FirstEnergy Corp. [stock price] fell after a report that engineers discovered cracks in the concrete shell of its Davis-Besse nuclear plant. Contractors on Oct. 10 discovered a hairline crack measuring about 30 feet (9.1 meters) long as they sliced a hole into the plant's outer shell in order to install a new reactor vessel head, said Jennifer Young, a FirstEnergy spokeswoman. The cracked shell is the outermost of several layers of steel and concrete that protect the reactor, which has been shut down since Oct. 1 in preparation for the repair work. Davis-Besse was shuttered for more than three months in 2010 after workers discovered cooling water leaking through cracks in some reactor-head nozzles, steel casings that hold fuel and control rods. Leaks and reactor corrosion prompted FirstEnergy to close the plant for two years, from 2002 to 2004, while the company retrained or replaced workers who ignored signs of damage, and eventually replaced the reactor head. The leaks found last year at the 900-megawatt plant prompted the Union of Concerned Scientists in April 2010 to demand that the plant remain closed until its owners established better controls to maintain health and safety standards.