TL;DR · 30-second read

The Short Version

Pennsylvania’s environmental regulator has approved how the Three Mile Island nuclear plant may use river water if it restarts. The plant’s power has been promised to Microsoft to help run its computer centers.

On its busiest days the plant could draw up to 73 million gallons from the Susquehanna River, enough to fill more than 100 Olympic swimming pools. About 21 million gallons a day would turn to steam and not return.

The takeaway: reviving old nuclear plants for artificial intelligence depends on rivers and fish, not just reactor safety.

The Pennsylvania Department of Environmental Protection (DEP) has issued a Section 401 Water Quality Certification for restarting Constellation Energy’s Three Mile Island nuclear plant in Londonderry Township, Dauphin County, PA Environment Digest reported on September 18, 2026. Notice of the decision appears in the September 19 edition of the Pennsylvania Bulletin, the state’s official register.

The certification covers water withdrawals from the Susquehanna River of up to 73.2 million gallons per day on a peak day, consumptive use of up to 21 million gallons per day, and heated-water discharges back to the river at a maximum of 110°F. Anyone aggrieved by the decision has 30 days from the notice to appeal to Pennsylvania’s Environmental Hearing Board.

Executive Summary

A Section 401 certification is the state’s formal finding that a federally licensed project will meet state water quality standards. For a nuclear plant, the federal license comes from the Nuclear Regulatory Commission (NRC), and under the federal Clean Water Act that license cannot take effect for an activity discharging into U.S. waters unless the state certifies it or waives its right to do so. DEP’s action therefore removes a state-level condition on the path to restarting Unit 1, the reactor Constellation has agreed to run for Microsoft under a 20-year power purchase agreement announced in 2024.

The certification also puts hard numbers on the restart’s physical footprint. The plant expects to withdraw up to 73.2 million gallons a day at peak and consume up to 21 million gallons, water lost to evaporation in cooling towers rather than returned to the river. DEP concluded that closed-cycle cooling will minimize harm to fish drawn into or trapped against the intake.

The decision is not yet final in a practical sense: the 30-day appeal window is open, and the certification is one of several approvals a restart needs. But it shows that the fastest route to firm, carbon-free power for AI, reviving a plant that already exists, still runs through water regulators.

Why a Water Permit Gates a Reactor Restart

Most coverage of nuclear power for AI focuses on the NRC, which judges whether a reactor is safe to operate. The Three Mile Island certification highlights a second, parallel track. A steam-cycle power plant is, in large part, a machine for moving heat into water: it pulls water from a river, uses it to condense steam, and either evaporates it through cooling towers or sends it back warmer. Every one of those steps touches state water quality law.

DEP’s notice identifies three impacts it had to weigh: withdrawal from the Susquehanna, consumptive use, and thermal discharge into a river designated for warm water fishes and migratory fishes. The figures are specific. Peak withdrawal of up to 73.2 million gallons a day, consumptive use of up to 21 million gallons a day, and a discharge temperature ceiling of 110°F. Those numbers define what the restarted plant is permitted to do, and they are set by the state, not by the NRC or the power buyer.

This is the mechanism behind the headline. A restart deal with a technology customer creates the demand; a reactor license makes operation legally possible; but without the water certification, the federal license cannot rest on a finding that state water standards will be met. For the developers, utilities and hyperscalers (the largest cloud operators) counting on restarts to bridge the AI power gap, water approvals belong on the critical path alongside nuclear safety reviews.

The Consumptive-Use Number Is the One to Watch

The 73.2-million-gallon withdrawal figure is the larger number, but the 21-million-gallon consumptive-use figure is arguably the more consequential. Consumptive use is water that does not return to the river, mostly because it evaporates from cooling towers. In a closed-cycle system, the plant recirculates water through towers instead of passing river water through once and discharging it, which sharply cuts withdrawals and the number of fish pulled into the intake (entrainment) or pinned against screens (impingement). The trade-off is that evaporation is built into the design.

That matters most in dry, hot periods, when river flows drop and every user of the basin competes for the same water. The release does not describe any low-flow conditions attached to the certification, which is why the full certification document, available from DEP’s Southcentral Regional Office, will be of interest to other water users downstream and to anyone modeling the plant’s summer availability.

Pennsylvania’s Grid Stress Raises the Stakes

The certification arrives in a week that illustrates why firm generation is valuable in Pennsylvania. PJM Interconnection, the grid operator for Pennsylvania and 12 other states plus Washington, D.C., issued maximum generation and load management alerts for September 17 amid hot weather and 35,000 megawatts of maintenance outages, and a Pennsylvania Public Utility Commission study warned that rapid data center growth could cause PJM to fail a reliability standard in 2027 absent further action. The same week, DEP opened comment on a water quality permit for an Amazon Data Services campus in Luzerne County and on stormwater permits for PPL Electric transmission projects intended to serve large loads.

Against that backdrop, a restart that returns an existing nuclear unit to service is one of the few supply additions that can arrive on a data-center timeline. But the pattern across these permits is consistent: whether the project is a reactor, a data center or a transmission line, state environmental permits for water and land disturbance are a gating step. Buyers of AI capacity who treat power as purely a contracting or interconnection question are likely to underestimate schedule risk.

Background

Three Mile Island, on the Susquehanna River south of Harrisburg, is best known for the 1979 partial meltdown at Unit 2, the most serious accident in U.S. commercial nuclear history. Unit 2 never reopened, but Unit 1 continued operating until 2019, when it closed for economic reasons. In 2024 Constellation Energy, the largest U.S. operator of nuclear plants, announced a 20-year power purchase agreement with Microsoft tied to restarting Unit 1, renamed the Crane Clean Energy Center.

The deal became a reference point for a broader trend: technology companies seeking around-the-clock, carbon-free electricity for AI data centers are turning to existing nuclear assets because new reactors take many years to build. Pennsylvania sits inside PJM, the largest U.S. grid region, where data center demand growth has become a central reliability and cost concern for regulators.

Sources

Source: DEP Issues Section 401 Water Quality Certification For Restart Of Constellation Energy Three Mile Island Data Center Nuclear Power Plant In Dauphin County (PA Environment Digest), summarizing DEP’s notice in the September 19, 2026 Pennsylvania Bulletin.