.. currentmodule:: openmc
Materials in OpenMC are defined as a set of nuclides/elements at specified densities and are created using the :class:`openmc.Material` class. Once a material has been instantiated, nuclides can be added with :meth:`Material.add_nuclide` and elements can be added with :meth:`Material.add_element`. Densities can be specified using atom fractions or weight fractions. For example, to create a material and add Gd152 at 0.5 atom percent, you'd run:
mat = openmc.Material()
mat.add_nuclide('Gd152', 0.5, 'ao')
The third argument to :meth:`Material.add_nuclide` can also be 'wo' for weight percent. The densities specified for each nuclide/element are relative and are renormalized based on the total density of the material. The total density is set using the :meth:`Material.set_density` method. The density can be specified in gram per cubic centimeter ('g/cm3'), atom per barn-cm ('atom/b-cm'), or kilogram per cubic meter ('kg/m3'), e.g.,
mat.set_density('g/cm3', 4.5)
The :meth:`Material.add_element` method works exactly the same as :meth:`Material.add_nuclide`, except that instead of specifying a single isotope of an element, you specify the element itself. For example,
mat.add_element('C', 1.0)
This method can also accept case-insensitive element names such as
mat.add_element('aluminium', 1.0)
Internally, OpenMC stores data on the atomic masses and natural abundances of all known isotopes and then uses this data to determine what isotopes should be added to the material. When the material is later exported to XML for use by the :ref:`scripts_openmc` executable, you'll see that any natural elements were expanded to the naturally-occurring isotopes.
The :meth:`Material.add_element` method can also be used to add uranium at a specified enrichment through the enrichment argument. For example, the following would add 3.2% enriched uranium to a material:
mat.add_element('U', 1.0, enrichment=3.2)
In addition to U235 and U238, concentrations of U234 and U236 will be present and are determined through a correlation based on measured data.
It is also possible to perform enrichment of any element that is composed of two naturally-occurring isotopes (e.g., Li or B) in terms of atomic percent. To invoke this, provide the additional argument enrichment_target to :meth:`Material.add_element`. For example the following would enrich B10 to 30ao%:
mat.add_element('B', 1.0, enrichment=30.0, enrichment_target='B10')
In order to enrich an isotope in terms of mass percent (wo%), provide the extra argument enrichment_type. For example the following would enrich Li6 to 15wo%:
mat.add_element('Li', 1.0, enrichment=15.0, enrichment_target='Li6',
enrichment_type='wo')
Often, cross section libraries don't actually have all naturally-occurring isotopes for a given element. For example, in ENDF/B-VII.1, cross section evaluations are given for O16 and O17 but not for O18. If OpenMC is aware of what cross sections you will be using (through the :envvar:`OPENMC_CROSS_SECTIONS` environment variable), it will attempt to only put isotopes in your model for which you have cross section data. In the case of oxygen in ENDF/B-VII.1, the abundance of O18 would end up being lumped with O16.
If you have a moderating material in your model like water or graphite, you
should assign thermal scattering data (so-called S(\alpha,\beta)) using
the :meth:`Material.add_s_alpha_beta` method. For example, to model light water,
you would need to add hydrogen and oxygen to a material and then assign the
c_H_in_H2O thermal scattering data:
water = openmc.Material()
water.add_nuclide('H1', 2.0)
water.add_nuclide('O16', 1.0)
water.add_s_alpha_beta('c_H_in_H2O')
water.set_density('g/cm3', 1.0)
OpenMC uses the GNDS naming convention for nuclides, metastable states, and compounds:
| Nuclides: | SymA where "A" is the mass number (e.g., Fe56) |
|---|---|
| Elements: | Sym0 (e.g., Fe0 or C0) |
| Excited states: | SymA_eN (e.g., V51_e1 for the first excited state of
Vanadium-51.) This is only used in decay data. |
| Metastable states: | SymA_mN (e.g., Am242_m1 for the first excited state
of Americium-242). |
| Compounds: | c_String_Describing_Material (e.g., c_H_in_H2O). Used for
thermal scattering data. |
Important
The element syntax, e.g., C0, is only used when the cross
section evaluation is an elemental evaluation, like carbon in
ENDF/B-VII.1! If you are adding an element via
:meth:`Material.add_element`, just use Sym.
Some Monte Carlo codes define temperature implicitly through the cross section data, which is itself given only at a particular temperature. In OpenMC, the material definition is decoupled from the specification of temperature. Instead, temperatures are assigned to :ref:`cells <usersguide_cells>` directly. Alternatively, a default temperature can be assigned to a material that is to be applied to any cell where the material is used. In the absence of any cell or material temperature specification, a global default temperature can be set that is applied to all cells and materials. Anytime a material temperature is specified, it will override the global default temperature. Similarly, anytime a cell temperatures is specified, it will override the material or global default temperature. All temperatures should be given in units of Kelvin.
To assign a default material temperature, one should use the temperature
attribute, e.g.,
hot_fuel = openmc.Material() hot_fuel.temperature = 1200.0 # temperature in Kelvin
Warning
MCNP users should be aware that OpenMC does not use the concept of cross section suffixes like "71c" or "80c". Temperatures in Kelvin should be assigned directly per material or per cell using the :attr:`Material.temperature` or :attr:`Cell.temperature` attributes, respectively.
In OpenMC it is possible to mix any number of materials to create a new material with the correct nuclide composition and density. The :meth:`Material.mix_materials` method takes a list of materials and a list of their mixing fractions. Mixing fractions can be provided as atomic fractions, weight fractions, or volume fractions. The fraction type can be specified by passing 'ao', 'wo', or 'vo' as the third argument, respectively. For example, assuming the required materials have already been defined, a MOX material with 3% plutonium oxide by weight could be created using the following:
mox = openmc.Material.mix_materials([uo2, puo2], [0.97, 0.03], 'wo')
It should be noted that, if mixing fractions are specifed as atomic or weight fractions, the supplied fractions should sum to one. If the fractions are specified as volume fractions, and the sum of the fractions is less than one, then the remaining fraction is set as void material.
Warning
Materials with S(\alpha,\beta) thermal scattering data cannot be used in :meth:`Material.mix_materials`. However, thermal scattering data can be added to a material created by :meth:`Material.mix_materials`.
The :ref:`scripts_openmc` executable expects to find a materials.xml file
when it is run. To create this file, one needs to instantiate the
:class:`openmc.Materials` class and add materials to it. The :class:`Materials`
class acts like a list (in fact, it is a subclass of Python's built-in
:class:`list` class), so materials can be added by passing a list to the
constructor, using methods like append(), or through the operator
+=. Once materials have been added to the collection, it can be exported
using the :meth:`Materials.export_to_xml` method.
materials = openmc.Materials() materials.append(water) materials += [uo2, zircaloy] materials.export_to_xml() # This is equivalent materials = openmc.Materials([water, uo2, zircaloy]) materials.export_to_xml()
OpenMC uses a file called :ref:`cross_sections.xml <io_cross_sections>` to
indicate where cross section data can be found on the filesystem. This file
serves the same role that xsdir does for MCNP or xsdata does for
Serpent. Information on how to generate a cross section listing file can be
found in :ref:`create_xs_library`. Once you have a cross sections file that has
been generated, you can tell OpenMC to use this file either by setting
:attr:`Materials.cross_sections` or by setting the
:envvar:`OPENMC_CROSS_SECTIONS` environment variable to the path of the
cross_sections.xml file. The former approach would look like:
materials.cross_sections = '/path/to/cross_sections.xml'