Finishing the MAVRIC section

CAAScapability.rst

+.. _CAAScapability:
+
+MAVRIC Appendix A: CAAS Capability
+==================================
+
+Introduction
+------------
+
+Modeling criticality accident alarm systems (CAAS) presents challenges
+since the analysis consists of both a criticality problem and a
+deep-penetration shielding problem :cite:`peplow_criticality_2009,peplow_criticality_2010`. Modern codes
+are typically optimized to handle one of those types of problems but
+usually not both. The two problems also differ in scale—the criticality
+problem depends on materials relatively close to the fissionable
+materials while the shielding problem can cover a much larger range.
+SCALE now contains fully three-dimensional tools to perform both parts
+of a CAAS analysis.
+
+CAAS analysis can be performed with SCALE using the KENO-VI criticality
+code and the MAVRIC shielding sequence. First, the fission distribution
+(in space and energy) is determined by KENO-VI. This distribution is
+saved to a file using a user-specified three-dimensional mesh grid and
+an energy structure from the cross section library (or a user-defined
+energy structure). MAVRIC then uses the fission distribution as the
+source for a shielding calculation. MAVRIC is designed to implement
+advanced variance reduction methods to calculate dose rates or detector
+responses for difficult shielding problems.
+
+For different types of shielding and different combinations of sources
+and detector locations, different strategies can be used within
+SCALE :cite:`peplow_methods_2012,wetzel_caas_2012`. Due to the way that cross sections for
+neutron reactions that create photons are stored in ENDF, some of the
+parameters used in the CAAS capability have changed since SCALE 6.1.
+Please be sure to understand and follow the new guidance for the correct
+accounting of secondary photons from neutron
+reactions :cite:`miller_guidance_2013,miller_guide_2013,miller_corrected_2013`.
+
+Methods
+-------
+
+The CAAS capability in SCALE is a two-step approach using KENO-VI and
+MAVRIC. The first step is the determination of the source distribution,
+done with the CSAS6 sequence which uses the KENO-VI functional module.
+Along with calculating the system k\ :sub:`eff`, KENO-VI has been modified to
+now accumulate the fission distribution over the non-skip generations.
+This information is collected on a three-dimensional Cartesian mesh that
+overlays the physical geometry model and is saved as a Monaco mesh tally
+file. A utility program is used to convert the mesh tally into a Monaco
+mesh source.
+
+The mesh source is then used in the second step as the source term in
+MAVRIC. The absolute source strength is set by the user based on the
+total number of fissions (based on the total power released) during the
+criticality excursion. Further neutron multiplication should be
+prevented in the MAVRIC transport calculation. Because the fission
+neutrons have already been accounted for in the KENO-VI calculation,
+failure to suppress neutron multiplication in the MARVIC sequence would
+lead to incorrect flux estimates. In addition, if further fissions were
+allowed, Monaco would add neutrons to its particle bank faster than they
+could be removed (since the system is at or above critical) and the
+simulation may never finish.
+
+For the transport part of a CAAS analysis, MAVRIC can be optimized to
+calculate one specific detector response at one location using CADIS or
+can be optimized to calculate multiple responses/locations with roughly
+the same relative uncertainty using FW-CADIS. For calculating mesh
+tallies of fluxes or dose rates, MAVRIC also uses FW-CADIS to help
+balance the Monaco Monte Carlo calculation such that low flux voxels are
+computed with about the same relative uncertainty as high flux voxels.
+
+With this two-step approach, users will have a great deal of flexibility
+in modeling CAAS problems. The CSAS6 step and the MAVRIC step could both
+use the same geometry and materials definitions or could have different
+levels of detail included in each. The fission source distribution from
+one CSAS6 calculation could be used in a number of different MAVRIC
+building/detector models, with each MAVRIC calculation being optimized
+for a given type of detector.
+
+User Input
+----------
+
+The user can create either one input file containing both the CSAS6 and
+MAVRIC calculations or can create two input files—one for each sequence.
+The materials and geometry for these two models could be the same but do
+not have to be. For example, the CSAS6 sequence might only contain the
+materials and geometry important to the criticality calculation. Note,
+however, that the critical source geometry and materials should be
+modeled identically in both problems. This allows users greater
+flexibility in modeling their specific problems.
+
+KENO-VI input
+~~~~~~~~~~~~~
+
+For the criticality problem, the only extra input a user needs to
+supply is the keyword “cds=yes” in the parameter block and a spatial
+mesh around all of the fissionable materials of the problem in its own
+gridGeometry block. Standard input for KENO-VI is described in the
+KENO-VI chapter and CSAS6 chapter. The mesh used for the fission
+source distribution is input using the “read gridGeometry *id*\ ”
+block, where *id* is an identification number for that grid. Note that
+only one grid can be specified, but that may change in the future. The
+cells of the mesh are specified in each dimension separately by either
+(1) listing all of the planes bounding the cells (keyword “xplanes”
+followed by an “end”), (2) using keyword “xLinear *n* *a* *b*\ ” to
+specify *n* cells between *a* and *b*, or by (3) specifying the
+minimum plane, the maximum plane, and how many cells to make in that
+dimension (“xmin=”, “xmax=”, “numXCells=”). The keywords “xplanes” and
+“xLinear” can be used together and multiple times. Similar keywords
+are used for the *y*- and *z*-dimensions. An example CSAS6 input file
+that collects the fission distribution information would be as
+follows:
+
+
+.. code:: scale
+
+  =csas6
+  CAAS Example
+  v7-238n
+  read composition
+      …
+  end composition
+  read parameters
+      …
+      cds=yes
+  end parameters
+  read geometry
+      …
+  end geometry
+  read gridGeometry 1
+      title="Mesh for Collecting Fission Source"
+      xLinear 13 0.0 78.0
+      yplanes 0 8 16 24 32 34 36 38 40 48 56 64 72 end
+      zLinear 10 -2.54 77.46
+  end gridGeometry
+  end data
+  end
+
+The fission source distribution collected by KENO-VI is saved to a
+Monaco mesh tally file and copied back to the home area with the name
+“\ *problemName*.fissionSource.3dmap”. This file can be viewed with the
+Mesh File Viewer capability of Fulcrum that comes with SCALE. Note that
+the finer the mesh spacing is the more generations/histories will have
+to be simulated by the criticality calculation in order to reduce the
+stochastic uncertainty in each mesh voxel of the distribution.
+Regardless of the mesh size, creation of a fission mesh source file will
+take more iterations than the number required to find k\ :sub:`eff`. KENO-VI
+also saves the value of the system the average number of neutrons per
+fission, in a file called “\ *problemName*.kenoNuBar.txt”. This value is
+needed to properly determine the source strength.
+
+Mesh tally to mesh source conversion
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+A utility program is used to convert the Monaco mesh tally file into a
+Monaco mesh source file. It can be part of the CSAS6 input file. The
+user then needs to copy the resulting \*.msm file back to his home area.
+
+.. code:: scale
+
+  =csas6
+  …
+  end
+
+  =mt2msm
+  'fissionSource.3dmap'    ! existing Keno fission source mesh tally
+  1                        ! which family (for Keno files, there is only 1)
+  -1                       ! use the whole family (keep all energy groups)
+  1                        ! particle type for *.msm file (1-neutron, 2-photon)
+  'fissionSource.msm'      ! name of newly created mesh source map file
+  end
+
+  =shell
+    copy fissionSource.msm “C:\mydocu~1\caasExample”
+  end
+
+Details on the conversion utility program are contained in :ref:`appendixb` of
+the MAVRIC manual.
+
+In SCALE 6.1, the fission source distribution mesh tally produced by
+KENO contained data representing the number of fissions in each mesh
+cell in each energy group. In SCALE 6.2, the data stored was changed to
+be the fissions per unit volume – the fission density. This is more
+consistent with other mesh tallies from Monaco which store flux or dose
+rates that represent averages over the mesh cells. This change also
+allows the Mesh File Viewer to display the KENO fission source
+distribution better. The mt2msm utility program also changed from SCALE
+6.1 to SCALE 6.2 to account for the change in what is stored in the Keno
+mesh tally file. Therefore, **KENO-produced fission source mesh tallies
+and the mt2msm utility should not be mixed-and-matched across versions
+of SCALE**. Doing so would result in the final Monaco mesh source file
+being improperly normalized, which would not represent the KENO fission
+source distribution and would give incorrect results in subsequent
+MAVRIC calculations. Because there is not a specific ‘version flag’ in a
+mesh tally file or mesh source map file, the user must ensure that they
+have used the same version of SCALE for both the CSAS6 and MAVRIC
+sequences any time the CAAS capability is used.
+
+MAVRIC input
+~~~~~~~~~~~~
+
+The input for the MAVRIC portion of the CAAS problem should include the
+materials and geometry of the criticality model, use the fission
+distribution as a source, set the source strength, and set any optional
+modifiers to the source to change its location or add fission photons.
+The cross section library used by the MAVRIC calculation does not need
+to have the same group structure as the fission distribution. MAVRIC
+will automatically convert the fission source group structure to match
+the group structure of its cross section library.
+
+The shielding calculation needs to specify that the source is the
+fission distribution file, which is typically “fissionSource.msm”. The
+total source strength can be specified by either the number of fissions
+in the criticality accident (fission rate or total number) or by the
+number of released neutrons (the fission rate multiplied by :math:`\over{v}` per
+fission). The value of :math:`\over{v}` will be read from the file “kenoNuBar.txt” in the
+SCALE temporary directory if it is not given in the source input with
+the keyword “nu-bar=”. The mesh source can also be placed at different
+coordinates in the geometry using the “origin x=\ *x* y=\ *y* z=\ *z*\ ”
+keywords, if a different reference frame was used with the criticality
+geometry model that created the mesh source. Rotations of mesh sources
+are not available at this time. It is also recommended to use filters in
+the source block to define the source, such as the ‘mixture=’ filter to
+only allow source sampling from a specific mixture since the mesh source
+can be transformed from it’s original origin or meshes can cover
+non-fissionable materials.
+
+For example, using a KENO-VI fission distribution, placing it somewhere
+in the MAVRIC model and setting the source strength (in neutrons/s) to
+correspond to 10\ :sup:`17` fission/s would look like
+
+.. code:: scale
+
+
+  =shell
+      copy “C:\mydocu~1\caasExample\kenoInput.kenoNuBar.txt”  kenoNuBar.txt
+  end
+
+  =mavric
+  …
+  read sources
+      src 1
+          meshSourceFile “C:\mydocu~1\caasExample\fissionSource.msm"
+              origin x=600 y=650 z=400
+          fissions=1.0e17
+      end src
+  end sources
+  …
+  end data
+  end
+
+The source strength in neutrons/s will be calculated by MAVRIC to be the
+fission rate multiplied by the value of read from the “kenoNuBar.txt”
+file. The neutron strength could have alternatively been specified using
+the standard source strength keyword “strength=2.5e17” (for an example
+with the average number of neutrons per fission of 2.5).
+
+The Monte Carlo functional module used by MAVRIC, Monaco, is a
+fixed-source code. Unless told otherwise, neutrons will multiply in
+fissionable materials. Since all of the neutrons were part of the
+source, neutron multiplication should not be allowed and MAVRIC should
+be run with the keyword “fissionMult=0” in the parameters block. For
+systems at or near critical without the “fissionMult=0” keyword, Monaco
+simulations may not end since neutrons will be added to the particle
+bank at the same rate they leave the system or get killed.
+
+The shielding calculation can be run using standard variance reduction
+methods (such as path length stretching, user-defined weight windows
+based on geometry regions, and user-defined source biasing) or using the
+automated tools which employ approximate discrete-ordinates calculations
+to determine the space/energy weight windows as well as a biased source
+distribution in space and energy. The automated tools can be used to
+optimize the shielding calculation to determine one specific tally using
+CADIS or several separate tallies or a mesh tally over a large volume of
+the problem space using FW-CADIS. When using these advanced variance
+reduction methods, remember to include planes in the discrete‑ordinates
+mesh definition that correspond to the planes in the fission
+distribution that the source is based on. If they are not included,
+MAVRIC will resample the fission source on the discrete-ordinates mesh
+it is using for the importance map, possibly smearing or reducing the
+original resolution of the fission distribution.
+
+Example problem
+---------------
+
+Consider the Jezebel critical plutonium sphere experiment, shown in
+:numref:`fig4a1`, taking place inside a simple fictitious building, shown
+in :numref:`fig4a2`. The building has two rooms: an experiment room and a
+control room. In the control room there is a criticality alarm detector,
+and it is positioned furthest from the entry to the experiment room. For
+this example, assume that a criticality excursion results in a total of
+10\ :sup:`18` fissions. This example will calculate the neutron and
+photon doses seen by a detector in the control room, as well as
+calculate a dose map for the entire building.
+
+.. _fig4a1:
+
+.. figure:: figs/fig4a1.png
+  :align: center
+  :width: 60 %
+
+  Cutaway view of Jezebel.
+
+.. _fig4a2:
+.. figure:: figs/fig4a2.png
+  :align: center
+  :width: 60 %
+
+  Simple two-room building.
+
+KENO-VI criticality and fission source distribution
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+For the criticality calculation, consider just a bare sphere of
+plutonium, with a radius of 6.38493 cm. Atom densities (atoms/b·cm) for
+the sphere material are :sup:`239`\ Pu 0.037047; :sup:`240`\ Pu
+0.0017512; :sup:`241`\ Pu 0.00011674; and Cu 0.0013752. This can be
+easily modeled as a sphere at the origin. For collecting the fission
+distribution, a uniform mesh grid can be constructed around the sphere,
+extending 7 cm in each direction, with a 1×1×1 cm voxel size. The first
+portion of the input files ``mavric.caasA.inp`` and ``mavric.caasB.inp`` looks
+like the following:
+
+.. code-block:: scale
+
+  =csas6
+  Dose Rates from a Jezebel Accident in a Block Building
+  v7-238n
+  '-------------------------------------------------------------------------------
+  ' Composition Block
+  '-------------------------------------------------------------------------------
+  read composition
+      Pu-239 1 0 0.037047   end
+      Pu-240 1 0 0.0017512  end
+      Pu-241 1 0 0.00011674 end
+      Cu     1 0 0.0013752  end
+  end composition
+
+  '-------------------------------------------------------------------------------
+  ' Parameters Block
+  '-------------------------------------------------------------------------------
+  read parameters
+      gen=250 npg=200000 nsk=50 htm=no
+      cds=yes
+  end parameters
+  '-------------------------------------------------------------------------------
+  ' Geometry Block - SCALE standard geometry package (SGGP)
+  '-------------------------------------------------------------------------------
+  read geometry
+      global unit 2
+          sphere 1  6.38493
+          media 1 1 1 vol=1090.3277
+      boundary 1
+  end geometry
+
+  '-------------------------------------------------------------------------------
+  ' Grid Block
+  '-------------------------------------------------------------------------------
+  read gridGeometry 1
+      title="Mesh for Collecting Fission Distribution"
+      xLinear 14 -7.0 7.0
+      yLinear 14 -7.0 7.0
+      zLinear 14 -7.0 7.0
+  end gridGeometry
+
+  end data
+  end
+
+  =mt2msm
+  'fissionSource.3dmap'
+  1
+  -1
+  1
+  mavric.caas[A/B].fissionSource.msm'
+  end
+
+The results of this 26 minute calculation are shown in :numref:`tab4a1`,
+and details about the calculated fission distribution are shown in
+:numref:`fig4a3` and :numref:`fig4a4`.
+
+.. _tab4a1:
+
+.. table:: Results of the CSAS6 calculation
+
+  +-----------------+----------------------------+-----------------+-------------+
+  | **Quantity**    | **Value**                  | **Uncertainty** |             |
+  +=================+============================+=================+=============+
+  | k\ :sub:`eff`   | best estimate system k-eff | 1.00024         | 0.00014     |
+  +-----------------+----------------------------+-----------------+-------------+
+  | :math:`\over{v}`| system nu bar              | 3.15671         | 4.77938E-05 |
+  +-----------------+----------------------------+-----------------+-------------+
+
+.. _fig4a3:
+.. figure:: figs/fig4a3.png
+  :align: center
+  :width: 60 %
+
+  Fission source spatial distribution for the center horizontal slice.
+
+.. _fig4a4:
+.. figure:: figs/fig4a4.png
+  :align: center
+  :width: 60 %
+
+  Fission source energy distribution for the center voxel.
+
+MAVRIC transport calculations
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Two MAVRIC calculations will be done—one that calculates the doses seen
+at the detector and one that computes mesh tallies of doses over the
+entire building. They will share the same materials, geometry, and
+source but will have different tally and variance reduction options.
+
+The two-room building will be a simple model using concrete-block walls,
+a concrete floor, and a steel roof, with dimensions shown in
+:numref:`fig4a5`. The building exterior dimensions are 1200 cm long,
+600 cm wide, and 300 cm high above the ground. The exterior and interior
+walls are all made of a double layer of typical concrete blocks (total
+of 40 cm thick.) Concrete blocks are typically 39×19×19 cm and weigh
+~13.5 kg, since they have a volume fraction of 33.2%. The floor is made
+of poured concrete, extending 60 cm into the ground. The roof and the
+exterior door (120 cm wide and 210 cm tall) are made of 1/8 in. (0.3175
+cm) thick steel. The experiment room on the left connects to the control
+room on the right through a maze that prevents radiation streaming.
+Assume that the critical experiment was in the center of the experiment
+room, 100 cm above the floor. Assume the detector in the control room is
+a 30 cm diameter sphere located at position (1145, 55, 285).
+
+.. _fig4a5:
+.. figure:: figs/fig4a5.png
+  :align: center
+
+  Coordinates of the floor, walls, ceiling, and door of the simple block building model (in cm).
+
+If the MAVRIC transport calculation is not in the same file as the CSAS6 calculation, the MAVRIC input would start by moving the KENO-VI results into the SCALE temporary area:
+
+.. code:: scale
+
+  =shell
+      copy %RTNDIR%\caas.kenovi.fissionSource.msm  fissionSource.msm
+      copy %RTNDIR%\caas.kenovi.kenoNuBar.txt      kenoNuBar.txt
+  end
+
+The materials and geometry blocks of the two MAVRIC input files for each of the two calculations,
+``smplprbs/caas.mavricA.inp`` and ``smplprbs/caas.mavricB.inp``, look like the following:
+
+.. code:: scale
+
+  '-------------------------------------------------------------------------------
+  ' Composition Block
+  '-------------------------------------------------------------------------------
+  read composition
+      pu-239 1 0 0.037047   end
+      pu-240 1 0 0.0017512  end
+      pu-241 1 0 0.00011674 end
+      cu     1 0 0.0013752  end
+      orconcrete 2 1.0     293.0 end
+      orconcrete 3 0.33198 293.0 end
+      ss304      4 1.0     293.0 end
+  end composition
+
+  '-------------------------------------------------------------------------------
+  ' Geometry Block - SCALE standard geometry package (SGGP)
+  '-------------------------------------------------------------------------------
+  read geometry
+      global unit 1
+          com="jezebel"
+          sphere   1  6.38493  origin x=280  y=300  z=100
+
+          com="exterior of the building, roof, floor"
+          cuboid   10 1200 0    600 0     300.3175 -60.0
+          cuboid   11 1200 0    600 0     300.3175 300.0
+          cuboid   12 1200 0    600 0     0.0        -60
+
+          com="air space in building - two rooms and maze"
+          cuboid   20 1160 40   560 40    300 0
+
+          com="interior walls to form maze to prevent streaming"
+          cuboid   21  560 520  560 120   300 0
+          cuboid   22  680 640  480  40   300 0
+
+          com="exterior door"
+          cuboid   30 1200 1160       360 240   210 0
+          cuboid   31 1200 1199.6825  360 240   210 0
+
+          com="detector sphere"
+          sphere  40   15.0  origin x=1145 y=55 z=285
+
+          com="jezebel"
+          media  1 1   1   vol=1090.3277
+
+          com="walls, roof, floor"
+          media  3 1   10 -20 -11 -12  -30
+          media  4 1   11
+          media  2 1   12
+
+          com="air space (void) and maze walls"
+          media  0 1   20 -21 -22   -40  -11 -12  -1
+          media  3 1   21                -11 -12
+          media  3 1   22                -11 -12
+
+          com="exterior door"
+          media  0 1   30 -31
+          media  4 1   31
+
+          com="detector"
+          media  0 1   40   vol=14137.167
+
+      boundary 10
+  end geometry
+
+The response functions used to compute the doses will be the standard
+flux-to-dose rate conversion factors for neutrons and photons. These are
+defined in the definitions block. Note that these responses have units
+of (rem/hr)/(/cm :sup:`2`/s).
+
+.. code:: scale
+
+  '-------------------------------------------------------------------------------
+  ' Definitions Block
+  '-------------------------------------------------------------------------------
+  read definitions
+      response 5
+          title="ANSI (1977) neutron flux-to-dose-rate"
+          specialDose=9029
+      end response
+      response 6
+          title="ANSI (1977) photon flux-to-dose-rate"
+          specialDose=9504
+      end response
+  end definitions
+
+The source used by each MAVRIC simulation will be based on the fission
+distribution mesh source determined by KENO-VI. The strength of the
+source can be specified by the total number of fissions that occurred in
+the criticality event. Fission photons will be added for :sup:`239`\ Pu.
+MAVRIC will determine the total source strength, including the fission
+photons, from the value of saved by KENO-VI and the multiplicity data
+from the fission photon data file.
+
+.. code:: scale
+
+  '-------------------------------------------------------------------------------
+  ' Sources Block
+  '-------------------------------------------------------------------------------
+  read sources
+      src 1
+          meshSourceFile=”fissionSource.msm"
+          origin x=280  y=300  z=100
+          fissions=1.0e18
+          mixture=1
+      end src
+  end sources
+
+Note that further multiplication needs to be turned off in MAVRIC using
+the “fissionMult=0” keyword in the parameter block as shown below.
+
+For the responses from the tallies, MAVRIC usually calculates dose rates
+(rem/hr) using a source strength in particles/s. For this example
+problem, instead of a source rate, we used a total number of particles
+(by specifying the number of fissions). Hence, the computed fluxes will
+have units of particles/cm\ :sup:`2` and the computed responses using
+the standard dose responses from the cross section libraries will have
+units of rem s/hr. To get a dose in rem, the responses need to be
+multiplied by (3600 s/hr)\ :sup:`-1`. This can be done using the MAVRIC
+tally multiplier keyword.
+
+Each MAVRIC simulation will need a discrete-ordinates mesh. The planes
+in each dimension where there are material changes are listed in
+:numref:`tab4a2`. In addition to these planes, the discrete-ordinates mesh
+should also subdivide the thick shields in the direction of particle
+travel. For example, the walls of the maze should be divided to better
+model the radiation attenuation through the walls in the Denovo
+calculation. The interior walls of the building will reflect particles,
+so the first few centimeters are the most important to capture in the
+importance map. Mesh planes should also be added that correspond to the
+mesh source after it is placed into the geometry model.
+
+.. _tab4a2:
+.. table:: Main planes in the building
+  :align: center
+
+  +---------+-------+---------+
+  | **x**   | **y** | **z**   |
+  +=========+=======+=========+
+  | 0       | 0     | -60     |
+  +---------+-------+---------+
+  | 40      | 40    | 0       |
+  +---------+-------+---------+
+  | 520     | 120   | 210     |
+  +---------+-------+---------+
+  | 560     | 240   | 300     |
+  +---------+-------+---------+
+  | 640     | 360   | 300.318 |
+  +---------+-------+---------+
+  | 680     | 480   |         |
+  +---------+-------+---------+
+  | 1160    | 560   |         |
+  +---------+-------+---------+
+  | 1199.68 | 600   |         |
+  +---------+-------+---------+
+  | 1200    |       |         |
+  +---------+-------+---------+
+
+
+Detector doses using CADIS
+^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The grid geometry for this calculation should also include planes that bound the adjoint source,