You’ve heard about Chernobyl. You’ve heard about Three Mile Island. As reported today in Wired News, the Davis-Besse reactor in Oak Harbor, Ohio came very close to joining this rogue’s gallery when it was literally discovered to be a reactor with a hole in its head.
What happened? The Davis-Besse reactor core is cooled with water that has concentrated boric acid powder mixed in to help absorb neutrons produced by the reactor. This reactor coolant is presurized to 2500 pounds per square inch, allowing it to stay liquid at the operating temperature of 500-600 degrees. If this pressurization is lost, the water-boric acid mixture would instantly flash into radioactive steam. This would be very, very bad. Keeping the core pressurized is the job of the containment vessel which has walls 6 inches thick constructed of strong carbon steel. Boric acid easily eats through carbon steel, tho, so the containment vessel is lined with less than a half-inch of weaker stainless steel. This stainless steel liner is not a pressure barrier; instead, it is impervious to the boric acid and is only intended to protect the carbon steel walls from boric acid.
Too bad this plan got implemented by Homer Simpson. As determined by subsequent investigation, one of the water lines developed a leak in 1995 that went unnoticed for years. An incredible 900 pounds of dried boric acid residue subsequently collected on the top of the carbon steel vessel dome, or “head”, that was not noticed during visual inspections in 1998 and 2000. The resulting corrosion continuously contaminated the cooling water to a point where filters were being changed daily instead of quarterly by 1999; this anomaly became accepted as a routine instead of raising an alarm. These years of boric acid exposure resulted in a 6-by-5 inch hole that penetrated the full six inches of the containment vessel, dissolving 70 pounds of carbon steel and reaching the 3/8 inch thick stainless steel liner. Unbelievably, this liner did a job it was never intended to do and maintained the 2500 PSI pressurization required to prevent a core breech and subsequent nuclear disaster. The liner was bowed outward in an eighth-inch high blister when it was discovered in March 2002, the only thing separating the Davis-Besse radioactive core from the Ohio countryside.
Unfortunately this story is far from over. Ongoing inspections of other US power reactors have uncovered similar problems, tho not as severe, at two other reactors as well.