.. _sec-xsproc.mcdancoff:
MCDancoff: Monte-Carlo based Dancoff Factor Calculation
=======================================================
*L. M. Petrie, B. T. Rearden, A. M. Holcomb, and K. S. Kim*
ABSTRACT
The MCDancoff program is used to calculate Dancoff factors in
complicated, three-dimensional (3-D) geometries using Monte Carlo
integrations. The geometries are standard SCALE geometry descriptions,
with the current restriction that Dancoff factors can only be calculated
for regions bounded by cuboids, spheres, or cylinders. Multiple Dancoff
factors can be calculated with one input file. New AMPX one group
library for MCDancoff was developed for SCALE 6.3 based on ENDF/B-VII.1.
ACKNOWLEDGMENT
This work was sponsored in part by Atomic Energy of Canada, Ltd.
Development of new ENDF/B-VII.1 based AMPX one group library for
SCALE 6.3 was sponsored by the US Nuclear Regulatory Commision (NRC). The
contribution of S. J. Poarch in preparing this document is gratefully
acknowledged.
Introduction
------------
MCDancoff (Monte Carlo Dancoff) is a program that calculates Dancoff
factors for complicated, three-dimensional geometries. Its input is a
slight modification of a CSAS6 input file which uses the standard SCALE
geometry as detailed for KENO-VI. The modifications to the input involve
different input in the START data block describing which Dancoff factors
are to be calculated. The calculation involves starting histories
isotropically on the surface of the region for which the Dancoff factor
is to be calculated and following the path of each history until it has
encountered all the elements of the material in the region, or until it
has exited the system. A one group cross-section library is used to
determine the total cross sections of the mixtures in the problem. The
one group library has been developed based on ENDF/B-VII.1 for SCALE 6.3
for which various reaction cross sections were obtained by energy group
collapsing and elastic cross sections (MT=2) were replaced by potential
cross sections.
A current restriction of MCDancoff is that it can only calculate Dancoff
factors for regions bounded by cylinders, spheres, or cuboids. Other
simple bodies could be added in the future, but a general bounding
surface would be impractical.
The Dancoff factors are used in SCALE to correctly self-shield
multigroup cross sections for a given problem; either as input to BONAMI
or to determine an equivalent cell for CENTRM. This is most typically
accomplished through the MORE DATA and CENTRM DATA blocks.
The Dancoff factors are actually calculated by a modified version of the
KENO-VI code called KENO_Dancoff. All printed output from these
calculations is suppressed by default. If there is a need to see this
output (for example, to find an error message), it can be turned on by
setting an environment variable **print_dancoff=yes**.
Input data description
----------------------
MCDancoff input data is the same as CSAS6 input data with the following
exceptions. A special one group cross-section library will be used. It
can be specified as **xn01** in the input but will be set to this if
anything else is entered for the library. Because MCDancoff is running a
fixed source problem, and the Dancoff factor doesn’t need to be
calculated with the same accuracy as an eigenvalue, there are useful
changes that can be made to the parameters in the PARAMETER data block.
:numref:`7-8-3` discusses this in more detail. Finally, the START data
block is used to define which Dancoff factors will be calculated. This
data block is defined below.
::
READ START Begins the data block
1. dancoff
begins defining a new Dancoff factor. Always start
relative to the global unit in the geometry.
2. array
step into an array contained in the current unit – followed
by karray, nbx, nby, nbz where karray is the
region containing the array in the current unit, nbx is the x
position in the array of the next unit, nby is the y position
in the array of the next unit, and **nbz** is the z position in
the array of the next unit.
3. hole
step into a hole contained in the current unit – followed by
nhole, the hole number relative to the current unit.
4. unit
final unit in the nesting chain – followed by **nn**, the
unit number
5. region
region to calculate the Dancoff factor for – followed by
k, the relative geometry word in unit nn defining the
outer bound of the region.
6. nst
if input, must be 0 (defaults to 0).
Repeat 2 and 3 to get from the global unit to the final unit nn.
Repeat 1–5 for each Dancoff factor to be calculated.
END START Ends the data block
.. _7-8-3:
Calculation and use of 3D Dancoff factors
-----------------------------------------
1. The 3-D Dancoff factors are computed with KENO-VI geometry. If
beginning with CSAS5 model, use C5TOC6 to convert to CSAS6.
2. Change sequence name from CSAS6 to MCDancoff and change cross-section
library to **xn01**.
3. Input appropriate parameter data.
..
Since the Dancoff calculation is fixed source integration, there is
no need to skip generations, and **nsk** should be set to 0. Since
small changes to the Dancoff have very minor effects on the cross
sections, fewer histories are probably needed for calculating the
Dancoff than for calculating *k\ eff*. Thirty thousand histories
divided as 100 generations of 300 histories per generation has
produced Dancoff factors with deviations of less than 1 percent. It
may be advantageous to turn off plots at this point. Since the same
parameters can be entered more than once, with the final entry being
the one used, adding a separate record with these values immediately
before the **end parameter** keywords would override the original
KENO-VI parameters.
Example:
.. highlight:: scale
..
::
read param
.........
nsk=0 npg=300 gen=100 nub=no fdn=no flx=yes plt=no
end param
4. Identify the region for which Dancoff factors are desired in START
data.
..
The start type needs to be set to **0** for the Dancoff calculation
(this is the default). All KENO-VI START data should be removed or
commented out by placing an apostrophe in column 1. Each region for
which a Dancoff calculation is desired then starts with the keyword
**dancoff**. This is followed by data that specify the relationship
of the global unit to the specific geometry description of the
region. If the region is nested inside an array, then the keyword,
**array**, is specified, followed by four integers. The first integer
is the indices of the media record specifying the array relative to
the current unit. The next 3 integers are the X, Y, and Z indexes of
the position of the next unit in the array. If the region is nested
in a hole, then the keyword, **hole**, is specified, followed by the
relative count of the correct hole in the unit. The preceding data
are repeated (in the correct nesting order starting with the global
unit) until reaching the unit where the region is located. Then the
keyword, **unit**, followed by the unit number is given, followed by
the keyword, **region**, followed by the relative index of the
geometry keyword describing the desired region with respect to that
unit. Currently, only cylinders, spheres, and cuboids are programmed
for calculating Dancoff factors.
Examples:
::
read start
nst=0
dancoff hole 1 unit=1 reg=1
end start
read start
dancoff array 1 1 1 1 array 1 17 17 2 unit 10 region 1
end start
5. Execute MCDANCOFF *filename.*\ input file like any other SCALE input
file.
6. Examine *filename*.dancoff file, which will contain Dancoff factors
for each nuclide in the specified region
::
index nuclide dancoff deviation
1 92234 3.36340E-01 1.81134E-03
2 92235 3.36340E-01 1.81134E-03
3 92236 3.36340E-01 1.81134E-03
4 92238 3.36340E-01 1.81134E-03
5 8016 1.00000E+00 0.00000E+00
7. Once all desired Dancoff factors are obtained, return to original model
and enter CENTRM DATA for each cell with dan2pitch(mix) specified.
::
read celldata
latticecell triangpitch fuelr=0.633 1 gapr=0.637 0 cladr=0.675 10 hpitch=0.867 14 end
centrm data
dan2pitch(1)=0.336
end centrm
8. If executing TSUNAMI-3D, additional steps are necessary because
TSUNAMI-3D does not treat the dan2pitch input parameter.
..
Return to the original TSUNAMI-3D input file and replace the sequence
name to "CSAS-MG PARM=CHECK" and delete all data after the unit cell
data to quickly obtain revised pitch values. (Note: CSAS will not
modify cell dimensions to more than 20 cm, so a revised moderator
density may need to be entered to obtain the desired Dancoff factor.)
Search for the word "\ *desired"* in output file to find new pitch
values for each cell.
::
unit cell = 1
original pitch = 1.7340E+00
Dancoff for orig pitch = 2.9728E-01
desired Dancoff = 3.3600E-01
pitch to produce desired Dancoff= 1.6845E+00
9. Enter revised pitch and revised moderator density (for cell
calculation only, not for geometry model) in TSUNAMI model.
Example Case
------------
The following is a contrived case to illustrate an input file using both
holes, arrays, and multiple sets of Dancoff factors (although both
factors apply to the same pin, so only one set can be used). The case
represents two fuel assemblies in a cylindrical tank, each assembly
having a poisoned central pin, and four water holes. The Dancoff factors
are calculated for each central pin. The input file is listed in
:numref:`list7-8-1`.
.. code-block:: scale
:name: list7-8-1
:caption: Example input file.
=mcdancoff
sample case demonstrating calculating Dancoff factors
xn01
read composition
uo2 1 den=10.38 1 294 92234 .0303 92235 4.7378 92236 .1364 92238 95.0955 end uo2
zirc4 2 1 294 end zirc4
h2o 3 1 294 end h2o
uo2 4 den=10.08 1 294 92234 .0303 92235 4.7378 92236 .1364 92238 95.0955 end uo2
gd 4 den= 0.3 1 294 end gd
end composition
read param
nsk=0 gen=100 npg=300
end param
read geometry
unit 1
com=!fuel pin!
cylinder 10 0.395 40.0 -40.0
cylinder 20 0.410 40.0 -40.0
cylinder 30 0.470 40.0 -40.0
cuboid 40 4p0.65 2p40.0
media 1 1 10
media 0 1 20 -10
media 2 1 30 -20
media 3 1 40 -30
boundary 40
unit 2
com=!water hole!
cuboid 40 4p0.65 2p40.0
media 3 1 40
boundary 40
unit 3
com=!unit containing a 2x2 array of fuel pins!
cuboid 10 4p1.30 2p40.0
array 1 10 place 1 1 1 -0.65 -0.65 0.0
boundary 10
unit 4
com=!unit containing a 1x2 array of fuel pins!
cuboid 10 2p0.65 2p1.30 2p40.0
array 2 10 place 1 1 1 0.0 -0.65 0.0
boundary 10
unit 5
com=!unit containing a 2x1 array of fuel pins!
cuboid 10 2p1.30 2p0.65 2p40.0
array 3 10 place 1 1 1 -0.65 0.0 0.0
boundary 10
unit 6
com=!unit containing a 5x5 array of fuel pins!
cuboid 10 4p3.25 2p40.0
array 4 10 place 2 2 1 0.0 0.0 0.0
boundary 10
unit 7
com=!unit containing a 5x5 array of fuel pins - water hole in the middle!
cuboid 10 4p3.25 2p40.0
array 5 10 place 2 2 1 0.0 0.0 0.0
boundary 10
unit 8
com=!unit containing a 5x5 array of fuel pins - poisoned pin in the middle!
cuboid 10 4p3.25 2p40.0
array 6 10 place 2 2 1 0.0 0.0 0.0
boundary 10
unit 9
com=!poisoned fuel pin!
cylinder 10 0.395 40.0 -40.0
cylinder 20 0.410 40.0 -40.0
cylinder 30 0.470 40.0 -40.0
cuboid 40 4p0.65 2p40.0
media 4 1 10
media 0 1 20 -10
media 2 1 30 -20
media 3 1 40 -30
boundary 40
unit 10
com=!unit containing a 15x15 fuel assembly!
cuboid 10 4p9.75 2p40.0
array 7 10 place 2 2 1 0.0 0.0 0.0
boundary 10
global
unit 11
com=!global unit with 2 fuel assemblies!
cylinder 10 25.0 60.0 -60.0
hole 10 origin x=-10.0
hole 10 origin x= 10.0
media 3 1 10
boundary 10
end geometry
read array
ara=1 typ=square nux=2 nuy=2 nuz=1 fill f1 end fill
ara=2 typ=square nux=1 nuy=2 nuz=1 fill f1 end fill
ara=3 typ=square nux=2 nuy=1 nuz=1 fill f1 end fill
ara=4 typ=square nux=3 nuy=3 nuz=1 fill 3 4 3 5 1 5 3 4 3 end fill
ara=5 typ=square nux=3 nuy=3 nuz=1 fill 3 4 3 5 2 5 3 4 3 end fill
ara=6 typ=square nux=3 nuy=3 nuz=1 fill 3 4 3 5 9 5 3 4 3 end fill
ara=7 typ=square nux=3 nuy=3 nuz=1 fill 7 6 7 6 8 6 7 6 7 end fill
end array
read start
' first Dancoff - calculate for the poisoned fuel pin in unit 9 for the x=-10 assembly
dancoff
' hole 1 is unit 10 at x=-10
hole 1
' array in first region of unit 10 is array 7 - 2 2 1 position is unit 8
array 1 2 2 1
' array in first region of unit 8 is array 6 - 2 2 1 position is unit 9
array 1 2 2 1
' cylinder labeled 10 in unit 9 is the first region
unit 9 region 1
' second Dancoff - calculate for the poisoned fuel pin in unit 9 for the x=+10 assembly
dancoff
' hole 2 is unit 10 at x=+10
hole 2
' array in first region of unit 10 is array 7 - 2 2 1 position is unit 8
array 1 2 2 1
' array in first region of unit 8 is array 6 - 2 2 1 position is unit 9
array 1 2 2 1
' cylinder labeled 10 in unit 9 is the first region
unit 9 region 1
end start
end data
end
This input file creates two files of Dancoff factors. The first such file is listed in :numref:`list7-8-2`.
.. code-block:: scale
:name: list7-8-2
:caption: Output file of Dancoff factors.
Unit 9 at global x -1.00000E+01 y 0.00000E+00 z 0.00000E+00
index nuclide dancoff deviation
1 92234 2.20873E-01 1.03436E-03
2 92235 2.20873E-01 1.03436E-03
3 92236 2.20873E-01 1.03436E-03
4 92238 2.20873E-01 1.03436E-03
5 8016 9.64748E-01 4.28121E-04
6 64000 2.82254E-10 3.61320E-11
The second file is statistically the same, as it solved for the mirror image Dancoff factor.