The discovery of cracks last year in an uncoated nuclear shield structure raised questions among federal regulators that were not publicly disclosed at the time, according to newly released documents.
One Nuclear Regulatory Commission official questioned the structural integrity of the Shield Building at the Davis-Besse Nuclear Power Station near Toledo, OH, and questioned the NRC’s own staff conclusions on the damage, according to agency documents recently released under a Freedom of Information Act request.
The 2.5-foot-thick, 250-foot-tall, reinforced concrete shield building was the only above-grade structure at Davis-Besse that did not receive a white Thoroseal finish before the facility went online in 1970. The project’s builder, Bechtel, says no coating was required at the time.
|The Shield Building at the Davis-Besse Nuclear Power Station was the only building in the complex that received no protective exterior coating before the facility went online in 1970. |
However, cracks in the shield structure were discovered last year during an unrelated major repair. Facility owner FirstEnergy Nuclear Operating Co. (FENOC) and the NRC have said that the blizzard of 1978 and moisture penetration over time caused the cracking in the uncoated structure.
FirstEnergy has called the cracks superficial and says a new coat of weatherproofing now being applied over the building’s 100,000 square feet of surface area will address all of the problems.
The anti-nuclear group Beyond Nuclear, which made the FOIA request, contends that the integrity of the Shield Building has been compromised.
‘Greatly Downplay the Issue’
NRC documents show that internal questions were raised about the damage in the wake of last year’s revelations.
In an email Nov. 4, 2011, Pete Hernandez, of NRC’s Office of Nuclear Reactor Regulation, submitted detailed questions to colleagues about a report on the cracking prepared by NRC’s Division of Operating Reactor Licensing.
Hernandez writes that the staff description “seems to greatly downplay the issue.”
Capturing the Problem
“This description makes me think that they are looking at a single crack going in a circle,” Hernandez writes.
“From what I understood, the crack is pervasive along the entire surface, spidering in all directions, similar to a pane of tempered glass breaking. The description in Attachment B addresses only the crack at the opening and assumes that the crack is right along the rebar line.
“The core bores have shown that the cracks are at different depths, so this doesn’t seem to capture the current situation. Throughout the calculation, the word Crack, singular, is used. They also mention that the extent of the crack is only 10’-12’. This seems to greatly downplay the issue.”
The staff report notes “(#1) Extensive cracking in the shoulder region, (#2) Cracking in the structural region outside the flute shoulder region near the main steam piping penetrations, (#3) Cracking indication via Impact Response (IR) mapping in the cylindrical portion of the building near the top of the building at the interface between the domed roof and the cylindrical wall.”
‘Ignoring All That Concrete’
Questioning a calculation used to quantify the problem, Hernandez writes: “At this point, core bores of only the shoulders have been taken. So the only crack widths we are aware of are those in the shoulders, which are not being addressed. How can an analysis be done on the structurally credited concrete if no data from that area, in the form of core bores, has been taken? Shouldn’t the structural integrity of the shoulders be calculated as well?”
|Nuclear Regulatory Commission photos taken in late 2011 show the laminar subsurface cracking (left) at the Shield Building and core bore samples from the Shield Building.|
Regarding the methodology for a staff calculation that “focuses on the structural integrity of the reinforced concrete ... once it is restored,” Hernandez asks: “This seems to say that they are just doing calculations for the new concrete that is and ignores the rest of the building altogether. Is that right?”
The staff report also notes that “the vertical reinforcement next to each flute (i.e., in a vertical strip approximately 10 ft wide) is conservatively ignored for evaluating the structural integrity of the SB [Shield Building] under mechanical loads.”
To which Hernandez replies: This says to me, that they are ignoring the shoulders, if they are ignoring all that concrete, it seems to be the opposite of conservative for evaluating the mechanical loads.”
‘Will the Building Stay Standing’
Hernandez also challenged a staff goal to determine the seismic stability of what FirstEnergy has called decorative concrete on the structure.
“I think the greater concern is will the SB stay standing and not whether or not the decorative concrete will fall off,” he wrote. “Because the licensee has not performed core bores to see if there is cracking in the credited concrete, do they have a basis to say that the structural concrete will maintain a Seismic II/I condition?”
He adds: “This use of singular terminology also discounts this calculation because it seems that they are looking at only 1 crack and 1 shoulder or 1 flute. Because cracks have been found through multiple core bores, shouldn’t the appropriate calculations account for the combined effects of cracks in all the shoulders and not just one by the opening and not just individually?”
‘Must Be Validated’
Finally, he questions the use of Impulse Response testing to support the staff’s conclusion that “the actual crack length is 10 to 12 feet long.”
“From what I understand, IR mapping is only an indicator, but must be validated by core bores,” writes Hernandez.
“Does basing all the calculations on a length of a 12 foot crack discount the calculations altogether, because we have indications of cracks at distances greater than 12 feet. This also seems to assume that there is only 1 crack and not many as the core bores seem to prove. Isn’t IR mapping only useful at a limited depth too, so that using it to evaluate a 48” thick piece of concrete is not realistic?”
Almost two weeks later, in another email, Hernandez notes that FirstEnergy's own concrete/rebar bonding technical consultant has informed the operator “that with the assumptions they are making, no credit for the rebar impacted by the cracks is warranted. In light of this, the licensee has started to do more mapping and core bores to better analyze the SB.”
The NRC did not reply Monday (Oct. 15) to a request for more information. An email to Hernandez was returned Monday as undeliverable.
The documents, released in response to a Freedom of Information Act request, include pages of NRC emails, inspection reports, the Commission’s requests for additional information from plant owner FirstEnergy, and engineering reports on the cracks found in the containment structure.
The cracks have become an issue in the operator’s application for a 20-year extension on the plant’s license, which is due to expire in 2017.
The NRC says its license renewal staff is “evaluating the implications of” the cracking issue and developing an inspection protocol to ensure that FirstEnergy corrects the problems.