first criticality

The point at which a nuclear reactor achieves a self-sustaining nuclear chain reaction.

First criticality is a major operational milestone for a nuclear reactor, marking the transition from a construction project to an active nuclear facility. It occurs when the nuclear fuel sustains a continuous fission chain reaction without requiring an external neutron source. At this point, the reactor's neutron population remains stable and self-sustaining, meaning the rate of neutron production from fission exactly balances the rate of neutron loss from absorption and leakage. For investors in advanced nuclear technology startups, small modular reactor (SMR) developers, or regulated utilities, achieving first criticality is a crucial de-risking event. It proves that the physical, mechanical, and engineering designs of the reactor core function as intended under real-world conditions. Successfully reaching this milestone often unlocks subsequent phases of low-power testing, regulatory approvals, and eventually, commercial power generation and revenue recognition. While first criticality is a highly positive technical catalyst, investors should not mistake it for immediate commercialization. A reactor at first criticality is operating at extremely low thermal power and is not yet generating electricity for the grid. Significant commissioning phases, safety reviews, and grid-connection steps remain before the asset can generate cash flow, and any operational anomalies discovered during this phase can lead to costly delays.

A utility company builds a new small modular reactor with a target capacity of 300 megawatts. After years of construction and regulatory reviews, the engineering team pulls the control rods to the precise height where the fission rate becomes self-sustaining, achieving first criticality at 0.05% of full thermal power. This milestone triggers a $50 million milestone payment from its joint-venture partner and allows the utility to begin the 6-month process of scaling up to 100% power for grid connection.