Three Mile Island (USA)

Map of Three Mile Island

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Unit-1: 800 MW PWR, constructed by B&W; grid connection in 1974
Unit-2: 900 MW PWR, constructed by B&W; grid connection in 1978; shutdown after severe accident in 1979

Facilities in Three Mile Island

plantreactor typconstruction startoperation startshut down
Three Mile Island-1PWR19681974
Three Mile Island-2PWR196919781979

TMI-1, which sits on an island in the middle of the Susquehanna just downriver from Harrisburg, was protected by a sea wall built when the plant was under construction. The wall rises 93 m above sea level and was not breached. The river reached 88 m above sea level, but a flood alert isn't even declared until it reaches 88,5 m.


During outage unusual buildup of corrosion products on 40 of the 177 fuel assemblies was found. Some of the fuel rods show some thinning from the corrosion. This rods have to be replaced. Some rods have pin holes.


As cause of TMI fuel thinning low pH in the reactor cooling water and "hot spots", caused by a lower coolant flow rate in certain parts of the core, caused an unusual build-up of corrosion products that led to thinning of 94 Babcock & Wilcox (B&W) fuel rods in Three Mile Island-1's core, GPU Nuclear Corp. (GPUN) says.
The lower pH was caused by relatively high boric acid - tied to use of higher-enriched fuel - and low lithium hydroxide concentrations.
TMI-1 went back on line October 12, ending a 34-day maintenance and refueling outage.
A recent NRC report found that the cladding defects occurred only in first-cycle fuel with 4.75% enrichment.
In all, 266 fuel rods were inspected and 87 rods were replaced with solid-stainless steel "dummy" rods. The other problem rods were in fuel assemblies discharged to the spent fuel pool.


RCS-leak > 55 l/min from a transmitter into the containment.