OWEN

Open Workspace for Engineered Neutronics

Created by Aaron W. Calhoun

The nuclear reactor modeling toolkit for VS Code & Cursor — a Monte Carlo language server, visual lattice and input builders, full-core 3D geometry preview, native OpenMC rendering and verification, a cross-code results viewer, and workflow automation for MCNP, OpenMC, Serpent, and SCONE.

A BelvoirDynamics product · part of ReactorMC

---

OWEN brings first-class editor support for the four major Monte Carlo neutron-transport codes to VS Code and Cursor. Write decks faster with smart snippets, catch physics and cross-reference mistakes as you type with the MC Language Server, build lattices and full input decks visually, preview and verify geometry in 3D, convert decks between codes, launch solvers and parameter sweeps, and analyze the results — without leaving your editor.

See it in action

Visual Lattice Builder → MCNP 17×17 assembly, with live syntax highlighting. Pick fuel, guide-tube, and instrument-tube positions on a grid and OWEN writes the lattice deck for you.

▶ Watch full-quality MP4

3D geometry preview of a full Serpent core — component toggles, Disc/Layers fidelity, and X / Y / Z slice planes. Inspect ~56,900 pins interactively: peel away vessel and guide tubes, switch to concentric pin layers, and slice through the core.

▶ Watch full-quality MP4

Features

Write

Feature	Description
Syntax highlighting	TextMate grammars for MCNP (.i, .mcnp, .inp), Serpent (.serp), and SCONE (.scone), plus an OpenMC injection grammar for Python. Four switchable palettes per language (Classic / Solarized / High Contrast / Pastel) via OWEN: Choose Highlight Palette.
Snippets	Ready-to-edit decks: PWR pin cell, 17×17 PWR assembly, criticality array, and shielding slab for MCNP; full OpenMC pin/assembly Python scripts; SCONE fuel pin, 5×5 assembly, and shielding tutorials.
MC Language Server	A real language server for MCNP, Serpent, and SCONE: real-time diagnostics as you type — density-sign and fraction-sign conventions, S(α,β) thermal scattering on non-hydrogenous materials, ZAID format, macrobody parameter counts, MCNP line length, and cross-reference errors (a cell referencing an undefined surface/material/universe/transform is flagged; defined-but-unused entities are faded hints) — plus hover, go-to-definition, find-references, and a grouped document outline (Cells / Surfaces / Materials / Universes / Transforms / Tallies). Ships as a self-contained out/server.js, reusable by other editors over stdio. OpenMC Python files keep Pylance plus OWEN: Validate Input File.
MCNP cross-reference tracker	Role- and position-aware hover, Go-to-Definition, Find-All-References, occurrence highlight, and a MCNP References tree for MCNP decks. A number is resolved by what it is and where it sits on the card — cell id (1st field), material number (2nd field; 0 = void), geometry surface refs (signed entries), surface id (1st field of a surface card), u= universe, fill/lat (lattice fill arrays are decoded so universe references inside them resolve), trcl/tr transforms, and mt/mx material-data cards. Clicking surface 3 finds only the references to surface 3 — never material 3, cell 3, or the digit 3 inside a fill= index.
Deep validation	On-demand language-aware diagnostics with codes — ZAID format, density/fraction sign conventions, mt/S(α,β) hydrogen checks, macrobody parameter counts (MCNP); IndependentSource/RectangularPrism API checks (OpenMC); cuboid vs rect, trcl, CLI omp (Serpent); aceNeutronDatabase, temperature-suffix matching, pinUniverse radii/fills (SCONE).

Build

Feature	Description
Lattice Builder	A visual grid editor that generates MCNP / OpenMC / Serpent / SCONE lattice code from a few clicks.
Input Builder	Five-step wizard: pick code, add materials from an 18-entry curated library or the searchable PNNL-15870 Rev. 2 compendium (411 materials), pin-cell or lattice geometry, run settings, preview — then insert or open as a new file.
Materials (NRDP + PNNL)	OWEN: Insert Material from Database inserts reactor materials rendered for the detected deck language — the curated Nuclear Reactor Data Project set (bundled snapshot, optional live refresh from reactormc.net) plus the full PNNL-15870 Rev. 2 compendium with correct per-code conventions (isotopic ZAIDs with weight fractions for MCNP/Serpent, add_element/add_nuclide for OpenMC, atom densities for SCONE; S(α,β) only on hydrogenous moderators).
Prebuilt models	OWEN: Open Prebuilt Model… opens bundled, offline reactor decks in a new editor with the correct language. Ships the complete BEAVRS Cycle-1 full core (all 193 assemblies, full axial pin stacks, baffle/barrel/shields/RPV) for all four codes — MCNP, OpenMC, Serpent, and SCONE — plus 17×17 PWR assembly starters and a Reflected UO2 Pin Cell teaching model in all four codes (the OpenMC twin is run-verified: k-inf 1.2256 ± 0.0010). The SCONE full-core deck is the author-verified source of truth; the MCNP/OpenMC/Serpent decks are geometry/materials-faithful translations of it.
Cross-code converter	OWEN: Convert Deck… (owen.convertDeck) converts MCNP ↔ OpenMC — a high-fidelity engine with a full boolean region AST, multi-level universes and rect/hex lattices, transforms, graveyard handling, and tally/source mapping, validated against the bundled BEAVRS full core in real OpenMC — plus MCNP → Serpent / SCONE (experimental). Anything that can't be mapped emits a clearly marked TODO(owen-convert) comment instead of being silently dropped, and results open in a Rosetta diff view — source and converted deck side-by-side with aligned cells/surfaces/materials sections and TODO highlights.

Visualize & verify

Feature	Description
3D geometry preview	Three.js webview rendering of MCNP / OpenMC / Serpent / SCONE geometry with component / material / axial-layer toggles, slice planes, and a Disc/Layers fidelity control. Renders a full BEAVRS core (all 193 assemblies) across every code — including OpenMC cores whose lattices are built programmatically (comprehension/dict-driven assembly maps are statically expanded, no Python executed) — without dropping pins, and shows the full axial stack for OpenMC too — each pin is reconstructed as its real z-column from the deck's _SHELLS/STACKS/R[key] tables, so grid spacers, plena, end plugs and SS nozzles render with their own per-band shells/materials over the complete 0→460 cm assembly height, matching MCNP/Serpent/SCONE. Geometry is instanced (so draw calls stay low) and a configurable instance budget (owen.preview.maxInstances, default 1.5M) auto-simplifies detail (shells→discs, then collapses axial) instead of hiding pins when a deck is huge. Hover any part to read its layer, material, axial index, radius/diameter and z-range; solo a layer to isolate it; and measure distances (with Δx Δy Δz), included angles, and pin/shell radii directly in the view.
Render with OpenMC	OWEN: Render with OpenMC (authoritative) shells out to your actual OpenMC installation and shows OpenMC's own slice plots (xy/xz/yz, origin/width controls, material/cell coloring, optional 3D ray trace on OpenMC ≥ 0.15) in a panel — ground truth straight from OpenMC's geometry kernel, ideal for verifying OWEN's built-in preview or debugging geometry. Finds your interpreter automatically (settings → ms-python → PATH → WSL) and falls back to the built-in preview when OpenMC isn't installed.
Verify Geometry with OpenMC	OWEN: Verify Geometry with OpenMC runs an OpenMC model through your local OpenMC installation and checks for overlapping cells (slice plots with overlap detection at several sampled planes) and lost particles (a short capped probe run). The results panel shows per-plane images with overlap highlights, the lost-particle report, or a green all-clear — with the honest caveat that sampled planes are evidence, not proof.
ALLEN σ(E) explorer + Doppler Studio	Built-in cross-section webview: log-log σ(E) plots from ENDF/B-VIII.0, nuclide/reaction picker, multi-overlay, hover readout — with nuclides auto-detected from the active deck. Doppler Studio adds multi-temperature overlays (294/600/900/1200 K), a resonance-integral readout, and a Bondarenko σ₀ self-shielding slider. Cross-library comparison (e.g. ENDF/B-VIII.0 vs JEFF-3.3) lives on the companion reactormc.net ALLEN pages, one click away.

Run & analyze

Feature	Description
Simulation runner	One-command launcher that starts the right solver (MCNP / OpenMC / Serpent / SCONE) in a dedicated terminal, with per-code executable settings and WSL support for SCONE on Windows.
Results Viewer	OWEN: View Results parses the outputs of all four codes (OpenMC statepoint.h5 via h5wasm + stdout fallback, MCNP mctal, Serpent _res.m, SCONE .out) and shows k-eff convergence, flux spectrum (log-log), a tally table, and mesh heatmaps — mesh tallies can be overlaid on the 3D geometry preview as a colored slice plane.
Parametric sweep + dashboard	JSON-described parameter sweeps with per-run input mutation, output capture, k-eff parsing, and a manifest + TSV summary — then OWEN: View Sweep Results plots k-eff vs the swept parameter with error bars, per-run convergence small-multiples, and a run table.
Community Library	Browse and insert community-approved models (opt-in via owen.community.enabled; you supply your own Supabase backend).
Tutorials	Deep-links into the reactormc.net learning material via OWEN: Open Tutorial.

Install

From the VS Code Marketplace:

1. Open the Extensions view (Ctrl+Shift+X / Cmd+Shift+X).
2. Search for OWEN and click Install — or install belvoirdynamics.owen-neutronics.

From Open VSX (Cursor, VSCodium, etc.): install belvoirdynamics/owen-neutronics.

From a VSIX (GitHub Releases):

code --install-extension owen-neutronics-1.0.0.vsix
# Cursor:
cursor --install-extension owen-neutronics-1.0.0.vsix

Or in the editor: Extensions view → ... menu → Install from VSIX….

Commands

Open the Command Palette (Ctrl+Shift+P / Cmd+Shift+P) and type OWEN:

Command	Description
OWEN: Open ALLEN Cross-Sections	σ(E) webview — nuclide/reaction picker, log-log plot, multi-overlay, Doppler Studio
OWEN: Open Input Builder	Wizard: materials (curated + PNNL compendium) + geometry + settings → full starter deck
OWEN: Open Lattice Builder	Visual lattice grid editor
OWEN: Validate Input File	Deep MCNP / OpenMC / Serpent / SCONE checks on demand
OWEN: Run Simulation	Launch the appropriate solver in a dedicated terminal
OWEN: Run Parameter Sweep	Generate and run a JSON-described sweep
OWEN: View Sweep Results (Dashboard)	k-eff vs parameter, per-run convergence, run table
OWEN: View Results	k-eff convergence, flux spectrum, tallies, mesh heatmaps for all four codes
OWEN: Open 3D Geometry Preview	Three.js webview — full-core BEAVRS, layer toggles, measurement tools
OWEN: Render with OpenMC (authoritative)	Native OpenMC slice plots of the active OpenMC Python model (requires OpenMC installed)
OWEN: Verify Geometry with OpenMC	Overlap + lost-particle checks through your local OpenMC
OWEN: Convert Deck… (MCNP↔OpenMC)	MCNP ↔ OpenMC (stable), MCNP → Serpent / SCONE (experimental), with Rosetta diff view
OWEN: Open Prebuilt Model…	Load a bundled BEAVRS full-core, assembly, or pin-cell deck
OWEN: Show MCNP References	Open the MCNP cross-reference tracker dock
OWEN: Insert Material from Database	NRDP + PNNL-15870 material picker, language-aware
OWEN: Open Tutorial	Jump to a reactormc.net tutorial page
OWEN: Choose Highlight Palette	Switch between Classic / Solarized / High Contrast / Pastel
OWEN: Toggle Invisible Characters	Reveal tabs/trailing whitespace that break fixed-format decks
OWEN: Search Reactor Library	Community Library browser (disabled by default)

Configuration

All settings live under the OWEN section (Ctrl+, → search "owen"):

Key	Default	Notes
owen.mcnp.executable	mcnp6	Path to the MCNP executable
owen.mcnp.lineLengthLimit	80	MCNP card-image column limit (set 128 for MCNP6.2+); drives diagnostics and the editor ruler
owen.serpent.executable	sss2	Path to the Serpent executable
owen.openmc.executable	openmc	Non-Python OpenMC entry point only
owen.openmc.pythonExecutable	python	Interpreter for OpenMC model scripts; when explicitly set it is also the first candidate for Render with OpenMC
owen.scone.executable	scone	On Windows, SCONE typically requires WSL
owen.preview.maxInstances	1500000	Max cylinder instances in the 3D preview; auto-simplifies detail (not pins) above this. Raise (e.g. 4000000) for full shell+axial detail on a full core
owen.simulation.workingDirectory	""	Empty = the input file's directory
owen.nrdp.live	true	Live-fetch NRDP snapshots when online
owen.nrdp.endpoint	https://reactormc.net/data	Base URL for live NRDP JSON
owen.allen.dataBaseUrl	https://reactormc.net/data/allen	Base URL for ALLEN σ(E) JSON; override for offline use
owen.community.enabled	false	Enable the Community Library browser
owen.supabase.url	""	Supabase project URL (you supply this)
owen.supabase.anonKey	""	Supabase anon/public key (you supply this)

The Community Library is off by default and ships with no credentials. To use it, point owen.supabase.url / owen.supabase.anonKey at your own Supabase project.

Requirements

OWEN is an editor toolkit — it does not bundle the Monte Carlo solvers. To run simulations, render/verify with OpenMC, install and point the settings above at your own builds of:

• MCNP (Los Alamos National Laboratory — export-controlled, requires a license)
• OpenMC (open source; run via the Python interpreter you configure)
• Serpent (VTT — requires a license)
• SCONE (University of Cambridge — open source; on Windows it typically runs under WSL)

Syntax highlighting, snippets, the language server, validation, the lattice/input builders, the converter, prebuilt models, ALLEN, and the built-in geometry preview all work without any solver installed.

Supported languages

Language	Highlighting	Snippets	Diagnostics	Runner
MCNP	Yes (4 palettes)	Yes	Real-time (LSP) + on-demand	mcnp6 inp=…
OpenMC (Python)	Injection grammar (4 palettes)	Yes	On-demand (deep) + Pylance	python <file>
Serpent	Yes (4 palettes)	Yes	Real-time (LSP) + on-demand	sss2 <file>
SCONE	Yes (4 palettes)	Yes	Real-time (LSP) + on-demand	scone <file> (WSL on Windows)

Acknowledgements

OWEN integrates with OpenMC (MIT License, © OpenMC contributors) for the Render with OpenMC (authoritative) and Verify Geometry with OpenMC features — the images and checks in those panels are produced by your locally installed OpenMC, not by OWEN. OpenMC itself is not bundled or redistributed.

Compendium material data derives from PNNL-15870 Rev. 2 (April 2021): R.S. Detwiler, R.J. McConn Jr., T.F. Grimes, S.A. Upton, E.J. Engel, Compendium of Material Composition Data for Radiation Transport Modeling, PNNL. https://doi.org/10.2172/1782721 — via the PyNE materials-compendium export (BSD-2-Clause).

Related

• ReactorMC — tutorials, the community library, ALLEN cross-section pages, and the NRDP material data that powers OWEN.
• GROVES — the companion desktop editor for the same input languages.
• NICHOLS — Sublime Text and Notepad++ packages for the same languages.

License

MIT © 2026 BelvoirDynamics.