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

i
n m≠n
i
n
i
n
i
n
i 1
i i g(
C )(1 − C )
σ
0, n
= ∑(
N
mσ
t,
m
) +
(1.6-2)
i
N
N L
i a
g(
Cn
) = (1.6-3)
1 + ( a −1)
C
i
n
where N is the atomic number density, L the mean chord length, C n the nuclide dependent Dancoff
correction factor 19) and a the Bell factor defined in the SRAC code by the geometry of absorbing
mixture.
The nuclide dependent Dancoff factor is specified optionally by the input value or the calculated
value assuming the black limit in case that the collision probability method is used. The nuclide
dependence of Dancoff factor is effective for the lattice including two or more different kinds of
resonant mixtures.
The infinite dilution cross-sections are given to the nuclide which has no f-table or to every
nuclide of the energy group out of range of tabulation. Current available libraries of SRAC have
f-tables for all resonant nuclides as far as resonance levels are evaluated in nuclear data file. f-table is
given for the reactions capture, fission, total, elastic, and elastic removal in the fast energy range, and
reactions capture, fission and total in the thermal energy range.
Dancoff correction by Tone’s method 20) is also available. In this method, the background
cross-section is calculated by the following formula;
∑∑
j j
N m Pj→iσ
t,
m
j m≠n
i
σ 0, n =
, (1.6-4)
j j
N P V
∑
j
n
j→i
where collision probability from region j to region i,
Pj
→ i
V
j
V j volume of region j.
The cell heterogeneity is evaluated so as to consider contributions from each region in a unit fuel cell
through collision probabilities. This method is expected to be effective for plate type fuel cell, where a
resonant nuclides exists in several composite mixtures with different number densities. It should not be
used for a normal pin type fuel cell.
1.6.2 IR Approximation
This method can be applied to the problem where a cell has only one resonant mixture (i=1
only). The microscopic effective resonance cross-sections are given by Eq.(1.6-5) by the interpolation
of f-table as well as the NR approximation.
13