Adding two dimensional axial cuts

Currently, homogenization calculations are performed on two dimensional slices, or cuts, through the three dimensional heterogeneous model. These cuts are used as building blocks for the homogeneous three dimensional model.

Things to consider when choosing cuts

The specification of a cut includes a height, that is, homogenization is not performed on a true plane cut, but rather an axial section. A two dimensional calculation is emulated by placing reflective boundary conditions on the top and bottom planes, but it will only be truly two dimensional if there are no axial changes within the section. Since nodal equivalence currently only covers the radial plane, with axial leakage assumed to be zero, it is best to try and limit the amount of axial heterogeneity within your sections. This only applies to cuts containing active (fissionable) material, as reflector regions are treated differently. However, especially in research reactors, there are frequently a lot of axial changes even in the active section of the core (irradiation rigs, beam tubes etc), making it impractical, or even impossible to fully capture everything in axially homogeneous slices. In this case, one has to use broader considerations than pure structural variation when placing cuts. The most important factor is to separate regions were large variations in average cross sections are expected, e.g. moving from a highly scattering to a highly absorbing region.

The system also allows you to refine your model by adding (or removing) cuts if the final error checking yields unacceptable results.

Note

The system will always correctly compute the average behavior over the region, you will just loose the axial shape, and true two dimensional equivalence will no longer be achieved.

Defining cuts that include fissionable materials

Cuts are defined after all the Homogenization Parameters have been set. Standard cuts, which includes source materials, are added using the following function:

parameters.add_cut(label, position=0.0 * units.cm, width=1.0 * units.cm, rng=None, description=None)

Defines a section on which homogenization will be performed.

Parameters:

label (str) – Name used to identify the section. It will be used to identify the cut on the command line. It is recommended to use a unique tag no longer than 12 characters.
position (length) – Center axial position of the section.
width (length) – Axial size of the section.
rng (tuple (length)) – Minimum and maximum axial position of the section.
description (str) – String giving more information about the cut (e.g. where it is located, why it was chosen, etc)

Returns:

A container with all the parameters configuring the section.

Attention

The label is also used when naming mixtures, so it is important that it is unique.

Attention

The rng parameter only makes sense if neither position nor width is given.

Typical examples are:

cut = parameters.add_cut('MY-CUT', 0.0 * units.cm, 10.0 * units.cm, description='My first axial cut')

or equivalently:

cut = parameters.add_cut('MY-CUT', rng=[-5 * units.cm, 5 * units.cm], description='My first axial cut')

Attention

The axial position coordinates are absolute, i.e. not relevant to the core axial center line!

Equivalence and replacement tests are available for these cuts.

The returned variable cut contains a copy of all the homogenization parameters, and can be used to modify the model, or any other generator parameter for that cut. These are described below.

Defining axial reflector cuts

For axial sections that do not contain source (fissionable) material, the following method is used to define the homogenization calculation.

parameters.add_axial_reflector_cut(label, position=0.0 * units.cm, width=1.0 * units.cm, rng=None, description=None)

Defines a section, without source material, on which homogenization will be performed.

Parameters:

label (str) – Name used to identify the section. It will be used to identify the cut on the command line. It is recommended to use a unique tag no longer than 12 characters.
position (length) – Center axial position of the section.
width (length) – Axial size of the section.
rng (tuple (length)) – Minimum and maximum axial position of the section.
description (str) – String giving more information about the cut (e.g. where it is located, why it was chosen, etc)

Returns:

A container with all the parameters configuring the section.

This method is typically used for regions above or below the active core. For example,

h = parameters.generator.model.active_height()
baf = parameters.add_axial_reflector_cut('BAFFLE', rng=(0.5 * h, 0.5 * h + 6 * units.cm),
                                         description='Baffle region above active fuel')

will homogenize a 6 cm region above the active core. Equivalence and replacement tests are not available for these cuts.

Note

Unlike normal active cuts, which tries to mimic two dimensional sections, reflector cuts uses the entire core as a driver region, and contains no axial boundary conditions. Thus, the amount of axial heterogeneity within the section is not an important factor.

The cut container interface

Each cut reference returned by parameters.add_cut and parameters.add_axial_reflector_cut has the following methods to access and modify data:

cut.get_parameters()

Returns the generator parameter set for the cut. This can be used to modify application parameters, like particles or max_iteration, from the defaults defined in the generator section. These modifications only affect to the current cut.

Example:

parm = cut.get_parameters()
parm.particles = 16000

cut.model()

Returns a reference to the underlying heterogeneous model. This can be used to modify the model’s state, change bank positions etc.

Example:

cut.model().set_banks(control.fully_inserted())

Attention

Any modifications made to the model via this call only applies to the current cut.

cut.__getitem__(label)

Returns a reference to a particular mixture in the homogenization_grid for this cut.

Parameters:: label (int or str) – Either a row index or a position label in the homogenization grid.
Returns:: A reference to the mixture at this position. Can be used when defining replacement cases, and material meshes.

For example, suppose the homogenization grid’s first row and column label is ‘A’ and ‘1’ respectively, then

mix = cut[0][0]

or

mix = cut['A1']

will return the mixture that is in top left corner of the grid.