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

Get it from the VS Marketplace, Open VSX, or GitHub Releases — the badges above show the current version on each.

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

A full BEAVRS core from a SCONE deck, at 1.2 million primitives, with the real axial stack. OWEN resolves the nested lattice — 17×17 assemblies of 17×17 pins — and draws every one of the 55,809 pin positions. Turn on axial segments and each pin expands into its true z-column, so the 36 levels of active fuel, plena, grid spacers, end plugs and nozzles are individually toggleable. Peel off the vessel, solo a layer, or slice through the core on any axis.

Geometry checked against your own OpenMC, not against OWEN's opinion of it. OWEN: Verify Geometry with OpenMC runs the model through your real OpenMC install — local or WSL — and reports overlapping cells on five sampled planes plus a short lost-particle probe. Overlap pixels come back highlighted in magenta, and the panel says plainly that sampled planes are evidence rather than proof.

Five highlight palettes per code, previewed side by side before you commit. OWEN: Choose Highlight Palette opens them on the language you picked — Classic, Solarized, High Contrast, Pastel, and Custom (your own colors) — rendered from the same color table the editor uses, so what you compare is what you get. Click a card to apply it. For OpenMC each built-in palette uses its own hue family (blue/teal, olive/blue, orange/yellow/green/cyan, lilac/rose/mint), and the palette governs the whole deck: openmc.* names, method calls (fuel.add_nuclide) and attributes (settings.batches) on any variable, and the Python layer around them — comments, strings, numeric literals, keywords, and the name a def/class introduces. That last part is what makes a switch obvious rather than subtle: on the bundled BEAVRS full-core deck the palette reaches 69% of the non-whitespace characters, against 10% when only the API names are colored. Set owen.highlight.openmc.coverage to OpenMC API only for accents alone. Custom colors live in owen.highlight.customColors — set any of the ten token roles to a hex color (or {foreground, fontStyle}); roles you leave out fall back to Classic.

Features

Write

Build

Visualize & verify

Run & analyze

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-<version>.vsix
# Cursor:
cursor --install-extension owen-neutronics-<version>.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:

Configuration

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

The Community Library now points at ReactorMC out of the box. The bundled key is a Supabase publishable key, which is public by design — Row Level Security restricts it to reading approved models, and OWEN's client is created with persistSession: false, so it cannot sign in or write anything. Set owen.community.enabled to false to stop OWEN contacting the backend at all; the bundled prebuilt models and snippets are unaffected either way.

Uploading, rating, and commenting happen on the website — OWEN browses and inserts only.

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

Why OpenMC is highlighted twice

MCNP, Serpent, and SCONE have their own language ids, so a grammar plus editor.tokenColorCustomizations is all a palette needs. OpenMC decks are Python, and Python belongs to Pylance.

VS Code colors a semantic token — which is what Pylance publishes — from the scopes that token's type maps to. openmc.Material arrives as a class and resolves against entity.name.type.class; the support.class.openmc that OWEN's injection grammar put at the same position is never consulted. The palette was applied, correct, and invisible on any machine with a Python language server installed.

So OWEN also draws the palette as editor decorations, which render above both the TextMate and semantic layers. The scanner behind them (src/highlight/openmcTokens.ts) matches the injection grammar's four patterns exactly, so both routes produce the same colors; npm run verify:openmc-tokens holds them together. Set owen.highlight.openmc.decorate to false to leave Python files entirely to Pylance.

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.

The optional adapter convert backend (OWEN: Convert to OpenMC XML (openmc adapters)) drives the OpenMC team's own converters, openmc_mcnp_adapter and openmc_serpent_adapter (both MIT License, © OpenMC contributors), through your Python environment. Neither adapter is bundled or redistributed — OWEN offers the pip install command when one is missing. OWEN's built-in converter is an independent implementation; the adapters serve as a second opinion (docs/ADAPTER_COMPARISON.md).

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.