
i,
gλi,
g 3∑
It follows at once from the fundamental theorem of algebra
D t, g ≥ D h,
g .
(7.5-11)
The use of D t, g to the cell with voided regions falls into the drawback that the diffusion
coefficient is unreasonably large to be used in practical calculation.
(2) Optional Process IC17 = 2
The isotropic diffusion coefficient is used and defined by
1
D 0 , g = ∑ D k , g .
(7.5-12)
3
k
Then, from the definition of the directional collision probability by Eq.(7.1-16), this diffusion
coefficient can be calculated using the isotropic collision probability defined by Eq.(7.1-13), i.e.,
D ϕ )
(7.5-13)
0,
g
/( Viϕi,
g
i j i
= ∑∑Vi
i,
g λ j,
g Pij,
g 3∑
(3) Optional Process IC17 = 3
The anisotropic diffusion coefficients defined by Eq.(7.5-5) are calculated. These coefficients
are used for the 2-D or 3-D diffusion calculation using the CITATION code.
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