In mid June, NRC said the cause of the excessive wear of the tubes in the steam generators at San Onofre Nuclear Power Plant could be a faulty computer modeling by the manufacturer, Mitsubishi Heavy Industries, which underpredicted the velocity of the steam and water.
Mitsubishi Heavy Industries sold 4 replacement steam generators to Southern California Edison. Each has over 10,000 tubes, instead of 4,000 or so for a standard steam generator that the company manufactures. Mitsubishi Heavy Industries is one of the top manufacturers of steam generators in the world.
From 10News.com (7/13/2012):
Tube Wear At San Onofre Blamed On Settling, Vibration
SoCal Edison Says Wear On Steam Generator Tubes Did Not Reach Federal Limits
SAN ONOFRE, Calif. -- The wear in the walls of steam generator tubes that forced the shutdown of the San Onofre Nuclear Generating Station earlier this year was caused by a settling of support equipment and vibrations, Southern California Edison announced Friday.
A leak from a tube forced the utility to take Unit 3 off-line in January, and thinning was found in a large number of them, which are less than two years old.
The plant's other unit had already been closed for routine maintenance. Neither has been returned to service.
SCE reported to the Nuclear Regulatory Commission last week that most of the wear in the tube wall was less than 20 percent, below the federal safety threshold of 35 percent. A tube over the limit must be plugged.
The cause of the wear on the tubes was vibration, which is unusual, and settling of the support structures, which sometimes occurs, SCE spokeswoman Jennifer Manfre said. She said the number of tubes affected by the settling was higher than normal.
"We're using this information and additional detailed data collected through testing to develop our repair plans according to best practices and industry standards, particularly the data on the unexpected tube-to-tube wear," said Pete Dietrich, senior vice president and chief nuclear officer.
SCE reported that in Unit 3 -- where the leak occurred -- 1,806 tubes showed wear of some type and 807 tubes were ultimately plugged. Of those, 381 tubes were plugged for wear of more than 35 percent and the rest as a precaution.
In Unit 2, 1,595 tubes showed wear of some type and 510 tubes were ultimately plugged. Six were for showing wear of more than 35 percent and the rest for preventative measures.
The units each have thousands of steam generator tubes.
An anti-nuclear group called Friends of the Earth released a statement Thursday, claiming that the utility and the NRC were down-playing the extent of damage. The FOE statement calls the steam generator problems at San Onofre the "most severe" of any nuclear plant in the United States.
San Onofre has more than three and a half times the number of steam tubes plugged as a safety measure than at 31 other similar U.S. reactors combined, according to the group. They contend that the number of tubes plugged in Unit 2 -- which did not have a leak -- is nearly five times that of any other reactor.
The San Onofre plant generates about 20 percent of the power for San Diego Gas and Electric. The utility is importing power to cover for the San Onofre shutdown, much of it coming over a transmission line from Imperial County that recently began operating.
Well, if each unit has two steam generators which have 10,000 tubes each, there are 20,000 tubes. If the number of tubes plugged in Unit 2, 510, was five times that of any other reactor in the US, what I'd like to know is the total number of tubes in the steam generator(s) in a typical PWR in the US. (Does anyone know? I'm being lazy on weekend...)
And yes, the tubes got worn out in 2 years instead of 20...
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5 comments:
NRC steam generator fact sheet says:
"...each steam generator can contain anywhere from 3,000 to 16,000 tubes..."
And, there is some information that looks contradictory to me, at least at first reading
"Typically, the tube wall thickness may be degraded by up to 40 percent of the initial wall thickness before the tube must be repaired. This allows an adequate margin against leakage and bursting. This criteria can be overly conservative for localized flaws such as short stress corrosion cracks. As a result, use of the 40-percent through-wall repair criteria may result in tubes being unnecessarily removed from service even though they would continue to satisfy the existing regulatory guidance for adequate structural and leakage integrity."
This might have been a reason for implementing voltage-based inspection to circumvent the 40-(or 35?)-percent rule, to allow for continued usage of tubes which wear exceed 40%:
"In 1995, NRC issued Generic Letter 95-05, “Voltage-Based Repair Criteria for Westinghouse Steam Generator Tubes Affected by Outside Diameter Stress Corrosion Cracking.” "
This is a very important economical matter, as many nuclear plants were shut down because replacement of the steam generators would have been too expensive for a nuclear plant that is already thoroughly worn, as are most of the western countries' NPPs.
So, reducing the safety margins is essential for maintaining and maximizing profits.
http://www.nrc.gov/reading-rm/doc-collections/fact-sheets/steam-gen.html
Have a nice weekend, and thank you again for your great reporting!
The NRC's Advisory Committee on Reactor Safeguards (ACRS) issued a report in February 2001.[2] The ACRS substantiated many of Dr. Hopenfeld's concerns. For example, the ACRS concluded:
"The techniques [used to look for cracked steam generator tubes] are not nearly so reliable for determining the depth of a crack, and in particular, whether a crack penetrates through 40% of the tube wall thickness." [NRC's regulations do not allow a nuclear plant to start up with any steam generator tube cracked more than 40 percent of its wall thickness, but the methods used to inspect the tubes for cracks cannot reliably determine the depth of cracks.
"Data available to the [ACRS] suggest that the constant probability of detection [of cracked steam generator tubes] adopted by the NRC staff is nonconservative for flaws producing voltage signals less than about 0.7 volts." [In other words, the NRC staff assumes that methods used to find cracked tubes are much better than the data shows them to be.]
http://epw.senate.gov/107th/loc_0508.htm
It seems to me that a problem in the reliability of the method to find cracks is more of concern than a component using a new design wearing out faster than expected.
As far as I understand steam generators are used in conventional powerplants too and pipes are closed when worn out in that case too (i.e. pipes are not expected to last indefinitely there either).
The operator of the plant reportedly lying to NRC when hiding the fact that a new design was being adopted is also a concern.
Finally, what are the consequences of a pipe rupture? Can anyone comment?
Beppe
@ Beppe
"what are the consequences of a pipe rupture?"
First I'd like to point out steam generators are used in pressurized water reactors, boiling water reactors do not use steam generators because the steam is produced inside the pressure vessel. Steam generators can also be found in the CANDU and BN-600 reactors.
As for the consequences steam generators are more than 50% of the pressure boundary they are found between the primary (radioactive) and secondary (non-radioactive) coolant loops but they are outside of the reactor containment. Failure of a significant number of these tubes during an accident could allow radiation to escape containment along with coolant. As far as the NRC is concerned doing nothing has solved the problem.
(From anon 8:46 link)
As the ACRS stated:
"Steam generators constitute more than 50% of the surface area of the primary pressure boundary in a pressurized water reactor."
"Unlike other parts of the reactor pressure boundary, the barrier to fission product release provided by the steam generator tubes is not reinforced by the reactor containment as an additional barrier."
"Leakage of primary coolant through openings in the steam generator tubes could deplete the inventory of water available for the long-term cooling of the core in the event of an accident."
In the decade since Dr. Hopenfeld first raised his safety concerns, the NRC has allowed many nuclear plants to continue operating nuclear power plants with literally thousands of steam generator tubes known to be cracked. The ACRS concluded that the NRC staff made these regulatory decisions using incomplete and inaccurate information. After receiving the ACRS's report, the NRC staff considered Hopenfeld's concerns "resolved" even though it had taken no action to address the numerous recommendations in the ACRS report
"The [ACRS] also finds that this contention of the DPO [namely, that an accident at a nuclear plant with cracked steam generator tubes could cause those tubes to completely break] has merit and deserves investigation."
"The [ACRS] concluded that the issue of the possible evolution of severe accident to involve gross failure of steam generator tubes and bypass of the containment is not yet resolved … [and] that the issue needs consideration regardless of the criteria adopted for the repair and replacement of steam generator tubes."
"what are the consequences of a pipe rupture?"
a possible not very nice outcome scenario:
A pipe breaks, causing a chain reaction of other pipes ripping open, leading to large or even complete destruction of the heat exchanger.
A 160 bar gigawatt-powered pressure cleaner would "work its way" from the initial broken pipe location through the secondary circuit which would pop open like an overpressurized tyre.
This creates a large bypass from the primary containment to the open air.
Which would first vent the steam from the water left in the reactor and then the vaporized and gaseous meltdown releases, and finally the fumes of the corium simmering at open air.
Resulting in an catastrophe that is probably between a Fukushima fizzle and a Chernobyl bang.
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