What does “going critical” actually mean?
Going critical means a reactor's chain reaction has become self-sustaining. It is not an accident, not a meltdown, and not a warning — it is a normal, controlled milestone, the point at which a reactor first sustains its own reaction.
The physics
When a uranium nucleus fissions, it releases neutrons. If those neutrons go on to cause, on average, exactly one further fission each, the reaction sustains itself indefinitely at constant power. Physicists express this with the effective neutron multiplication factor, k: below 1 the reaction dies out (subcritical), at exactly 1 it is critical, above 1 it grows (supercritical). When Chicago Pile-1 achieved the first human-made chain reaction on December 2, 1942, k reached about 1.0006 — barely self-sustaining, and entirely under control.
Why “critical” is not a warning
The word borrows from mathematics — a critical point, the threshold where behavior changes — not from the language of emergencies. Every power reactor on the grid is critical the whole time it operates; steady criticality is normal operation. When a company announces its reactor "achieved first criticality," it means the reactor worked: the physics, the fuel, and the controls all did what they were designed to do.
The milestone ladder
Reactor projects announce a sequence of milestones, and news coverage often blurs them:
- Fuel load — nuclear fuel is placed in the core. The reactor is still shut down.
- First criticality — the chain reaction sustains itself for the first time, usually at very low ("zero") power.
- First grid connection — the plant delivers electricity to the grid for the first time.
- Commercial operation — testing is complete; the plant runs as a business.
For microreactors and test reactors, criticality itself is often the headline goal — some are research machines that will never deliver power to the grid.
Reactor status
Every reactor on this site carries a status — the furthest rung it has reached on that ladder (or, at the end, how it left service). The two most easily confused are grid connected and operating: a grid-connected unit is already sending power to the grid, but is still in commissioning and testing, whereas an operating unit has been formally declared in commercial service.
- Announced
- Publicly announced — a site and design are proposed, but construction has not begun.
- Site preparation
- Early site and civil works are under way; the first safety-related ("nuclear") concrete has not yet been poured.
- Under construction
- Formal construction has begun, marked by the first nuclear concrete.
- Fuel loaded
- Nuclear fuel has been loaded into the core. The reactor is fuelled but has not yet gone critical.
- Critical
- Has reached first criticality — a self-sustaining chain reaction, usually at low power — but is not yet connected to the grid.
- Grid connected
- Synchronised to the electrical grid and exporting power for the first time, but still in commissioning and power-ascension testing — not yet in commercial service.
- In commercial operation — the operator has formally declared the unit in regular commercial service.
- Shut down
- Permanently shut down and no longer generating; may be defuelled, undergoing decommissioning, or preserved in place.
- Canceled
- Abandoned before completion.
Prompt critical: the dangerous exception
A tiny fraction of fission neutrons are "delayed" — emitted seconds to minutes after fission. Reactors are designed so the chain reaction depends on these delayed neutrons, which makes power changes slow and controllable. A reactor that becomes critical on prompt neutrons alone (prompt critical) multiplies power in milliseconds — this is what happened at Chernobyl. Commercial designs use physics (negative feedback: hotter fuel slows the reaction) and engineering to keep that regime out of reach.
Common questions
What does it mean when a nuclear reactor "goes critical"?
A reactor goes critical when its nuclear chain reaction becomes self-sustaining: each fission event produces, on average, exactly one further fission. Physicists call this k = 1, where k is the effective neutron multiplication factor. First criticality is a planned, controlled milestone — the point at which a reactor first sustains its own chain reaction.
Is going critical dangerous?
No. "Critical" sounds alarming but describes the normal operating condition of every running reactor. A reactor generating electricity on the grid is critical the entire time. The dangerous condition is prompt criticality — an uncontrolled, fast-multiplying reaction — which commercial reactor designs prevent through physics (delayed neutrons, negative temperature feedback) and engineered controls.
What is the difference between critical, subcritical, and supercritical?
Subcritical (k < 1): the chain reaction dies out on its own. Critical (k = 1): the reaction exactly sustains itself — steady power. Supercritical (k > 1): the reaction grows — operators make a reactor slightly supercritical, in a controlled way, to raise power, then return it to critical to hold steady.
What happens between first criticality and producing electricity?
First criticality is usually achieved at very low power. The plant then runs a program of low-power physics tests, ascends through power levels, connects its generator to the grid (first grid connection), and finally enters commercial operation. For large power reactors this typically takes several months.
When did a reactor first go critical?
December 2, 1942. Chicago Pile-1, built under the west stands of Stagg Field at the University of Chicago by Enrico Fermi’s team, achieved the first human-made self-sustaining chain reaction at 3:25 p.m.
See the record for yourself: every first criticality since 1942, and the reactors going critical next.