Candle in a Jar — a fission reactor that converts nuclear fire directly to electricity through light.
This simulation runs a 2-group neutron transport model calibrated to 35+ OpenMC Monte Carlo runs with ENDF/B-VIII.0 cross sections. Drag the sliders to change the reactor design and watch criticality respond in real time.
Each colored line is a neutron random-walking through the reactor. Fast neutrons (born from fission) scatter through the salt, slowing down until they're thermal. Thermal neutrons that hit a fuel pin can cause another fission (gold dot) — sustaining the chain reaction.
Try this: Drag the salt density to zero. Watch k-eff. In infinite lattice mode, it goes UP (+5,369 pcm — the Chernobyl direction). Switch to finite core mode. Now it plummets to ~0.3. The candle cannot burn without the jar.
The same physics gives opposite answers depending on whether you assume an infinite or finite reactor. Drag the slider and watch both sides simultaneously.
k∞ = η × f × p × ε. Each factor responds differently to salt density. Watch which ones drive the positive void coefficient.
21-point void sweep from actual Monte Carlo simulation. 50,000 particles × 150 batches. ENDF/B-VIII.0 cross sections. The orange dots are OpenMC; the line is our 2-group model.
The reactor IS the light bulb. LiCl vapor emits at 670.8nm (Li D-line) — a discrete spectral line matched to III-V photovoltaic cells. Not blackbody radiation. Monochromatic nuclear light.
Same physics, same architecture, same candle. Four orders of magnitude.
| Variant | Enrichment | Core | U-235 | W/kg |
|---|---|---|---|---|
| Open-source lantern | 0.72% | ~8 m | ~200 t | — |
| D₂O + table salt | 3% | ~1 m | 14 kg | — |
| HALEU compact | 19.75% | ~1.1 m | ~300 kg | ~6 |
| Moon basketball | 93% HEU | 20 cm | 39 kg | 384 |
The reactor breathes. Fission heats salt, salt boils, void fraction rises, reactivity drops, power drops, salt condenses, reactivity rises, power rises. A coupled nonlinear oscillation. Insert reactivity and watch it stabilize.
Try this: Push the reactivity slider to +200 pcm. Watch power spike, then self-stabilize as Doppler and void feedback pull it back. The reactor finds its own equilibrium. Now push to +500 pcm (near prompt critical at β = 650 pcm). The oscillation gets violent but still converges. The candle regulates itself.