.. _usersguide_depletion: =========================== Depletion and Transmutation =========================== OpenMC supports transport-coupled and transport-independent depletion, or burnup, calculations through the :mod:`openmc.deplete` Python module. OpenMC uses transmutation reaction rates to solve a set of transmutation equations that determine the evolution of nuclide densities within a material. The nuclide densities predicted at some future time are then used to determine updated reaction rates, and the process is repeated for as many timesteps as are requested. The depletion module is designed such that the reaction rate solution (the transport "operator") is completely isolated from the solution of the transmutation equations and the method used for advancing time. :mod:`openmc.deplete` supports multiple time-integration methods for determining material compositions over time. Each method appears as a different class. For example, :class:`openmc.deplete.CECMIntegrator` runs a depletion calculation using the CE/CM algorithm (deplete over a timestep using the middle-of-step reaction rates). An instance of :class:`~openmc.deplete.abc.TransportOperator` is passed to one of these Integrator classes along with the timesteps and power level:: power = 1200.0e6 # watts timesteps = [10.0, 10.0, 10.0] # days openmc.deplete.CECMIntegrator(op, timesteps, power, timestep_units='d').integrate() The depletion problem is executed, and once it is done a ``depletion_results.h5`` file is written. The results can be analyzed using the :class:`openmc.deplete.Results` class. This class has methods that allow for easy retrieval of k-effective, nuclide concentrations, and reaction rates over time:: results = openmc.deplete.Results("depletion_results.h5") time, keff = results.get_keff() Note that the coupling between the reaction rate solver and the transmutation solver happens in-memory rather than by reading/writing files on disk. OpenMC has two categories of transport operators for obtaining transmutation reaction rates. .. _coupled-depletion: Transport-coupled depletion =========================== This category of operator solves the transport equation to obtain transmutation reaction rates. At present, the :mod:`openmc.deplete` module offers a single transport-coupled operator, :class:`openmc.deplete.CoupledOperator` (which uses the OpenMC transport solver), but in principle additional transport-coupled operator classes based on other transport codes could be implemented and no changes to the depletion solver itself would be needed. The :class:`openmc.deplete.CoupledOperator` class requires a :class:`~openmc.Model` instance containing material, geometry, and settings information:: model = openmc.Model() ... op = openmc.deplete.CoupledOperator(model) Any material that contains a fissionable nuclide is depleted by default, but this can behavior can be changed with the :attr:`Material.depletable` attribute. .. important:: The volume must be specified for each material that is depleted by setting the :attr:`Material.volume` attribute. This is necessary in order to calculate the proper normalization of tally results based on the source rate. Fixed-Source Transmutation -------------------------- When the ``power`` or ``power_density`` argument is used for one of the Integrator classes, it is assumed that OpenMC is running in k-eigenvalue mode, and normalization of tally results is performed based on energy deposition. It is also possible to run a fixed-source simulation and perform normalization based on a known source rate. First, as with all fixed-source calculations, we need to set the run mode:: settings.run_mode = 'fixed source' Additionally, all materials that you wish to deplete need to be marked as such using the :attr:`Material.depletable` attribute:: mat = openmc.Material() mat.depletable = True When constructing the :class:`~openmc.deplete.CoupledOperator`, you should indicate that normalization of tally results will be done based on the source rate rather than a power or power density:: op = openmc.deplete.CoupledOperator(model, normalization_mode='source-rate') Finally, when creating a depletion integrator, use the ``source_rates`` argument:: integrator = openmc.deplete.PredictorIntegrator(op, timesteps, sources_rates=...) As with the ``power`` argument, you can provide a different source rate for each timestep in the calculation. A zero source rate for a given timestep will result in a decay-only step, where all reaction rates are zero. Caveats ------- .. _energy-deposition: Energy Deposition ~~~~~~~~~~~~~~~~~ The default energy deposition mode, ``"fission-q"``, instructs the :class:`~openmc.deplete.CoupledOperator` to normalize reaction rates using the product of fission reaction rates and fission Q values taken from the depletion chain. This approach does not consider indirect contributions to energy deposition, such as neutron heating and energy from secondary photons. In doing this, the energy deposited during a transport calculation will be lower than expected. This causes the reaction rates to be over-adjusted to hit the user-specific power, or power density, leading to an over-depletion of burnable materials. There are some remedies. First, the fission Q values can be directly set in a variety of ways. This requires knowing what the total fission energy release should be, including indirect components. Some examples are provided below:: # use a dictionary of fission_q values fission_q = {"U235": 202e+6} # energy in eV # create a Model object model = openmc.Model(geometry, settings) # create a modified chain and write it to a new file chain = openmc.deplete.Chain.from_xml("chain.xml", fission_q) chain.export_to_xml("chain_mod_q.xml") op = openmc.deplete.CoupledOperator(model, "chain_mod_q.xml") # alternatively, pass the modified fission Q directly to the operator op = openmc.deplete.CoupledOperator(model, "chain.xml", fission_q=fission_q) A more complete way to model the energy deposition is to use the modified heating reactions described in :ref:`methods_heating`. These values can be used to normalize reaction rates instead of using the fission reaction rates with:: op = openmc.deplete.CoupledOperator(model, "chain.xml", normalization_mode="energy-deposition") These modified heating libraries can be generated by running the latest version of :meth:`openmc.data.IncidentNeutron.from_njoy()`, and will eventually be bundled into the distributed libraries. Local Spectra and Repeated Materials ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ It is not uncommon to explicitly create a single burnable material across many locations. From a pure transport perspective, there is nothing wrong with creating a single 3.5 wt.% enriched fuel ``fuel_3``, and placing that fuel in every fuel pin in an assembly or even full core problem. This certainly expedites the model making process, but can pose issues with depletion. Under this setup, :mod:`openmc.deplete` will deplete a single ``fuel_3`` material using a single set of reaction rates, and produce a single new composition for the next time step. This can be problematic if the same ``fuel_3`` is used in very different regions of the problem. As an example, consider a full-scale power reactor core with vacuum boundary conditions, and with fuel pins solely composed of the same ``fuel_3`` material. The fuel pins towards the center of the problem will surely experience a more intense neutron flux and greater reaction rates than those towards the edge of the domain. This indicates that the fuel in the center should be at a more depleted state than periphery pins, at least for the fist depletion step. However, without any other instructions, OpenMC will deplete ``fuel_3`` as a single material, and all of the fuel pins will have an identical composition at the next transport step. This can be countered by instructing the operator to treat repeated instances of the same material as a unique material definition with:: op = openmc.deplete.CoupledOperator(model, chain_file, diff_burnable_mats=True) For our example problem, this would deplete fuel on the outer region of the problem with different reaction rates than those in the center. Materials will be depleted corresponding to their local neutron spectra, and have unique compositions at each transport step. The volume of the original ``fuel_3`` material must represent the volume of **all** the ``fuel_3`` in the problem. When creating the unique materials, this volume will be equally distributed across all material instances. .. note:: This will increase the total memory usage and run time due to an increased number of tallies and material definitions. Transport-independent depletion =============================== This category of operator uses multigroup microscopic cross sections along with multigroup flux spectra to obtain transmutation reaction rates. The cross sections are pre-calculated, so there is no need for direct coupling between a transport-independent operator and a transport solver. The :mod:`openmc.deplete` module offers a single transport-independent operator, :class:`~openmc.deplete.IndependentOperator`, and only one operator is needed since, in theory, any transport code could calculate the multigroup microscopic cross sections. The :class:`~openmc.deplete.IndependentOperator` class has two constructors. The default constructor requires a :class:`openmc.Materials` instance, a list of multigroup flux arrays, and a list of :class:`~openmc.deplete.MicroXS` instances containing multigroup microscopic cross sections in units of barns. This might look like the following:: materials = openmc.Materials([m1, m2, m3]) ... # Assign fluxes (generated from any code) flux_m1 = numpy.array([...]) flux_m2 = numpy.array([...]) flux_m3 = numpy.array([...]) fluxes = [flux_m1, flux_m2, flux_m3] # Assign microscopic cross sections micro_m1 = openmc.deplete.MicroXS.from_csv('xs_m1.csv') micro_m2 = openmc.deplete.MicroXS.from_csv('xs_m2.csv') micro_m3 = openmc.deplete.MicroXS.from_csv('xs_m3.csv') micros = [micro_m1, micro_m2, micro_m3] # Create operator op = openmc.deplete.IndependentOperator(materials, fluxes, micros) For more details on the :class:`~openmc.deplete.MicroXS` class, including how to use OpenMC's transport solver to generate microscopic cross sections and fluxes for use with :class:`~openmc.deplete.IndependentOperator`, see :ref:`micros`. .. note:: The same statements from :ref:`coupled-depletion` about which materials are depleted and the requirement for depletable materials to have a specified volume also apply here. An alternate constructor, :meth:`~openmc.deplete.IndependentOperator.from_nuclides`, accepts a volume and dictionary of nuclide concentrations in place of the :class:`openmc.Materials` instance. Note that while the normal constructor allows multiple materials to be depleted with a single operator, the :meth:`~openmc.deplete.IndependentOperator.from_nuclides` classmethod only works for a single material:: nuclides = {'U234': 8.92e18, 'U235': 9.98e20, 'U238': 2.22e22, 'U236': 4.57e18, 'O16': 4.64e22, 'O17': 1.76e19} volume = 0.5 op = openmc.deplete.IndependentOperator.from_nuclides(volume, nuclides, flux, micro_xs, chain_file, nuc_units='atom/cm3') A user can then define an integrator class as they would for a coupled transport-depletion calculation and follow the same steps from there. .. note:: Ideally, multigroup cross section data should be available for every reaction in the depletion chain. If cross section data is not present for a nuclide in the depletion chain with at least one reaction, that reaction will not be simulated. .. _micros: Loading and Generating Microscopic Cross Sections ------------------------------------------------- As mentioned above, any transport code could be used to calculate multigroup microscopic cross sections and fluxes. The :mod:`openmc.deplete` module provides the :class:`~openmc.deplete.MicroXS` class, which can either be instantiated from pre-calculated cross sections in a ``.csv`` file or from data arrays directly:: micro_xs = MicroXS.from_csv(micro_xs_path) nuclides = ['U234', 'U235', 'U238'] reactions = ['fission', '(n,gamma)'] data = np.array([[0.1, 0.2], [0.3, 0.4], [0.01, 0.5]]) micro_xs = MicroXS(data, nuclides, reactions) .. important:: The cross section values are assumed to be in units of barns. Make sure your cross sections are in the correct units before passing to a :class:`~openmc.deplete.IndependentOperator` object. Additionally, a convenience function, :func:`~openmc.deplete.get_microxs_and_flux`, can provide the needed fluxes and cross sections using OpenMC's transport solver:: model = openmc.Model() ... fluxes, micros = openmc.deplete.get_microxs_and_flux(model, materials) If you are running :func:`~openmc.deplete.get_microxs_and_flux` on a cluster where temporary files are created on a local filesystem that is not shared across nodes, you'll need to set an environment variable pointing to a local directoy so that each MPI process knows where to store output files used to calculate the microscopic cross sections. In order of priority, they are :envvar:`TMPDIR`. :envvar:`TEMP`, and :envvar:`TMP`. Users interested in further details can read the documentation for the `tempfile `_ module. Caveats ------- Reaction Rate Normalization ~~~~~~~~~~~~~~~~~~~~~~~~~~~ The :class:`~openmc.deplete.IndependentOperator` class supports two methods for normalizing reaction rates: .. important:: Make sure you set the correct parameter in the :class:`openmc.abc.Integrator` class. Use the ``source_rates`` parameter when ``normalization_mode == source-rate``, and use ``power`` or ``power_density`` when ``normalization_mode == fission-q``. 1. ``source-rate`` normalization, which assumes the ``source_rate`` provided by the time integrator is a flux, and obtains the reaction rates by multiplying the cross sections by the ``source-rate``. 2. ``fission-q`` normalization, which uses the ``power`` or ``power_density`` provided by the time integrator to obtain normalized reaction rates by computing a normalization factor as the ratio of the user-specified power to the "observed" power based on fission reaction rates. The equation for the normalization factor is .. math:: :label: fission-q f = \frac{P}{\sum\limits_m \sum\limits_i \left(Q_i N_{i,m} \sum\limits_g \sigma^f_{i,g,m} \phi_{g,m} \right)} where :math:`P` is the power, :math:`Q_i` is the fission Q value for nuclide :math:`i`, :math:`\sigma_{i,g,m}^f` is the microscopic fission cross section for nuclide :math:`i` in energy group :math:`g` for material :math:`m`, :math:`\phi_{g,m}` is the neutron flux in group :math:`g` for material :math:`m`, and :math:`N_{i,m}` is the number of atoms of nuclide :math:`i` for material :math:`m`. Reaction rates are then multiplied by :math:`f` so that the total fission power matches :math:`P`. This equation makes the same assumptions and issues as discussed in :ref:`energy-deposition`. Unfortunately, the proposed solution in that section does not apply here since we are decoupled from transport code. However, there is a method to converge to a more accurate value for flux by using substeps during time integration. `This paper `_ provides a good discussion of this method. .. warning:: The accuracy of results when using ``fission-q`` is entirely dependent on your depletion chain. Make sure it has sufficient data to resolve the dynamics of your particular scenario. Multiple Materials ~~~~~~~~~~~~~~~~~~ A transport-independent depletion simulation using ``source-rate`` normalization will calculate reaction rates for each material independently. This can be useful for running many different cases of a particular scenario. A transport-independent depletion simulation using ``fission-q`` normalization will sum the fission energy values across all materials into :math:`Q_i` in Equation :math:numref:`fission-q`, and Equation :math:numref:`fission-q` provides the normalization factor applied to reaction rates in each material. This can be useful for running a scenario with multiple depletable materials that are part of the same reactor. This behavior may change in the future. Time integration ~~~~~~~~~~~~~~~~ The values of the microscopic cross sections passed to :class:`openmc.deplete.IndependentOperator` are fixed for the entire depletion simulation. This implicit assumption may produce inaccurate results for certain scenarios. Transfer Rates ============== Transfer rates define removal or feed of nuclides to or from one or more depletable materials. This can be useful to model continuous fuel reprocessing, online fission products separation, etc. Transfer rates are defined by calling the :meth:`~openmc.deplete.abc.Integrator.add_transfer_rate()` method directly from one of the Integrator classes:: ... integrator = openmc.deplete.PredictorIntegrator(op, time_steps, power) integrator.add_transfer_rate(...) Defining transfer rates ----------------------- The :meth:`~openmc.deplete.abc.Integrator.add_transfer_rate()` method requires a :class:`~openmc.Material` instance (alternatively, a material id or the name) as the depletable material from which nuclides are processed, a list of elements that share the same transfer rate, and a transfer rate itself. .. caution:: Make sure you set the transfer rate value with the right sign. A positive transfer rate assumes removal, while a negative one assumes feed. The ``transfer_rate_units`` argument specifies the units for the transfer rate. The default is `1/s`, but '1/min', '1/h', '1/d' and '1/a' are also valid options. For example, to define continuous removal of xenon from one material with a removal rate value of 0.1 s\ :sup:`-1` (or a cycle time of 10 s), you'd use:: mat1 = openmc.Material(material_id=1, name='fuel') ... integrator = openmc.deplete.PredictorIntegrator(op, time_steps, power) # by openmc.Material object integrator.add_transfer_rate(mat1, ['Xe'], 0.1) # or by material id integrator.add_transfer_rate(1, ['Xe'], 0.1) # or by material name integrator.add_transfer_rate('fuel', ['Xe'], 0.1) Note that in this case the xenon isotopes that are removed will not be tracked. Defining a destination material ------------------------------- To transfer elements from one depletable material to another, the ``destination_material`` parameter needs to be passed to the :meth:`~openmc.deplete.abc.Integrator.add_transfer_rate()` method. For example, to transfer xenon from one material to another, you'd use:: ... mat2 = openmc.Material(name='storage') ... integrator.add_transfer_rate(mat1, ['Xe'], 0.1, destination_material=mat2)