Snow problem at Davis-Besse

A severe blizzard some 34 years ago was said to be the cause of cracks discovered last year in the shield building of the Davis-Besse nuclear power plant. Safety regulators are reviewing the analysis while normal operation continues.

A severe blizzard some 34 years ago was said to be the cause of cracks discovered last year in the shield building of the Davis-Besse nuclear power plant. Safety regulators are reviewing the analysis while normal operation continues.

Davis-Besse (Image: NRC)

'Tight cracks' were found within the reinforced concrete of the Davis-Besse reactor building in October last year during a major outage to replace the head of the reactor pressure vessel. Plant operator First Energy Nuclear Operating Company (Fenoc) notified the Nuclear Regulatory Commission (NRC), which decided the cracks did not represent an "immediate safety issue" and the reactor returned to service in December, while Fenoc embarked on an analysis to determine how the cracks came to be.

After considering an "exhaustive" range of potential causes including design, construction, environmental and operational factors, Fenoc yesterday announced that testing of concrete samples supported two conclusions: firstly that extreme weather had caused the cracking and also that safety performance of the building had not been affected.

During January 1978, said Fenoc, "three days of driving rain preceded a drastic temperature drop to around 0ºF (-18ºC)" while intense winds continued throughout the storm. This weather event, known as the Great Blizzard of 1978, resulted in 51 deaths in Davis-Besse's home state of Ohio. Neither the shield building nor other concrete structures on site were equipped with a weatherproof coating and similar cracks were found in both. Lab tests based on the weather conditions produced the same cracking in concrete samples, said Fenoc.

The NRC had four inspectors monitoring Fenoc while it compiled the study and the commission is now reviewing the resulting 119-page document which also details some steps that will be taken to manage the building. The company will apply a weatherproof coating, perform extra inspections to verify that cracks have not spread and develop what it called a long-term building monitoring plan.

The 30-inch (76.2 cm) thick shield building surrounds a 1.5-inch (3.8 cm) thick steel containment vessel that contains the reactor system. The two structures are separated by a 54-inch (137.1 cm) space and both have important roles in nuclear safety: The sealed containment vessel is a final safety barrier to contain radioactivity in the event of an accident while the shield building protects this from external forces.

Fenoc has a licence to operate the 908 MWe pressurized water reactor until 2017, subject to NRC's ongoing satisfaction with the company's safety performance. An application is in process for an extension of the licence to 2037.

Researched and written
World Nuclear News