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Davis Besse football size hole
In March 2002, plant staff discovered that the borated water that serves as the reactor coolant had leaked from cracked control rod drive mechanisms directly above the reactor and eaten through more than six inches (150 mm) of the carbon steel reactor pressure vessel head over an area roughly the size of a football (see photo)“. This “left only 3⁄8 inch (9.5 mm) of stainless steel cladding holding back the high-pressure (~2500 psi, 17 MPa) reactor coolant. A breach most likely would have resulted in a mass loss-of-coolant accident, in which reactor coolant would have jetted into the reactor’s containment building and resulted in emergency safety procedures to protect from core damage or meltdown. Because of the location of the reactor head damage, such a jet of reactor coolant might have damaged adjacent control rod drive mechanisms, hampering or preventing reactor shut-down.” After finding the hole, the Davis-Besse owner (First Energy of Akron) bought a used, yes USED, replacement head, which had cracks and problems! Talk about doing it on the cheap! (The used had problems too, so had to be replaced). http://en.wikipedia.org/wiki/Davis–Besse_Nuclear_Power_Station

Straight from the Nuclear Beast itself, US NRC:
(Appropriate, and easier on the eye, puke yellow and red highlights added).
Pressurized Water Reactor Embrittlement NRC p. 1
Pressurized Water Reactor Embrittlement NRC p. 2
PWR Stress Embrittlement, p. 3
Pressurized Water Reactor Embrittlement NRC p. 4

Click to access prv.pdf

Greenpeace on dangers of old reactors: http://www.greenpeace.org/switzerland/Global/switzerland/de/stromzukunft_schweiz/atom/ageing2014/Lifetime-extension-of-ageing-nuclear-power-plants-Entering-a-new-era-of-risk.pdf