|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.
Please see all the DAB Safety Team’s San Onofre Papers and Press Releases on this subject by using this link or the search phrase DAB Safety Team & Related Info on Yahoo or Google search engines.
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