[ RadSafe ] More on San Onofre - Engineer's input needed

Jaro Franta jaro_10kbq at videotron.ca
Thu Apr 12 10:58:27 CDT 2012

Published: April 11, 2012 Updated: 11:19 p.m. 

Odd tube wear seen in both San Onofre reactors


An extremely unusual pattern of steam-generator-tube wear that engineers had
previously seen in only one of the San Onofre nuclear plant's idled reactors
has now been seen in the second reactor.
The odd pattern was first seen in the Unit 3 reactor after it was shut down
Jan. 31 following a water leak and release of a small amount of radioactive

But it is too soon to draw any conclusions about the implications for the
second reactor, Unit 2, which also has been shut down since January, said
Jennifer Manfre, spokeswoman for plant operator Southern California Edison.
Both reactors remain offline indefinitely, and will not be restarted until
the cause of the wear is determined and the restart is cleared by the
Nuclear Regulatory Commission.
"We've informed the NRC that we have identified additional minor tube wear
in one of the Unit 2 steam generators, and this newly identified wear is
similar to the type of wear previously seen in Unit 3," Manfre said.
The cause of the wear in hundreds of steam generator tubes in both reactors
remains a mystery. While the more common type of tube wear seen in Unit 2 is
typical throughout the life of steam generators, San Onofre's four steam
generator units -- two for reach reactor -- are only two years old.
And Unit 3 had shown more extensive wear even though it was in operation for
a shorter period than Unit 2: since February of last year for Unit 3 versus
February 2010 for Unit 2.
But now the unusual wear pattern -- along the length of steam-generator
tubes, indicating they were rubbing together -- has turned up in two of the
tubes in Unit 2, Manfre said.
The wear in both reactors is considered premature. Edison has plugged a
total of 321 of the worn tubes in both reactors, and while plugging of worn
tubes is common practice and does not affect the generators' performance, it
is unusual so early in the life of the generators.
Edison is continuing testing with NRC inspectors on site, although a special
inspection team sent in earlier by the nuclear regulatory agency has now
It is too soon to tell whether the latest wear-pattern discovery might help
shed light on the cause of the tube wear in the two units, Manfre said. The
steam generators were manufactured by Mitsubishi Heavy Industries in Japan.
In a meeting with reporters after touring the troubled plant on Friday,
Gregory Jaczko, the chairman of the NRC, said the cause of the premature
wear must be determined before the reactors can be restarted.
Water heated by the reactors flows through the tubes, nearly 20,000 for each
pair of generators, and converts water in the generators to steam, which
turns turbines to create electricity.
But normally, the water that circulates through the tubes is kept separate
from the water in the generators.
Plant operators chose to shut down Unit 3 after a water-leak was detected in
one of its steam-generator tubes, but the leak of radioactive water from the
tube into the generator caused a small release of radioactive gas.
A sensor was tripped in a building next to the reactor, but sensors
elsewhere on San Onofre property picked up no changes in radiation. Edison
said neither plant employees nor the public were placed in danger.
Unit 2 had been shut down for routine maintenance, but closer inspection
found premature wear in its steam-generator tubes.
The indefinite shutdown of the reactors prompted the California Independent
Systems Operator, which manages the state's power grid, to develop
contingency plans for electricity generation in case of an extended summer
heat wave.
The plans include possible reactivation of two power units at the AES power
plant in Huntington Beach that had been shut down


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