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| NRC No Restart at San Onofre |
Steam
generator tubes and their support structures need to be extremely carefully
designed and manufactured within the allowed design tolerances. These tubes preform a very important safety
function because they are the main barrier that prevents the loss of highly
radioactive reactor coolant from venting into the environment during a main
steam line break or other abnormal operational conditions. Therefore, the designers
and manufacturers have to ensure that these tubes, which are about the diameter
of a penny and much thinner than a dime, don’t excessively rattle, vibrate or hit
other tubes with violent impacts causing them to fail due to tube-to-tube wear during
normal power operation, main steam line breaks or other abnormal operational
conditions.
San Onofre Steam
Generators History
After two
decades of operation of the original San Onofre steam generators (OSGs), it became
evident to Edison Engineers that the steam generator tubes, made predominantly
of Alloy 600, were susceptible to primary water stress corrosion cracking
(PWSSC). This corrosion mechanism resulted in tube degradation necessitating
plugging large numbers of tubes after each inspection of the tubing. In addition, the San Onofre OSG design had shown to
be susceptible to tube through-wall wear and severe corrosion of the tube
supports. It became evident that the OSGs would have to be replaced much sooner
than stipulated by their design service life. Continuing to operate with highly degraded
steam generators involved substantial economic losses from forced outages and extended
refueling outages, as well as the direct costs of inspections and repairs.
The
financial considerations and mounting losses prompted Edison to make a rash decision
(despite objection by San Diego Gas & Electric) to replace the San Onofre OSGs
in both Units as soon as possible. The contract for fabrication and delivery of
the replacement steam generators (RSGs) was awarded to Mitsubishi Heavy
Industries Ltd. (MHI). As designed and specified by Edison, the RSGs were
supposed to be a replacement in-kind for the OSGs in terms of form, fit and
function. At the same time, however, the RSG specification included many new
requirements derived from both industry and San Onofre operating experience, and the
requirement to use the best and most suitable materials of construction. These
requirements were aimed at improving the RSG longevity, reliability,
performance and maintainability. Also, the Edison design specifications called
for very tight fabrication tolerances of the components and sub-assemblies,
especially the tube-sheet and the tube U-bend support structure. In addition, San Onofre steam generators are among
the largest in the industry, which called for innovative design solutions and
improved fabrication processes when working on the RSGs.
Rather
than simply rebuild the steam generators to their original design
specifications, Edison decided to extensively modify the original San Onofre
steam generator design for producing more heat than the “Industry NORM”,
thereby, to produce more electricity and make more money for Edison. None of
the design modifications, except change of tube alloy material, were necessary
for operation of either San Onofre Unit 2 or 3.
These numerous and untested changes were added by Edison without
adequate consideration for their impact upon the reliability and safety of the
RSGs and the reactors. San Onofre RSGs tubes
were designed by SCE with very close
spacing between the tubes and hundreds of
additional tubes, to generate more heat compared with the original steam
generators in a “hush-hush” manner without formal NRC Approval and Public
Hearings as required by Federal laws.
Furthermore the design and manufacturing were done without the use of “critical
questioning and investigative attitude”, “solid teamwork & alignment” and
“prudent industry and academic research benchmarking.” The entire process was performed in a rush without
proper consideration for adequately protecting the health and safety of the
public in case of a potential nuclear accident.
SONGS Steam Generator
Problems
Investigations
after the leak in Unit 3 on January 31, 2012 revealed that the unprecedented
tube-to-tube wear [TTW caused by fluid elastic instability (FEI)] in Unit 3
RSGs was caused by operating the RSGs at higher reactor power and lower steam
generator pressure than the OSGs (the more reactor power, the more money for
Edison). Poorly designed steam
generators such as San Onofre RSGs without in-plane support protection or proper damping
(a thin but crucial water film on the surface of the super hot tubes, which
acts as a shock absorber to reduce severe vibrations and tube ruptures) and with
narrow tube clearances experience severe vibrations at lower steam generator
pressures. High reactor thermal power also causes high steam flows and high
fluid velocities. This type of steam generator operation fulfilled Edison’s
desire to make more money, but caused the top of the U-tube area to be almost
dry (No damping because the crucial water film was missing and could not
transfer the heat from the super hot tubes) in some regions of the U-tube
bundle. Without liquid in the mixture, there is insufficient damping against
excessive tube rattling and/or severe in-plane vibrations (fluid elastic
instability). Therefore, due to high steam flows, high fluid velocities and
extremely low tube clearances, severe fluid-elastic instability and
flow-induced random vibrations developed in Unit 3 RSG’s. The tubes
rattled, vibrated and hit other tubes causing violent impact wear, like a
jackhammer. Unit 2 was operating at
higher than normal reactor power and at higher steam pressures. This different steam
generator operation in Unit 2 damaged hundreds of tubes (including one that had
90% tube wall wear) due to flow-induced random vibrations but did not cause the
U-tube bundle to dry out due to the higher steam pressures and steam saturation
temperatures, therefore no fluid elastic instability developed in Unit 2. This statement is consistent with
Westinghouse, the premier nuclear designer in the world.
San Onofre Unit 2 Restart
Risks
Now
Edison wants to restart Unit 2 with hundreds of damaged tubes at reduced power
by claiming that, “Reducing power to 70% eliminates the thermal hydraulic
conditions that cause FEI and associated [tube wear].” Then Edison contradicts
itself by saying, “Limiting power to 70% significantly reduces fluid velocity.
The reduction in fluid velocity significantly reduces the potential for FEI. Decreasing the void fraction in the upper
bundle region during power operation increases damping and reduces the
potential for FEI.” (Italics added.)
MHI in an official notice to NRC on October 5, 2012 stated, “The plugging of
the tubes that have the possibility of the fluid elastic instability and
thermal power output reductions were identified as potential corrective
actions. SCE [Southern California Edison] will run Unit 2 at 70% power for a
short duration as a corrective action. In addition, SCE and MHI will continue a
detailed analysis and investigation to this problem. Additional corrective
actions may be required as the analysis and investigation continue,” which
means the problems are not fully understood at this time. Unit 2 inspections
reveal that there are numerous U-bends in both RSGs with tube-to-tube
clearances as small as 0.05 inches (San Onofre RSGs Design 0.25 inches, Industry
NORM 0.35-0.55 inches)”.
If allowed
to restart San Onofre Unit 2 by NRC, even at 70% power operations for a trial period
of 5 months, a steam line break outside containment in Unit 2 and/or any
unanticipated operational transients at any time during the 5-month trial
period can result in the following potential scenarios: (1) the
depressurization of the steam generators with the failure of a main steam
isolation valve to close, and release of steam to the environment, or (2) over-pressurization
of the steam generators with the actuation of a main steam safety valve and
release of steam to the environment. Any of these adverse conditions
despite power reductions and higher steam pressures at any time during the
5-month trial period would still result in 100% void fractions in the degraded
Unit 2 U-Tube bundle due to extremely low tube-to-tube clearances and no
in-plane anti-vibration bar protection. This condition of ZERO Water in
the steam generators would cause fluid elastic instability (FEI) and flow-induced
random vibrations, which in all likelihood would then result in a massive
cascading of RSGs tube failures (tubes would excessively rattle or vibrate, hitting
other tubes with violent impacts). This Titanic effect would involve hundreds
of degraded but active SG tubes along with all the damaged inactive (all the
plugged /stabilized) SG tubes causing an undetermined amount of simultaneous
tube leaks/ruptures. Under one or more
of these adverse scenarios, approximately 60 tons of very hot high-pressure
radioactive reactor coolant would leak into the secondary system. The release
of this amount of radioactive primary coolant, along with an additional
approximately 200 tons of steam in the first five minutes from a broken steam
line would EXCEED the San Onofre NRC approved safety margins. So, in essence, the
RSG’s are loaded guns, or a nuclear accident like Fukushima, waiting to happen.
Any failure under these conditions would allow significant amounts of radiation
to escape to the atmosphere and a major Loss Of Coolant Accident (LOCA) could
easily result causing much wider radiological consequences and even a potential
nuclear meltdown of the reactor. Operator Action as claimed by Edison to
re-pressurize the steam generators is not feasible to stop a major nuclear
accident in Unit 2 in progress in the first 15 minutes of a MSLB, stuck open SG
safety valve, Earthquake, Steam Generator Tube Ruptures and other anticipated
operational transients during the 5-month trial period.
NRC’s Duty Is Public
Safety
Under no circumstances should the NRC permit SCE to
restart San Onofre unit 2 without re-tubing or replacing the defective replacement steam
generators & a full NRC 50.90 Licensing Amendment and transparent trial-like public
hearings.
Copyright December 13, 2012 by The
DAB Safety Team. All rights reserved. This material may not be published,
broadcast or redistributed without crediting the
DAB Safety Team. The contents cannot be altered without the Written Permission
of the DAB Safety Team Leader and or the
DAB Safety Team’s Attorneys.
Nuclear News – Assessment Of SONGS Replacement Steam Generators
December 13, 2012
