Physical Simulation

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Neutron Modelica is Neutron’s physical-simulation component. Its implementation lives in modelica/; the Python distribution is named neutron-sim.

What exists today

The neutron_sim package exposes Variable, Parameter, Equation, der, Connector, Component, System, connect, and simulate. Domain libraries cover electrical, mechanical, thermal, and fluid components. SciPy provides numerical integration.

Optional modules add FMI import and orchestration, a juliacall bridge, Nucleus result storage, plotting, MCP tools, and scikit-learn surrogate models. These require their own dependencies and, for live integrations, their runtimes or services.

The separate neutron_modelica source package contains FMI runtime, Kirchhoff circuit, and Julia bridge helpers. The current pyproject.toml includes only neutron_sim in the wheel. Source-tree availability does not establish that both packages are installed from a built distribution.

Develop from the repository

From modelica/:

python -m pip install -e ".[dev]"
python -m pytest

Select extras such as fmi, julia, and ai for the integrations you use. Julia also needs a working Julia runtime and its numerical packages. Nucleus integration tests require a configured database. Review skipped tests before treating a local test run as validation of those integrations.

Modelica and FMI boundaries

Neutron’s Python modeling API is not a general compiler for .mo source. Use an external Modelica compiler for Modelica-language models and validate the resulting FMU against the selected importer.

The current export function creates modelDescription.xml and serialized Python resources for Neutron round trips. It does not supply a native executable FMI implementation. General interchange requires a compatible implementation as well as metadata; see the FMI specification.

Keep model authoring, solving, and result storage independently usable. Validate numerical results against analytical references where available, document solver tolerances, and test optional backends explicitly. A portable FMU export needs an independent import-and-execute test, not only a Neutron round trip.

See the proposed modeling architecture for the shared artifact and application-integration approach. These additions are future work.