THREE MILE ISLAND
OperatingNuclear power plant
- Capacity
- 976 MW
- Primary fuel
- Nuclear
- County
- Dauphin County
- Operator
- EXELON NUCLEAR
- Status
- Operating
- Commissioned
- 1974
- EIA plant ID
- 8011
- Generators
- 1
- Technology
- Nuclear
- Data vintage
- EIA-860 2024 final; EIA-923 2022-2024 final
- Coordinates
- 40.1545, -76.7249
THREE MILE ISLAND, a 976 MW Nuclear power plant in Dauphin County, Pennsylvania. Operated by EXELON NUCLEAR, commissioned 1974.
How THREE MILE ISLAND compares
With 976 MW of capacity, THREE MILE ISLAND ranks 13th among the 170 operating power plants Bywatts maps in Pennsylvania and is the smallest of the 5 operating Nuclear plants in the state. It accounts for about 9.7% of Pennsylvania's operating Nuclear capacity and 2.5% of all operating capacity in the state. Nationally it is the 47th-largest of the 55 operating Nuclear plants in the database. Commissioned in 1974, it is older than 75% of the 168 operating Pennsylvania plants with a recorded commissioning year. Its operator, EXELON NUCLEAR, runs 3 operating plants in Pennsylvania totaling 5,572 MW. It shares Dauphin County with 2 other operating plants, HARRISBURG FACILITY and PAXTON CREEK COGENERATION.
EIA-860 lists 1 generator at the site with 981 MW of nameplate capacity, all nuclear technology.
Grid neighborhood
The nearest named substation is THREE MILE ISLAND (500 kV), about 0.7 miles away; the closest transmission line found within 10 miles is a 230 kV line owned by METROPOLITAN EDISON CO, under 0.1 miles away. Another 16 operating power plants totaling 3,091 MW sit within 25 miles; the largest is PPL BRUNNER ISLAND (1,567 MW, Coal).
Computed by Bywatts from HIFLD power plant, substation and transmission line records and EIA-860/923 survey data; distances are straight-line. Sources and methodology
Weather peril exposure
THREE MILE ISLAND is a 976 MW nuclear power plant in Dauphin County, PA, operated by EXELON NUCLEAR, commissioned in 1974. Dauphin County carries low modeled catastrophe exposure in the FEMA National Risk Index, scoring 1.3 of 5 at the 9th national percentile. Hurricane is the leading peril, rated Relatively Moderate by FEMA at the 77th percentile.
The Bywatts loss layer is calibrated to solar PV hardware, so it is not applied to a nuclear plant. The ratings below are the FEMA National Risk Index county hazard ratings for this location.
Dauphin County hazard ratings
| Peril | FEMA rating | National percentile | Peril score | Events per year |
|---|---|---|---|---|
| Hurricane | Relatively Moderate | 77th | 4.1 of 5 | 0.07 |
| Riverine flooding | Relatively High | 70th | 3.8 of 5 | 2.0 |
| Earthquake | Relatively Low | 58th | 3.3 of 5 | <0.01 |
| Winter weather | Very High | 50th | 3.0 of 5 | 12.3 |
| Tornado | Relatively Moderate | 29th | 2.1 of 5 | 0.28 |
| Strong wind | Relatively High | 16th | 1.6 of 5 | 4.7 |
| Wildfire | Very Low | 12th | 1.5 of 5 | <0.01 |
| Hail | Very Low | 11th | 1.4 of 5 | 2.2 |
| Lightning | Relatively High | 10th | 1.4 of 5 | 35.2 |
| Ice storm | Relatively Low | 9th | 1.3 of 5 | 0.58 |
| Heat wave | Relatively Moderate | 7th | 1.3 of 5 | 5.1 |
Perils not assessed in Dauphin County
These perils carry no usable National Risk Index figure for this county, so they are excluded from the scoring rather than scored zero. A zero would claim the peril was assessed and found benign.
| Peril | Why it is not scored |
|---|---|
| Coastal flooding | FEMA rating is "Not Applicable" for this county |
Model hail, wind and flood exposure for Dauphin County in the weather risk calculator
Ratings are the FEMA National Risk Index county hazard ratings; the peril score is a Bywatts transform of the FEMA national percentile. Full data sources and the solar vulnerability methodology are in the weather risk calculator.
Modeled estimate for screening and negotiation. Built from FEMA National Risk Index county hazard data and a solar vulnerability layer documented above. Not a quote, a binder, or actuarial advice, and not a substitute for a carrier's own model. Hazard data: FEMA National Risk Index December 2025, synced 2026-07-24. Resolution is county level, not site level.