Multigroup neutron diffusion solver for 1-D and 2-D geometries. Written in C++17; exposed to Python via pybind11.

• Arbitrary number of energy groups and material regions
• Vacuum, reflective, and albedo boundary conditions
• k-eigenvalue solver - matrix-free power iteration; Aφ = (1/k)Bφ
• Fixed-source solver - direct solve of Aφ = q for a user-supplied volumetric source
• Time-dependent solver - theta-weighted time stepping, unconditionally stable
• Per-group Thomas (TDMA) tridiagonal solver inside a Gauss-Seidel group sweep
• Harmonic-mean diffusion coefficients at material interfaces

• Delayed neutron precursors - any number of precursor groups, per-material delayed fractions and delayed fission spectra
• Implicit fission source, so the scheme stays unconditionally stable through a supercritical transient
• Second-order time differencing - theta = 0.5 is Crank-Nicolson; the default theta = 1 is backward Euler
• Mid-transient perturbation via update_materials, for reactivity insertions

• Finite-difference 5-point stencil on an nx x ny Cartesian grid
• Left (x=0) and bottom (y=0) boundaries hardcoded as reflective; right and top boundaries take user-specified Robin BCs per group
• k-eigenvalue solver - line-TDMA x-sweeps inside a Gauss-Seidel outer iteration
• Fixed-source solver - same spatial sweep; solves Aφ = q directly
• Time-dependent solver - theta-weighted stepping using the same line-TDMA sweep

• Cell-centred finite-volume method (FVM)
• Arbitrary Robin BCs per boundary tag; harmonic-mean interface diffusion coefficients
• k-eigenvalue solver - power iteration with point Gauss-Seidel inner solve
• Fixed-source solver - point SOR (successive over-relaxation) inner solve
• Time-dependent solver - theta-weighted stepping with point Gauss-Seidel

Requires a C++17 compiler, Python >= 3.9, and pybind11 >= 2.12.

pip install .

For development:

pip install -e ".[dev]"

The dev extra pulls in pytest, scipy, and ruff - everything CI runs. Installing without it leaves parts of the test suite silently skipped rather than failing (see Running tests).

Python edits under src/ndiffusion/ are picked up immediately; after editing C++ sources, re-run pip install -e ".[dev]" to rebuild the extension.

import numpy as np
import ndiffusion as nd

m = nd.Materials()
m.n_mat    = 1
m.n_groups = 1
m.D        = [3.850204978408833]
m.removal  = [0.1532]
m.scatter  = [0.0]
m.chi      = [1.0]
m.nusigf   = [0.1570]

cells = 50
edges = list(np.linspace(0.0, 100.0, cells + 1))

solver = nd.KEigenSolver(
    mats       = m,
    medium_map = [0] * cells,
    edges_x    = edges,
    geom       = nd.Geometry.Sphere,
    bc         = [nd.BoundaryCondition(A=1.0, B=0.0)],
    epsilon    = 1e-8,
    max_outer  = 500,
)
result = solver.solve()
assert result.converged
print(f"keff = {result.keff:.8f}")   # -> 1.00000475

Every result carries a converged flag; always check it before trusting the answer (an unconverged run returns the last iterate without raising).

solver = nd.KEigenSolver2D(
    mats       = m,
    medium_map = [0] * (nx * ny),
    edges_x    = list(np.linspace(0.0, R, nx + 1)),
    edges_y    = list(np.linspace(0.0, R, ny + 1)),
    geom       = nd.Geometry2D.XY,
    bc_x       = [nd.BoundaryCondition(A=1.0, B=0.0)],   # vacuum right
    bc_y       = [nd.BoundaryCondition(A=1.0, B=0.0)],   # vacuum top
)
result = solver.solve()
flux = np.array(result.flux).reshape(nx, ny, m.n_groups)

# Build an unstructured mesh (vertices + connectivity + boundary faces)
mesh = nd.UnstructuredMesh2D()
mesh.vx = vx; mesh.vy = vy
mesh.cell_vertices = cell_vertices
mesh.cell_offsets  = cell_offsets
mesh.material_id   = mat_ids
mesh.bface_v0      = bface_v0
mesh.bface_v1      = bface_v1
mesh.bface_bc_tag  = bface_bc_tag   # integer tag per face

bc = [nd.BoundaryCondition(A=1.0, B=0.0)]   # tag 0 -> vacuum

solver = nd.FixedSourceSolverUnstructured2D(
    mats      = m,
    mesh      = mesh,
    bc        = bc,
    epsilon   = 1e-10,
    max_inner = 1000,
    omega     = 1.9,    # SOR relaxation factor
)
result = solver.solve([q] * n_cells)   # volumetric source per cell

See examples/k_eigenvalue.py and examples/time_dependent.py for further examples.

Multigroup transport libraries tabulate a total (or absorption) cross section, a scatter matrix and fission data; the solvers want D, a removal cross section and a diagonal-free scatter matrix. make_materials_from_transport does the conversion and returns a ready-to-use Materials:

mats = nd.make_materials_from_transport([uo2, mox, moderator], G=7)

Each input dict holds SigT or Siga (the other is derived from the scatter matrix), a Scat matrix, nuSigf, and optionally chi, SigTr or Scat1:

transport_to_diffusion(data, G) exposes the same transform for a single material and returns a plain dict, useful for inspecting the derived D, Removal and SigTr before building a Materials.

See examples/transport_cross_sections.py for a runnable end-to-end example.

The time-dependent solvers model delayed neutron precursors:

(1/v_g) dphi_g/dt = -A_g phi_g + scatter
                  + (1-beta) chi_p,g F + sum_i chi_d,i,g lambda_i C_i
        dC_i/dt   = beta_i F - lambda_i C_i,   F = sum_g' nuSigf_g' phi_g'

The time discretisation weights the right-hand side between the two time levels with a factor theta (see Time differencing below). Whatever the weight, C^{n+1} still eliminates in closed form, which folds the delayed source into a dt-dependent effective fission spectrum plus a source known from the old precursors:

chi_eff,g = (1-beta) chi_p,g
          + sum_i chi_d,i,g beta_i lambda_i (theta dt) / (1 + lambda_i theta dt)
Q_d,g     = sum_i chi_d,i,g lambda_i
            [ C_i^n (1 - (1-theta) lambda_i dt) / (1 + theta lambda_i dt)
            + (1-theta) dt beta_i F^n           / (1 + theta lambda_i dt)
            + ((1-theta)/theta) C_i^n ]

At the default theta = 1 those collapse to the backward-Euler forms chi_eff,g = (1-beta) chi_p,g + sum_i chi_d,i,g beta_i lambda_i dt / (1 + lambda_i dt) and Q_d,g = sum_i chi_d,i,g lambda_i C_i^n / (1 + lambda_i dt).

As theta*dt -> 0 the effective spectrum tends to (1-beta) chi_p (prompt only); as theta*dt -> inf it tends to the total fission spectrum, so a critical system with equilibrium precursors is an exact fixed point at any step size and any theta. Fission is evaluated at the new time level inside the Gauss-Seidel sweep, so the scheme stays unconditionally stable.

delayed = nd.make_delayed_data(nd.DELAYED_U235_6GROUP, G=2, n_mat=3, chi=mats.chi)

# Start from a genuine steady state: a k-eigenvalue flux is only stationary
# once nusigf is divided by keff.
res = nd.KEigenSolver2D(mats, medium_map, edges_x, edges_y, geom, bc_x, bc_y).solve()
critical = nd.scale_to_critical(mats, res.keff)

solver = nd.TimeDependentSolver2D(
    mats=critical, medium_map=medium_map,
    edges_x=edges_x, edges_y=edges_y, geom=nd.Geometry2D.XY,
    bc_x=bc_x, bc_y=bc_y, initial_flux=res.flux,
    delayed=delayed,          # omit for prompt-only kinetics
)
solver.update_materials(perturbed)   # step insertion at t = 0
out = solver.run(dt=2e-3, n_steps=250)
out.precursors                        # [cells * n_precursor], per unit volume

Precursors default to equilibrium with the initial flux, which is what a transient starting from steady state needs; pass initial_precursors to override. Drive a ramp by calling update_materials once per step with interpolated cross sections.

DelayedNeutronData arrays are flat: lambda_ is [n_precursor], beta is [n_mat * n_precursor], chi_delayed is [n_mat * n_precursor * n_groups] (each spectrum must sum to 1), and chi_prompt is [n_mat * n_groups] or empty.

When chi_prompt is omitted it is derived rather than defaulted to Materials.chi:

chi_p = (chi - sum_i beta_i chi_d,i) / (1 - beta)

so the prompt and delayed parts always add back up to the total spectrum, and chi_eff still tends to chi as dt -> inf for any delayed spectrum. With the usual chi_delayed = chi this reduces to chi_p = chi. Pass chi_prompt explicitly only if Materials.chi is itself the prompt spectrum.

Fission-matrix mode is supported. There is no separable spectrum, so the split is applied to the matrix: the production cross section is the column sum P[g'] = sum_g F[g][g'] (total neutrons emitted per fission caused by a group-g' neutron), the delayed yield beta_i chi_d,i[g] P[g'] is subtracted from the tabulated matrix, and the part emitted within the step is added back. The two representations agree exactly when the matrix is separable. Note that Materials.chi is all zeros in this mode, so it cannot serve as the ChiDelayed fallback - supply one.

Time differencing. All three time-dependent solvers take a theta weight, as a constructor argument and as a settable property:

(1/(v dt)) (phi^{n+1} - phi^n) = theta * R(phi^{n+1}, C^{n+1})
                               + (1-theta) * R(phi^n,   C^n)

theta = 1 (the default) is backward Euler, first order in dt; theta = 0.5 is Crank-Nicolson, second order. The weighting is applied consistently to the flux equation and the precursor balance, so the whole transient is second order at theta = 0.5 - on the infinite-medium point-kinetics problem in tests/test_kinetics.py the observed order is 2.00, and at a fixed dt the error is ~140x smaller than backward Euler's:

solver = nd.TimeDependentSolver(..., delayed=delayed, theta=0.5)
solver.theta = 1.0    # or change it mid-transient

Values outside [0.5, 1] raise ValueError: that is exactly the A-stable range, and below it the fast spatial modes - the ones quantified below, decaying at ~1e5 to 1e6 per second - would diverge at any useful step size.

Both ends of the range are unconditionally stable, but only backward Euler damps the stiff modes. theta = 0.5 is A-stable and not L-stable: a mode with zeta = |lambda| dt >> 1 has amplification (1 - (1-theta) zeta)/(1 + theta zeta), which is ~1/zeta at theta = 1 but tends to -1 at theta = 0.5, so it decays slowly with an alternating sign instead of being killed.

Whether that ever shows up depends on how much stiff content the state actually contains. The stiff modes here are the mesh-scale spatial harmonics: on the 8-cell slab in tests/test_kinetics.py the checkerboard mode decays at ~6e5 / s, so dt = 1e-3 puts it ~560x beyond the resolved range and a localized flux bump still retains ~80% of its size after 20 steps, reversing sign on every one, while backward Euler annihilates it in a single step. A smooth, mode-shaped perturbation excites almost none of that - the TWIGL step insertion in tests/test_benchmarks.py moves the flux shape by only ~1e-5 and shows no ringing at all. Nothing diverges either way. When you do have a stiff perturbation, damp it first and then switch:

solver.update_materials(perturbed)   # step insertion at t = 0
solver.theta = 1.0
solver.run(dt, 2)                    # two damped steps
solver.theta = 0.5
solver.run(dt, n_steps)              # second order from here on

update_materials takes effect at the start of the next step, which is what a step insertion at t_n means: the new cross sections hold across the whole of [t_n, t_n + dt], the explicitly weighted term included.

Choosing dt and max_inner. An implicit fission source means the inner Gauss-Seidel sweep resolves the multiplication as well as the scatter coupling, and only the 1/(v_g*dt) diagonal term keeps that iteration contracting. When 1/(v*dt) << Sigma_r a near-critical problem converges at roughly k per sweep. The solvers apply Aitken extrapolation to that fixed point: the convergence ratio is estimated from successive iterate changes and, once it has held steady, the iterate jumps to the limit of the geometric series. In the worst case tested - exactly critical, zero leakage, 1/(v*dt) at 3% of Sigma_a - this cuts the iterations per step from thousands to a few dozen. The extrapolation is safeguarded (ratio stability judged against 1 - sigma, and the jump capped relative to ||phi||) and is self-correcting, since convergence is still measured across the sweep. Any step that nonetheless hits max_inner prints a warning to stderr naming the solver and the residual - never trust a transient that warned.

See examples/kinetics.py for a runnable end-to-end transient.

The ndiffusion.boundary_conditions(Dg, alpha) helper constructs the coefficient array from an albedo value alpha (0 = vacuum, 1 = reflective).

Two Python helpers layer on top of the compiled solvers (they reuse the existing solver classes, so no rebuild is involved):

• Adjoint - ndiffusion.make_adjoint_materials(mats) returns the adjoint cross sections (group scatter transposed, chi/nusigf swapped). Running any solver on them solves the adjoint (importance) problem; the k-eigenvalue is identical to the forward one, and the flux is the neutron importance function.

• Method of nearby problems (MNP) - a discretization-error estimator (ndiffusion.nearby_fixed_source, ndiffusion.nearby_k_eigenvalue). It fits a smooth curve through the numerical flux, substitutes it into the continuous diffusion operator to form a residual source, and re-solves the resulting "nearby problem" whose exact solution is the fit - so nearby - fit estimates the true error. Works in 1-D, 2-D structured, and 2-D unstructured (a high-order least-squares reconstruction supplies the Laplacian on the FVM mesh). Requires SciPy: pip install ndiffusion[nearby].

solver = nd.FixedSourceSolver(mats, medium_map, edges_x, nd.Geometry.Slab, bc)
result = nd.nearby_fixed_source(solver, mats, source,
                                medium_map=medium_map, edges_x=edges_x,
                                geometry=nd.Geometry.Slab)
# result.error_estimate estimates (numerical - exact) flux

To build and run the 1-D reference problems without Python:

cmake -B build -DCMAKE_BUILD_TYPE=Release
cmake --build build
./build/cpp/ndiffusion_driver

CMakeLists.txt              top-level CMake
pyproject.toml              build config (scikit-build-core)

cpp/
  CMakeLists.txt
  include/ndiffusion/
    types.hpp               shared types: Geometry, Materials, BoundaryCondition, results
    solver_1d.hpp           1-D solver class declarations
    solver_2d.hpp           2-D structured and unstructured solver declarations
    solver_3d.hpp           3-D solver declarations (placeholder)
  src/
    solver_1d.cpp               1-D solver implementation
    solver_2d_structured.cpp    structured 2-D implementation
    solver_2d_unstructured.cpp  unstructured 2-D implementation
    main.cpp                    standalone driver (1-D reference problems)
  python/
    bindings.cpp            pybind11 bindings -> ndiffusion._core

src/ndiffusion/
  __init__.py               re-exports from _core + create/mesh utilities
  create.py                 make_materials / make_medium_map / boundary_conditions
  transport.py              transport -> diffusion cross-section transform
  adjoint.py                make_adjoint_materials - forward -> adjoint transform
  kinetics.py               delayed neutron data + critical scaling helpers
  nearby.py                 method of nearby problems (fixed-source & k-eigenvalue)
  mesh.py                   load_gmsh - Gmsh .msh import for unstructured meshes

tests/
  test_1d_k_eigenvalue.py
  test_1d_time_dependent.py
  test_1d_fixed_source.py
  test_2d_k_eigenvalue.py
  test_2d_time_dependent.py
  test_2d_fixed_source.py
  test_kinetics.py
  test_benchmarks.py

examples/
  k_eigenvalue.py
  time_dependent.py
  kinetics.py
  transport_cross_sections.py
  c5g7_quarter_core.py

pip install -e ".[dev]"
pytest

The test suite lives in tests/ and is configured via pyproject.toml.

Install the extra. Optional dependencies are handled with pytest.importorskip, so a bare pip install -e . gives a run that looks clean while quietly skipping tests: without scipy that is the point-kinetics reference comparisons in tests/test_kinetics.py - the primary kinetics validation - plus all of tests/test_nearby_*.py. Run pytest -rs to see what skipped and why. (tests/test_mesh_gmsh.py needs pip install -e ".[mesh]", which is a heavier dependency and is not part of dev.)

The Testing/ directory is a generated CMake/CTest artifact and is not part of the source test suite.

Doxyfile configures Doxygen for the C++ sources under cpp/include/ndiffusion, cpp/src, and cpp/python. To generate the HTML documentation:

doxygen Doxyfile

Or, after configuring CMake:

cmake --build build --target docs

The output is written to docs/doxygen/html/.

Geometry

• 3-D structured geometry (x-y-z) and 3-D unstructured (tetrahedra/hexahedra)
• General boundary conditions on all edges (1-D currently hardcodes symmetry at the left/inner edge; 2-D structured hardcodes left and bottom as reflective)
• Non-orthogonal correction for the unstructured FVM two-point flux approximation (accuracy degrades on skewed meshes)

Physics

• Automatic time-step control, using the difference between the theta = 1 and theta = 0.5 answers as a local error estimate
• Improved quasi-static or adiabatic kinetics, factoring the flux into a point kinetics amplitude and a slowly varying shape
• Thermal-hydraulic feedback (Doppler / moderator density) driving update_materials from the power distribution
• Sensitivity and perturbation analysis built on the adjoint importance function (the adjoint materials transform make_adjoint_materials now exists)
• Depletion coupling - Bateman equations for nuclide inventory evolution

Solvers and performance

• Flip the default inner solver for the 2-D k-eigenvalue solvers to the within-group CG (now a use_cg constructor option; default remains Gauss-Seidel, overridable via NDIFFUSION_KEIG_CG=1); extend CG to the fixed-source and time-dependent solvers, replacing hand-tuned SOR
• Power-iteration acceleration (Wielandt shift or Chebyshev extrapolation); CMFD (Coarse Mesh Finite Difference) for unstructured k-eigenvalue convergence. The transient inner iteration already uses Aitken extrapolation (FissionAccelerator); the same idea would apply to the k-eigenvalue outer
• Zero-copy numpy arrays across the pybind11 boundary (fluxes and sources currently cross as Python lists)
• OpenMP parallelism for the spatial sweep loops

Testing

• Published two-group benchmark regressions are in tests/test_benchmarks.py (1-D Ringhals-4 slab, 2-D TWIGL, 2-D IAEA PWR on the stepped quarter core, and 2-D BIBLIS full-core PWR); the C5G7 quarter core runs end-to-end in examples/c5g7_quarter_core.py (mesh + 7-group transport cross sections + unstructured solver); still to add: a CI-sized C5G7 diffusion regression
• The TWIGL kinetics transient (TestTwiglKinetics) currently validates against its own static reactivity rather than the benchmark's published power history; digitising that history would turn it into a true published regression