March 20, 2002 — The well-chronicled reactor vessel degradation issues that have surfaced at FirstEnergy’s Davis-Besse nuclear plant appear to be confined to that facility.
However, the issue appears serious, and has the potential to affect a number of other pressurized water (PWR) nuclear operators, according to a report by Merrill Lynch.
Simply stated, the technical crux of Davis-Besse’s problem is corrosion of the reactor’s “lid,” which not only holds the control rods but also contains the pressurized primary coolant water. While surface corrosion of this component is common, Davis-Besse’s corrosion extended almost entirely through the reactor vessel head; failure of the head would be very bad, the report states.
The U.S. Nuclear Regulatory Commission (NRC), which held a meeting on this issue Wednesday, has characterized this issue as “extremely significant” and “not anticipated.”
During Besse-Davis’ planned outage, the nozzles were inspected with ultrasonic testing. Nozzles 1, 2, and 3 had through-wall cracks that leaked, and nozzles 5 and 47 had cracks that were not through-wall. Problems were encountered during the repair of nozzle 3. Nozzle 3 was removed from the reactor vessel head and boric acid deposits were removed. Inspectors found degraded areas adjacent to nozzles 1, 2, and 3, according to a report issued by the NRC Wednesday.
The NRC takes this issue very seriously, and has so notified PWR owners in a March 18 letter, but has not yet taken actions that would force extend outages or unplanned plant shutdowns. More extensive information is available at the NRC’s web site, https://nrc.gov/reactors/operating/ops-experience/vessel-head-degradation.html.
This issue only affects PWR units, and there are only seven with reactor vessels made by Babcock & Wilcox, who built Davis-Besse’s. However, there are a number of PWR plants of Davis-Besse’s vintage. The report also pointed out that two-thirds of the companies Merrill Lynch covers have nuclear exposure.
There are currently 10 PWR units with an aggregate capacity of 10,580 MW off-line for refueling. It may be reasonable to assume that at least some of these outages could be extended a bit to allow for closer inspection. If the problem turns up at another PWR, widespread inspection outages are almost assured.
Companies with big PWR nuclear fleets-especially those with unregulated plants-would be most exposed. Key names here are Entergy, Dominion, and Exelon, all of which have expanded into the “merchant” nuke business. Exelon is the biggest nuclear player, but its fleet is dominated by non-PWR plants-and all but one of its PWRs are pretty new. Duke, Constellation, and Progress Energy also have plants that could face closer scrutiny.
Apart from increased nuclear O&M costs, outages also drive up power prices-bad for those whose nuke plants are already down, but good for those who generate with natural gas, or particularly coal. If this becomes a widespread PWR issue, owners of boiling water reactors (BWR’s) could also profit nicely.
How bad is it?
That’s the question that the Nuclear Regulatory Commission (NRC) and the financial community are both trying to address. Last week’s disclosure of major reactor vessel head corrosion at FirstEnergy’s Davis-Besse plant in northwestern Ohio prompted Merrill Lynch to downgrade that stock, but also raised issues for other operators of pressurized water reactor (PWR) plants around the U.S.
Over the past couple of years, nuclear power plant operators have considerably improved the performance of their facilities. Capacity factors are way up; the industry average was 88% in 2001 vs. 70% or so five years ago. Refueling outage times have dropped to weeks from months, and protracted corrective outages have become almost unheard of. Even the so-called “troubled” plants of a few years ago, such as AEP’s Cook, PSEG’s Salem, and much of the Unicom (now Exelon) fleet have come back strong, churning out large quantities of megawatts and contributing nicely to their owners’ bottom lines.
FirstEnergy’s recent discovery of substantial reactor vessel head corrosion at its Davis-Besse plant bears watching. As it’s a new issue, it will likely lead the NRC to scrutinize other plants for similar problems. Accordingly, inspections and analysis will no doubt ensue at other facilities. Industry veterans will recall the PWR steam generator tube cracks and BWR core shroud metallurgical issues that cropped up at one plant and were then seen in many others in the early- to mid-1990s.
The reactor vessel head (RVH) is a big lid, over six inches thick and weighing hundreds of tons. Apart from keeping the pressurized, hot, radioactive coolant water surrounding the reactor core where it belongs, the vessel head also holds the control rods, which regulate the nuclear reaction taking place inside. Six inches is a lot of steel; it’s there for weight, strength, insulation, and to slow down the bombardment of neutrons that emanates from the reactor’s core.
The illustration at the top of this article highlights a particular section around a group of control rods where six inches of steel surrounding the stainless interior cladding was essentially reduced to useless rust.
If the RVH failed, there would be at least some loss of coolant, although it would be confined to the protective concrete containment structure and therefore not a public health risk.
Most inspections for this issue have been routine. RVH surface corrosion first became an issue at Duke’s Oconee 3 reactor in Seneca, South Carolina. The NRC issued a bulletin identifying the problem in August of last year, and directed PWR operators to look for it and report back. The industry has been doing that, and yesterday reported its findings at a meeting with NRC Staffers.
In a nutshell, the industry said it had narrowed the “root causes” of the problem down to: 1) leakage around the control rods (which would squirt out and settle on top of the RVH), 2) minor leakage from the associated plumbing above (which in turn dripped on the RVH), or 3) a combination of the first two.
These inspections have so far been routine, but the findings at Davis-Besse raise new issues.
The NRC Staff’s probing questions seemed to focus on the fact that the Davis-Besse RVH had corrosion that ate through six inches of steel. That’s a lot of steel, and doesn’t quite sound like “surface” corrosion.
In Tuesday’s meeting, the tone of questioning seemed to go along the lines of: 1) Did you consider the possibility that something else is afoot here, that perhaps there’s something other than drips or some hissing steam? 2) That it didn’t show up until the guys were up there grinding and welding and the darned thing just busted? 3) And that maybe that kind of damage wasn’t visible to the human eye or digital video camera?
While the industry has been focused on drips of boric acid, the NRC seems to be asking if there might not be another root cause.
The NRC Staff filed a twelve page bulletin Tuesday that asked each owner to address whether corrosion inspections had been conducted, to what extent, and whether or not findings and/or actions taken were sufficient to assure regulatory compliance. Operators were given 15 days to respond.
The NRC Staff had scheduled another meeting on this issue Wednesday at its headquarters. Merrill Lynch is not predicting a return to the difficult early 1990s, but is aware that nuclear plants have been back in the news since last year’s terrorist attacks and there is a heightened level of public concern over plant safety and security.
To read the NRC’s reports and news on reactor vessel head degradation, visit the commission’s web site at https://nrc.gov.