PROMPT CRITICAL
RU GitHub

Minecraft 1.21.1 · NeoForge · pre-alpha

A reactor is not a block that outputs RF.

Prompt Critical is a Minecraft mod about heavy industry, from the coal mine to the closed nuclear fuel cycle. Its physics core runs real equations. The desk on this page runs the same reactor kinetics in your browser.

Your shift

The reactor is shut down, rods fully inserted. Bring it to 100 % power and hold it within ±5 % for 15 seconds.

  • Pull the rods slowly. Power rises exponentially, and the period meter tells you how fast.
  • Protection trips the reactor if the period drops below 5 s or power passes 118 %. It does not ask.
  • Curious what prompt critical looks like? Bypass protection. Then don't.
SOURCE RANGE
PERIOD < 20 s
POWER > 110 %
PROMPT CRITICAL
AT POWER ±5 %
PROTECTION BYPASSED
REACTOR TRIP
FUEL DAMAGE
Power
—
Period
—
Reactivity
—
Rods out
—
Sim time
—
Time

Six delayed-neutron groups for U-235 (Keepin), β = 0.0065, Λ = 20 µs: the constants from pc-sim's DelayedNeutronData, integrated implicitly in 1 ms steps. Temperature feedback here is a one-line toy; in the mod it comes from the thermal-hydraulic model.

01

Why the mod is named after the scariest term in reactor physics

Only 0.65 % of neutrons from U-235 fission arrive late, seconds to a minute after the fission. Those few delayed neutrons are what makes a reactor controllable: they stretch the time between generations from microseconds to something a human, or a relay cabinet, can react to.

Once the reactivity reaches one dollar (β), the chain reaction no longer waits for them. That point is prompt criticality. Below it you steer the reactor. Above it, nothing steers it.

98.6 s vs 0.031 s
reactor period after a +0.1 $ step, with and without delayed neutrons

Reactor period vs reactivity. Inhour equation for U-235, Λ = 20 µs, computed by pc-sim. Log scale.

02

Test runs. Nobody scripted these curves.

Every chart below is a run of pc-sim, the pure-Java physics core of the mod: no Minecraft, no hand-drawn curves. The data regenerates with ./gradlew :pc-sim:siteData.

PR-300 test plant loses a pump. 300 MW, two loops. Pump B loses power at 5 s; coastdown, flow redistribution and the trip come out of the equations.
  1. 5.00 sPump B loses power and starts to coast down.
  2. 6.45 sProtection trips on low coolant flow, 2 out of 3 channels.
  3. 8 sPower is down to 6 %. Minimum DNBR in the transient: 4.16, the trip setpoint is 1.45.
  4. 60 sThe idle loop flows backwards: loop A pushes water through it at −16 %.
SCRAM: the reactor keeps making heat. Fission power from a pc-sim run with a manual trip at 5 s. Decay heat is a Way–Wigner estimate, not yet modeled in pc-sim. Log scale.
Why water boils at 345 °C in a PWR. Saturation line from pc-sim's IAPWS-IF97 tables. At 15.5 MPa, the pressure of a typical PWR primary loop, water boils at about 345 °C.

0.027 ms

mean per tick for the whole coupled PR-300 plant

03 ms budget

{n} ticks after warm-up: p50 {p50} ms, p95 {p95} ms, p99 {p99} ms. Measured on a developer machine, so treat it as an order of magnitude.

03

Five industries. Each one feeds the next.

You don't unlock a reactor with a recipe. You build the industry that can make its parts.

  1. I0

    Coal and steam

    A coal mine with methane and ventilation, coal boilers, a steam engine driving a Create shaft.

  2. I1

    Steel and power

    Coke ovens, blast furnace, converter, seamless pipes. Synchronous generators, power lines, transformers, protection relays. The first graphite pile.

  3. I2

    Oil and gas

    Seismic survey, drilling and well control, a refinery, gas processing, gas turbines.

  4. I3

    Chemistry

    Pyrolysis, polymers, chlor-alkali, syngas, ion-exchange resins, TBP for fuel reprocessing.

  5. I4

    Nuclear

    An enrichment cascade, fuel assemblies, reprocessing, isotopes and RTGs.

No ready-made reactor controller

The mod gives you sensors, function blocks and peripherals. The control system is yours to design, and it grows with the plant.

  1. A0

    Manual: valves, switches and a person watching gauges.

  2. A1

    Relay logic: interlocks wired in a cabinet.

  3. A2

    Analog computers: integrators and comparators.

  4. A3

    PLCs in IEC 61131-3 languages and safety instrumented systems.

  5. A4

    DCS and SCADA on CC: Tweaked computers.

  6. A5

    Plant-level optimization.

Block renders are placeholder art from the Blockbench sources in the repository.

04

Where it stands

Exists today

  • pc-sim: reactor kinetics, IAPWS-IF97 water and steam, the two-loop PR-300 test plant, hardware protection.
  • Era I0 in the game: a fire-tube boiler and a steam engine on a Create shaft; a mine with ventilation, methane and explosions; hoisting, drainage, a longwall, coal washing, a foundry and machine tools; the engineering handbook.
  • The NeoForge mod skeleton, CI, architecture tests that keep pc-sim free of Minecraft code.
  • Placeholder models for the blocks across all five industries.
  • The design document; the wiki summarizes it.

Doesn't exist yet

  • Eras I1–I4. Steel, oil, chemistry and a reactor in the world are still design only.
  • Releases. There is nothing to download.
  • Final art.