An educational, browser-based nuclear power plant simulator. It models three reactor types — a PWR (Pressurized Water Reactor), a BWR (Boiling Water Reactor), and an RBMK (the Chernobyl-era Soviet design) — accurately enough to teach real plant behavior, and to reproduce the conditions behind the three most famous nuclear accidents: Three Mile Island, Chernobyl, and Fukushima.
It runs entirely in the browser as vanilla JavaScript — no server, no downloads, no WebAssembly, no framework, no build step — so it works even in locked-down institutional environments.
▶ Play it: open index.html, or go straight to the control room at
ui/shell.html?engine=pwr.
⚠️ This is an educational simulation — not engineering software. It must never be used to operate, design, train for, or make any decision about a real nuclear facility, and is no substitute for licensed operator training. Seelegal.html.
The point of the simulator is in the name. A power plant is a set of components that push on each other, and the thing worth learning is how they are coupled — change one, and know what responds, and why.
The clearest example takes about a minute. Put the control rods in manual, then lower generator demand from 100 to 60 MWe. Nobody touches the reactor — and reactor power falls to 57.5 % on its own, while average coolant temperature rises from 579 °F to 602 °F (304 → 317 °C). The plant did that itself: less steam drawn means less heat removed, so the coolant warms, and a warmer moderator is a less effective one, which pushes power down until it matches the load again. Power follows load, and the negative moderator temperature coefficient is why.
Second comes operating the plant — running a real evolution in the right order, with the right systems lined up: an approach to criticality, a power ascension, a cooldown to cold shutdown. Some steps protect against things you cannot see happen at the moment you perform them, which is exactly why they are worth practising.
Instruments are part of this, but as the layer you observe it through: readings lag, carry noise, and — in failure scenarios — can be flatly wrong. Every gauge, alarm and automatic protection here reads the instrument rather than the true state, exactly as in a real plant. That is what makes Three Mile Island reproducible, where operators watched a valve indicator reading "closed" while the valve was stuck open. It matters most when something breaks; it is not what the simulator is about.
- Real reactor physics — point kinetics with six delayed-neutron groups; reactivity feedback from fuel and moderator temperature, boron, and xenon; decay heat; two-phase steam-generator behavior; and a complete balance of plant from the core through the turbine to the grid.
- Guided training campaigns — an instructor layer watches the board and gates your progress through real procedures (approach to criticality, power ascension, load maneuvers, cooldown, casualty response), culminating in a story module set during the night shift of March 28, 1979.
- Commercial-format operator manuals built into the simulator's Training tab.
- Break it on purpose — inject stuck valves, failed sensors, and pump trips, then work out what the plant is actually doing from the couplings that still hold.
Status: the PWR control room is playable today (functional alpha). The BWR and RBMK physics engines are complete; their control rooms are under construction.
No build step. Open index.html directly, or serve the folder with any static server:
npx serve .
# or
python3 -m http.server
The control-room UI lives at ui/shell.html (the PWR card on the landing page
deep-links to ui/shell.html?engine=pwr).
There is nothing to load at runtime — no fetch, no modules, no web fonts, no images —
so the sim runs straight from file:// with no server at all. To carry it as a single
attachment:
node tools/make_portable.js # -> dist/Reactor_Dynamics_Alpha_1.9.0.html (~2.5 MB)
On Windows you can also just double-click tools\make_portable.cmd, which runs the
above and zips the result for email. Do not double-click the .js — Windows hands a
.js file to Windows Script Host rather than Node, which fails with the misleading
Syntax error, Code: 800A03EA.
Either route inlines every script and stylesheet into one self-contained page you can
double-click on a machine with no network, no Node, and no install. (Send the .zip, not
the .html — some mail providers strip .html attachments.) test/run_portable.js is the
gate that keeps it buildable.
A layered stack — snapshots flow up, commands flow down; each layer talks only to the one directly below it:
User Interface
│ commands ↓ snapshots ↑
Instructor Layer (guided campaigns, story scenarios)
Control & Failure Layer (trips, protection, ESF logic, injected failures)
Physics Engines (PWR / BWR / RBMK)
A simulation service drives the step loop and assembles each snapshot. The control layer
owns instrument-based actuations; the engines own pure hydraulics and physics. It's
all plain global-namespace JavaScript loaded via <script> tags — the same files the
Node test runners execute for the test suite.
├── index.html, about.html, legal.html, privacy.html, changelog.html ← the website
├── engines/ physics engines (pwr/, bwr/, rbmk/)
├── layers/ control, simulation service, instructor, test runner
├── scenarios/ flagship + library training scenarios
├── ui/ the control-room interface (diagram, panels, manuals)
├── test/ Node CLI test runners + harnesses
├── tools/ build tooling
├── Blueprint/ the authoritative design specification
└── Manuals/ the PWR operator manual (also rendered in-app)
Note for contributors:
Blueprint/is a dense internal engineering specification and design log, andDiagnostic/holds the candid tuning/audit/playtest reports it references. A few notes and code comments also point to aninbox/working directory (design-import scratch) that isn't published — you can safely ignore those breadcrumbs.
Reactor Dynamics is dual-licensed, © 2026 Timothy Holt:
- Code — the physics engines, control/simulation/instructor layers, UI, tooling,
and tests — is licensed under the GNU Affero General Public License v3.0
(
LICENSE). AGPL is copyleft with a network clause: anyone who distributes the code or hosts a modified version as a network service must make their complete corresponding source available under the same license. - Operator manuals & training prose — the material under
Manuals/and the human-readable mission/instructor text — is licensed under Creative Commons Attribution 4.0 (LICENSE-CONTENT): reuse freely, including commercially, with attribution.
The full public terms — safety disclaimer, no-warranty, and limitation of liability — are on the legal page.
Source: https://github.com/TH462/Reactor-Dynamics — this is also the AGPL §13 network-source offer referenced in legal.html §5.
Questions, bug reports, or notes on the physics? Email reactordynamics@gmail.com, or use the in-simulator 💬 button to send a report with your session attached.