Three Mile Island 1
DecommissionedConstellation Energy·Middletown, PA·PWR·786 MWe
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Current reactor power level from the most recent NRC daily report.
Shut Down
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Days of continuous operation since the last outage ended. Ranked against all operating units.
2,033
Current Run
days
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Longest recorded period of continuous operation between outages.
2,033
Record Run
days
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Breaker-to-Breaker runs — continuous operation exceeding the cycle-adjusted threshold (619 days for this 24-month cycle unit) with no trips to 0% power.
4
B2B Runs
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Lifetime capacity factor: average power level across all recorded days (100% = always at full power). Ranked against all NRC-tracked units.
87%
Capacity Factor
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Net electricity actually generated over the most recent 12 months reported by the U.S. Energy Information Administration (Form EIA-923). These are authoritative actuals, not an estimate.
6.46 TWh
Net Generation
EIA-923 · 12 mo to 2019-10
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Total NRC-reported scram events (emergency reactor shutdowns). Ranked fleet-wide — fewer scrams = better rank.
2
Scrams
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Years since commercial operation began.
45
Operating Age
years
Excellence Score, Records & Plant Facts
Breaker-to-Breaker Runs 4 runs
Breaker-to-breaker runs measure continuous operation between refueling outages using a cycle-adjusted threshold (90% of cycle length minus RFO duration). An active streak means the unit has been running continuously since the last refueling with no trips to 0% power.
Three Mile Island 1 has logged 4 breaker-to-breaker runs, continuous stretches from one refueling to the next with no trip to zero power. Its longest ran 709 days, and its runs average 697 days.
709
Best Run (days)
T1R22 (2017-10-12) → 2019-09-21
697
Avg Run (days)
★ Double breaker-to-breaker
T1R14 → T1R15 → T1R16
All Runs
| 2017-10-12 | 2019-09-21 | 709 days | T1R22 → ? |
| 2007-11-21 | 2009-10-26 | 705 days | T1R17 → T1R18 |
| 2003-12-05 | 2005-10-24 | 689 days | T1R15 → T1R16 |
| 2001-12-05 | 2003-10-19 | 683 days | T1R14 → T1R15 |
About This Site
Written by NukeWorker from public records: facts that do not change. Operator, status and capacity come from the live data elsewhere on this page. Sources: Wikipedia: Three Mile Island Nuclear Generating Station.
Three Mile Island Nuclear Generating Station is on an island in the Susquehanna River in Londonderry Township, Dauphin County, Pennsylvania, just south of Harrisburg.
Read more
General Public Utilities Corporation built the plant, and its subsidiary Metropolitan Edison operated it. It has two pressurized water reactors designed by Babcock & Wilcox. Construction began on May 18, 1968, for Unit 1 and November 1, 1969, for Unit 2. Unit 1 entered commercial operation on September 2, 1974, and Unit 2 on December 30, 1978. On March 28, 1979, a cooling system malfunction caused a partial meltdown of the Unit 2 core, and that unit never operated again. Unit 1 was permanently shut down on September 20, 2019.
Facility Statistics
Key performance and lifecycle metrics calculated from NRC operational data. Includes operating age, construction timeline, capacity factor, efficiency, generation totals, and reliability records.
Three Mile Island 1 has operated for about 45 years and averaged a 87% lifetime capacity factor, generating roughly 133.0 TWh of electricity. Its longest unbroken run reached 2,033 days.
Lifecycle & Tenure
Operating Age: 45 Yrs
Build Time: 6.3 Yrs
Lic. Remaining: N/A Yrs
Production & Power
Lifetime CF: 87%
Efficiency: 30.6%
Generated: 133.0 TWh
Reliability & Outages
Record Run: 2,033 Days
Refuelings: 10
Avg Startup: 4.5 Days
Ownership & Construction
The utility or company that owns and operates the facility, the architect-engineer responsible for plant design, and the primary construction contractor. Click names to view all facilities by that entity on the map.
Ownership, operation and management history
| Date | Change | From | To |
|---|---|---|---|
| Dec 20, 1999 | Sold | GPU | AmerGen (PECO Energy / British Energy) |
| Dec 22, 2003 | Ownership stake | AmerGen (Exelon / British Energy) | AmerGen (wholly owned by Exelon) |
| Jan 2009 | Merger | AmerGen | Exelon |
| Feb 1, 2022 | Spin-off | Exelon | Constellation Energy |
CEG
Constellation Energy Corporation
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Scram History 2 events since 2011
Automatic and manual reactor scram (emergency shutdown) events reported to the NRC since 2011. Includes event dates, trip type, and average recovery time. Click the title to view the full fleet scram dashboard.
Three Mile Island 1 has recorded 2 reactor scrams, or emergency shutdowns, since 2011 (2 automatic, 0 manual). On average the unit was back to full power about 9 days after each trip.
Technical Specs
Reactor design specifications including type (PWR/BWR), containment design, nuclear steam supply system manufacturer, thermal and electrical power ratings, and current operational status.
Reactor Type:
PWR
Containment:
PWR-DRYAMB
NSS Supplier:
B&W LLP
Turbine Supplier:
GENERAL ELECTRIC CO. IAEA
Thermal Power:
2568 MWt
Net Capacity:
786 MWe
Gross Capacity:
880 MWe IAEA
Design Capacity:
819 MWe IAEA
Efficiency:
30.6%
Status:
Decommissioned
Lifetime Output:
245.1 TWh through 2025 IAEA
Licensing
NRC licensing history including docket number, license type and number, construction permit date, operating license issuance, commercial operation date, license renewal, and expiration date.
Docket #:
05000289
License #:
DPR-50
License Type:
OL-FP
Const. Permit:
1968-05-18
Construction Start:
1968-05-18 IAEA
Operating Lic:
1974-04-19
First Criticality:
1974-06-05 IAEA
Grid Connection:
1974-06-19 IAEA
Commercial Op:
1974-09-02
License Renewed:
2009-10-22
License Exp:
2034-04-19
Market & Region
EIA market and grid-region classifications: NERC reliability region, U.S. Census region, balancing authority responsible for grid operations, and whether the unit operates as a rate-regulated utility or a competitive merchant generator. Hover any value for the full name.
Balancing Authority:
PJM
— PJM Interconnection (ISO/RTO)