Failed Bolts Bedevil a Nuclear Plant
Posted by on June 26, 2014
Truly novel issues are, generally speaking, few and far between at U.S. nuclear power plants. Whether it’s a specific type of pipe that springs a leak or an electrical relay that goes on the fritz, chances are good that the problem has been experienced before somewhere across the nation’s fleet of commercial power reactors during the many decades they have been in operation.
An issue that has drawn attention at the Salem Unit 2 nuclear power plant, a pressurized-water reactor in southern New Jersey, has to do with the failure of small bolts contained in four reactor coolant pumps. The bolts, measuring 1 inch in diameter and 4 inches in length, are used to secure a turning vane inside the pumps.
These pumps stand about 30 feet tall and provide forced flow of coolant, or water, through the reactor to transport heat from the fuel to the steam generators. The steam generators, in turn, make use of that heat by converting it to steam. The steam is then piped to the turbine to spin it and generate electricity.
As
can be seen in the graphic, water is drawn upward through the suction
nozzle at the bottom of the pump via an impeller. The turning vane
directly above the impeller then redirects the water toward an opening
on the side, from which it flows into the reactor vessel.When a refueling and maintenance outage began at the plant this spring and evaluation and maintenance work got under way, a number of turning vane boltheads were found in piping associated with one of the reactor coolant pumps and in the reactor vessel. (Similar discovery of these boltheads, albeit just a handful of them, had been observed in two prior outages.) Subsequent reviews, which have now included the examination of all of the pumps, have identified dozens of failed or sheared turning vane bolts in all of them.
Each pump has 20 such bolts. (The arrow shows the approximate location of the bolts.) A majority of the failed boltheads, though separated from the bolt shanks, remained in place thanks to mechanical restraints or tack welds.
While this is not a significant safety concern in terms of potentially causing a reactor core damage accident, there are several related operational issues. For one, the boltheads are considered foreign material that could have an adverse impact on reactor coolant system performance if they were to impact key components inside the system. For another, the turning vane could conceivably drop down and come into contact with the impeller and impede or halt its functioning.
The cause of the bolts’ failure remains under review, but one possibility is stress-corrosion cracking. Indeed, the NRC issued Information Notices to the industry in the 1990s regarding this phenomenon.
A 1994 Information Notice put out by the agency was designed to make the industry aware of stress-corrosion cracking that caused turning vane cap screws to fail at the Millstone Unit 3 nuclear power plant. Also, a 1990 Information Notice discussed the failure of turning vane bolts at a foreign reactor.
In a 1995 Information Notice, the NRC made plant owners aware of the loss of integrity for bolt-locking devices in the turning vanes of reactor coolant pumps at the Seabrook nuclear power plant but for a different reason: flow-induced vibrations.
PSEG, the owner and operator of the Salem and Hope Creek plants, will have to not only repair the Salem Unit 2 pumps but evaluate what went wrong. For now, the plant remains out of service while this work is taking place. NRC inspectors and specialists will closely follow these activities.
One area for consideration will be whether the problem could have been avoided based on previously available information.
No comments:
Post a Comment