OWEN

Open Workspace for Engineered Neutronics

VS Code Extension for Nuclear Monte Carlo

Monte Carlo language server, MCNP cross-file workspace validation, Input Builder snippet wizards, cross-code deck converter, native OpenMC render and geometry verification, ALLEN Doppler Studio, 3D full-core preview, results dashboards, lattice builder, and simulation automation — inside the editor you already use.

MCNP workspace validation

Set a project root — validate cells, surfaces, materials, and universes across read/copy includes; undefined refs and duplicate IDs surface in Problems

Input Builder wizards

Material, surface, cell, lattice, source, and settings snippet wizards with validation preview — Ctrl+Shift+I

Cross-code converter

MCNP↔OpenMC stable, MCNP→Serpent/SCONE experimental — Rosetta diff webview, TODO markers for gaps

OpenMC render & verify

Native slice plots, overlap and lost-particle checks — validate converted geometry before you run

Results viewer & sweeps

k-eff convergence, flux spectra, tally tables, mesh heatmaps; parametric sweep dashboard over run dirs

PNNL-15870 materials

411-material compendium — search by name or ZAID, insert per-code cards from the command palette

Monte Carlo language server

Real-time diagnostics, hover, go-to-definition, find-references, and document symbols for MCNP/Serpent/SCONE

Syntax highlighting

4 palettes (Classic, Solarized, High Contrast, Pastel) for MCNP, OpenMC (Python), Serpent, SCONE

MCNP reference tracker

Role-aware Go-to-Definition (F12), Find References (Shift+F12), hover tooltips, lattice fill-array decoder dock

Snippets

Ready-to-edit pin cell, assembly, criticality, and shielding decks for all four codes

Input validation

Deep per-language checks: density sign, fraction sign, ZAID suffixes, S(α,β) hydrogen rules, macrobody counts

Simulation runner

Launch MCNP, OpenMC, Serpent, or SCONE in a dedicated terminal without leaving the editor

Parametric sweeps

JSON-configured parameter sweeps with per-run mutation, k-eff capture, and TSV summary

Lattice tab (Input Builder)

Integrated visual grid editor inside Input Builder — paint pin types, W 17×17 / BWR presets, emits MCNP / OpenMC / Serpent / SCONE

Prebuilt Models

Bundled BEAVRS full-core (193 assemblies) and 17×17 assembly starters for all four codes

NRDP material insert

Command-palette search by name or ZAID, insert ready-to-use material cards

Community Library

Browse and insert approved ReactorMC models — on by default; disable with owen.community.enabled

ReactorMC search panel

Search tutorials and NRDP from the command palette — results stay in-editor; open reactormc.net only when you need the full page

Highlight palette picker

Switch between four color palettes per language without restarting — applied live

3D geometry preview

Three.js instanced rendering — full-core BEAVRS (193 assemblies), component/material/axial layer toggles, fidelity modes (Auto/Disc/Layers), axial slice, solo/isolate, hover inspect, measurement tools (distance, angle, radius), BEAVRS baffle/barrel/RPV/shield pads

ALLEN Doppler Studio

Built-in σ(E) webview from ENDF/B-VIII.0 — multi-temperature overlay, resonance integral, self-shielding

v1.1.6 • VS Code & Cursor

A BelvoirDynamics Product | ReactorMC

Install OWEN v1.1.6

VS Code Marketplace Open VSX (Cursor / VSCodium)VSIX v1.1.6 (GitHub)

Install from the VS Code Marketplace in VS Code, or from Open VSX in Cursor and VSCodium — Cursor users should use the Open VSX link. Marketplace, Open VSX, and the GitHub VSIX are all on v1.1.6(Community Library browse/insert in-editor, OpenMC highlight palettes that survive Pylance).

Or install from the command line:

code --install-extension belvoirdynamics.owen-neutronics

Or via the Command Palette (Quick Open):

ext install belvoirdynamics.owen-neutronics

Source on GitHub.

Key settings

owen.mcnp.executable — path to mcnp6 for the run command.
owen.openmc.pythonExecutable — Python interpreter for OpenMC model scripts.
owen.serpent.executable — path to the Serpent 2 binary (sss2).
owen.scone.executable — path to the SCONE binary (WSL on Windows).
owen.mcnp.projectRoot — absolute path to the MCNP project root for cross-file workspace validation.
owen.mcnp.workspaceValidation.enabled — run project-wide MCNP validation when a root is set (default on).
owen.mcnp.workspaceValidation.warnUnused — emit unused-definition hints project-wide (default off).
owen.mcnp.lineLengthLimit — column limit for the MCNP line-length guard (default 80).
owen.preview.maxInstances — 3D preview instance budget (default 1,500,000; raise for full BEAVRS at max fidelity).
owen.community.enabled — browse the ReactorMC Community Library in-editor (on by default; turn off to stop contacting the backend).
owen.nrdp.live — live-fetch NRDP material snapshots when online.
owen.allen.dataBaseUrl — base URL for ALLEN cross-section JSON (override for offline use).

Looking for the desktop app? Visit /groves.

Cross-code deck converter

Port decks between MCNP, OpenMC, Serpent, and SCONE

Run OWEN: Convert Deck… from the Command Palette, pick source and target code, and review the output in a Rosetta diff webview. MCNP↔OpenMC is stable(BEAVRS-validated); MCNP→Serpent and MCNP→SCONE remain experimental. Unconvertible constructs are never silently dropped — they carry TODO(owen-convert) markers with a reason.

MCNP ↔ OpenMC — stableMCNP → Serpent / SCONE — experimental

After converting, use Render with OpenMC or Verify Geometry with OpenMC to confirm the model before production runs.

Full converter guide — limitations, workflow, examples

Roadmap

OWEN v1.1.6 — what's next

OWEN v1.1 adds live validation of OpenMC XML inputs (materials, geometry, settings, tallies, model.xml — with cross-file reference checks), PHITS syntax highlighting and snippets, an optional second-opinion convert backend that drives the OpenMC team's own MCNP and Serpent adapters through your Python, and a fifth highlight palette — Custom — colored from your own per-role settings, alongside visibly distinct OpenMC palettes. That sits on top of language-server diagnostics, cross-code deck conversion (MCNP↔OpenMC stable), native OpenMC render/verify, ALLEN Doppler Studio, 3D full-core BEAVRS preview, results dashboards, PNNL materials, prebuilt models, and the simulation runner. Here's where it's headed next.

Converter breadth

MCNP↔OpenMC stable already handles BEAVRS-scale geometry. Next: broader tally and source coverage, richer Serpent/SCONE targets, and tighter round-trip guarantees on assembly decks.

Language server

Cross-file workspace validation, Input Builder wizards, and the integrated Lattice tab ship in v1.0.3. Next: richer completions across include files and shared material libraries.

Broader geometry coverage

The 3D preview today renders cylinders and the BEAVRS radial structure. Next up: broader macrobody support and geometry-faithful rendering across more input styles and codes.

Validation depth

v1.0.3 validates undefined refs and duplicate IDs across MCNP includes. Next: workspace-level ZAID audits, Serpent/SCONE cross-file checks, and optional unused-definition hints.

Community models — live in v1.1.6

Search approved ReactorMC decks from the Command Palette and insert them into the editor. Upload, rate, and comment stay on this Community Library page — OWEN opens it in a browser for that path. Disable with owen.community.enabled if you want a fully offline install.

Need a fully-offline editor for air-gapped work? GROVES desktop returns as v2.0.0 by Christmas 2026.