Wednesday, February 6, 2013

Media Alert: What caused the destruction of San Onofre’s new replacement steam generators?


Media Alert

 

The DAB Safety Team: February 6, 2013

 

Media Contact: Don Leichtling (619) 296-9928 or Ace Hoffman (760) 720-7261

 

What caused the destruction of San Onofre’s new replacement steam generators?

 

The DAB Safety Team has transmitted the following request to the Offices of Chairman of the NRC, California Attorney General and Senator Barbara Boxer’s Committee on Environment and Public Works (EPW).

 

Subject: Human Performance Tool “Critical Questioning & Investigative Attitude”

A. SONGS Unit 3 FEI Root Cause: Lack of “Critical Questioning & Investigative Attitude” by SCE/MHI
·          Design Causes: Narrow tube pitch/diameter ratio, too many tall tubes and lack-of in-plane supports, Low Frequency Retainer Bars
·          Operational Causes: High steam flows, high fluid interstitial velocities and low steam pressures
·          Primary Mechanistic Cause: Tube-to-Tube Wear caused by Fluid Elastic Instability aka vapor fraction >99.6% aka steam dry-outs, lack of tube damping (no thin water film on tube to release heat)
Secondary Mechanistic Causes: Tube-to-AVB/TSP Wear, Retainer Bar Wear
1  - caused by flow-induced random vibrations & excessive hydrodynamic pressures (“Mitsubishi flowering effect”)
·          Human Performance Errors:
1. Lack of solid teamwork, alignment and design reviews
2. Avoidance of 10CFR 50.90 amendment process
3. Financial and time pressures
4. Lack of academic2 and industry benchmarking (ANO-2, PVNGS 1, 2 & 3)
5. Complacency and negligence
6. Lack of NRC oversight
7. Lack of skills, experience and technology to design & build a CE Replacement Steam Generator

B. SONGS OSGs Extensive Wear Probable Root Cause: SONGS 2001 Power Uprate, which was approved by NRC Region IV, without the use of “Critical Questioning & Investigative Attitude” allowed SCE to increase its profits, but PROBABLY also increased the need for tube-plugging and/or shortened the remaining service life of SONGS Original Combustion Engineering Steam Generators (OSGs).

DAB Safety Team Investigation of OSGs and RSGs Operational Data and Unit 3 FEI

NRC AIT report stated, “The combination of unpredicted, adverse thermal hydraulic conditions and insufficient contact forces3 in the upper tube bundle caused a phenomenon called “fluid-elastic instability” which was a significant contributor to the tube to tube wear resulting in the tube leak. The NRC team concluded that the differences in severity of the tube-to-tube wear between Unit 2 and Unit 3 may be related to the changes to the manufacturing / fabrication of the tubes and other components which may have resulted in increased clearance between the anti-vibration bars and the tubes; Due to modeling errors4, the SONGS replacement generators were not designed with adequate thermal hydraulic margin to preclude the onset of fluid-elastic instability. Unless changes are made to the operation or configuration of the steam generators, high fluid velocities and high void fractions in localized regions in the u-bend will continue to cause excessive tube wear and accelerated wear that could result in tube leakage and/or tube rupture.” John Large states, I note here that there are three clear conflicts of findings between the OAs: From AREVA that AVB-to-tube and TTW result from in-plane FEI, contrasted to Westinghouse that there is no in-plane FEI but most probably it was out-of-plane FEI, and from MHI that certain AVB-to-tube wear results in the absence of in-plane FEI from just turbulent flow. My opinion is that such conflicting disagreement over the cause of TTW reflects poorly on the depth of understanding of the crucially important FEI issue by each of these SCE consultants and the designer/manufacturer of the RSGs.”

Based on examination of the Westinghouse or WEC data in Tables 8-2 and 8-3 (Ref. SCE Unit 2 Return to Service Report, Enclosure 2), and SCE Unit 3 Root Cause Evaluation, DAB Safety Team concludes that SONGS Original Combustion Engineering steam generators (OSGs) were operated since 2001 power uprate at a secondary side operating pressure of 900 psi, thermal power 1729 MWt, void fraction of 96.1% and maximum interstitial velocity of 22.90 feet/sec5.  This lower maximum interstitial velocity is why the OSGs did not suffer FEI (tube-to-tube wear), but only experienced tube-to-TSP wear from flow-induced random vibrations in the out-of-plane directions.  SONGS Unit 3 RSGs were operating at a secondary side operating pressure of 833 psi, thermal power 1729 MWt, void fraction of 99.6% and maximum interstitial velocity of 28.30 feet/sec. That is why SONGS Unit 3 RSGs suffered FEI (unprecedented tube-to-tube wear) in the in-plane direction and experienced significant tube-to-AVB/TSP wear from excessive flow-induced random vibrations and Mitsubishi Flowering Effect in the out-of-plane directions. Based on information from anonymous RCE Root Cause Member, DAB Safety Team investigation of Unit 2 plant operational/design data, calculations and NRC AIT Report, SONGS Unit 2 RSGs were operating at a secondary side operating pressure between 892-942 psi, thermal power 1724 MWt, void fraction between of 96-98% and maximum interstitial velocity of ~ 23 feet/sec.  That is why SONGS Unit 2 RSGs did not suffer FEI and experienced limited tube-to-AVB/TSP wear compared with Unit 3 from flow-induced random vibrations and Mitsubishi Flowering Effect in the out-of-plane directions.  If we compare the Westinghouse data, steam flows and fluid velocities were high enough in both the OSGs and the RSGs to cause excessive out-of-plane vibrations.  Therefore, we can ignore those effects, as well as errors in computer modeling (unresolved at this time) and insufficient contact forces (discussed below) on Unit 3 FEI.  We can then see that the lower secondary steam pressure of 833 psi in Unit 3 caused formation of high vapor fraction aka steam dry-outs (resulting from lower steam saturation temperature) in the region of high tube-to-tube wear (FEI), and that because Unit 2 was operating at higher pressure (resulting in higher steam saturation temperature), no FEI occurred in that Unit.

1Document Title 2006 - Fluid-elastic instability of an array of tubes preferentially flexible in the flow direction subjected to two-phase cross flow. (http://yakari.polytechnique.fr/people/revio/masters_research_subject.html)  Violette R., Pettigrew M. J.  &  Mureithi N. W.  state, “In nuclear power plant steam generators, U-tubes are very susceptible to undergo fluid elastic instability because of the high velocity of the two-phase mixture flow in the U-tube region and also because of their low natural frequencies in their out of plane modes. In nuclear power plant steam generator design, flat bar supports have been introduced in order to restrain vibrations of the U-tubes in the out of plane direction. Since those supports are not as effective in restraining the in-plane vibrations of the tubes, there is a clear need to verify if fluid elastic instability can occur for a cluster of cylinders preferentially flexible in the flow direction.  Almost all the available data about fluid elastic instability of heat exchanger tube bundles concerns tubes that are axisymmetrically flexible. In those cases, the instability is found to be mostly in the direction transverse to the flow. Thus, the direction parallel to the flow has raised less concern in terms of bundle stability.”

 

 

 

 

Figure 1 – Vibrations amplitude as a function of flow pitch velocity for a flexible cylinder in a rigid cluster (taken from Pettigrew et al. 1991). The cylinder is free to vibration in the cross-flow direction.

 

Note: SONGS AVB Structure low frequency Retainer Bars can cause potential failure of Unit 2’s preventatively non-stabilized tubes (without cable stabilizers) by turbulence-induced vibrations at pitch flow velocity as low as 3 meters/sec or 10 feet/ sec (See Figure 1).  At 70 percent power, the maximum interstitial velocity projected in SONGS Return to Service Report, Enclosure 2, Table 8-3 varies between 11.91 and 13.28 feet/sec.  Potential failure and ramifications of Unit 2 preventatively non-stabilized tubes (without cable stabilizers) will be discussed in a future DAB Safety Team Media Alert.


2Ibid.
3Insufficient contact tube-to-AVB gaps is just an SCE excuse to collect insurance money from MHI. Westinghouse3A & AREVA3B OAs contradict SCE/NRC claims.  MHI3C statements are contradicting and confusing, therefore, are not considered reliable. But, as John Large3D says, “I find that the ‘zero-gap’ AVB assembly, which features strongly in the onset of TTW, is clearly designed to cope only with out-of-plane tube motion since there is little designed-in resistance to movement in the in-plane direction.”  

3AWestinghouse states, “Test data shows that the onset of IP vibration requires much higher velocities than the onset of Out of Plane (OP) fluid-elastic excitation. Hence, a tube that may vibrate IP would definitely be unstable OP. A small AVB gap that would be considered active in the OP mode would also be active in the IP mode because the small gap will prevent significant in-plane motion due to lack of clearance (gap) for the combined OP and IP motions. Thus, a contact force is not required to prevent significant IP motion. The above conclusion was demonstrated by a series of tests described in Reference 24. The tests were conducted with U-bends under controlled conditions at different AVB gaps. It was found that “the effect of flat bar supports with small clearance is to act as apparent nodal points for flow-induced tube response. They not only prevented the out-of-plane modes as expected, but also the in-plane modes. No in-plane instabilities were observed, even when the flow velocity was increased to three times that expected to cause instability in the apparently unsupported first in-plane mode. These tests clearly demonstrated that a contact force is not required to prevent in-plane vibration. A small gap (e.g., 3 mils) is sufficient to prevent in-plane vibration.” There are several potential manufacturing considerations associated with review of the design drawings based on Westinghouse experience. None were extensively treated in the SCE root cause evaluation.”

3BAREVA States, “Contact forces significantly increase at normal operating temperature and pressure due to diametric expansion of the tubes and thermal growth of the AVBs. In contrast, after instability develops, the amplitude of in-plane motion continuously increases and the forces needed to prevent in-plane motion at any given AVB location become relatively large. Hence shortly after instability occurs, U-bends begin to swing in Mode 1 and overcome hindrance at any AVB location.”

3CMHI investigated field experience with U-bend tube degradation using INPO, NRC and NPE databases, and concluded that tube wear in the operating U-tube SGs was mostly being caused by out-of-plane tube motion. Consistent with this and Reference 7 (JSME, S016-2002, Guideline for Fluid-elastic Vibration Evaluation of U-bend Tube in SG, March 2002), only out-of-plane vibration of the SG U-tubes was evaluated because tube U-bend natural frequency in the out-of-plane direction is lower than the natural frequency in the in-plane direction and out-of-plane vibration is more likely to occur than in-plane vibration. No SG problems stemming from in-plane tube motion were identified by MHI and thus MHI concluded that the design and fabrication processes described above were sufficient for minimizing tube wear in the SONGS RSGs.  In the design stage, MHI assumed that the tube support in the out-of-plane direction with “zero” tube-to-AVB gap in hot condition was sufficient to prevent the tube bundle from becoming fluid-elastic unstable during operation. But, the recent SONGS experience shows that the flat bar AVBs do not provide friction forces required to prevent tubes from vibrating in the in-plane direction and eventually become fluid-elastic unstable under high local secondary thermal-hydraulic conditions such as in the SONGS RSGs.  Secondary side thermal-hydraulic conditions in the SONGS SGs during operation appear to be such that the effective fluid flow velocities are higher than the critical velocities in the U-bend in-plane direction for several tubes in a particular region of the tube bundle where the void fraction is very high. MHI Observations Common to Unit-2 and Unit-3:  The AVBs, end caps, and retainer bars were manufactured according to the design. It was confirmed that there were no significant gaps between the AVBs and tubes which might have contributed to excessive tube vibration because the AVBs appear to be virtually in contact with tubes.”

3DJohn Large states, “Causes of Tube and Restraint Component Motion and Wear: My study of the various OAs leads me to the following findings and opinion that: (i) degradation of the tube restraint localities (RBs, AVBs and TSPs) occurs in the absence of fluid elastic instability (FEI) activity; (ii) TTW, acknowledged to arise from in-plane FEI activity, generally occurs where the AVB restraint has deteriorated at one or more localities along the length of individual tubes; (iii) the number of tube wear sites or incidences for AVB/TSP locations outstrips the TTW wear site incidences in the tube free-span locations. I find that the ‘zero-gap’ AVB assembly, which features strongly in the onset of TTW, is clearly designed to cope only with out-of-plane tube motion since there is little designed-in resistance to movement in the in-plane direction - because of this, it is just chance (a combination of manufacturing variations, expansion and pressurization, etc.) that determines the in-plane effectiveness of the AVB; (iv) Uniquely, the SONGS RSG fluid regimes are characterized by in-plane activity, which is quite contrary to experience of other SGs used in similar nuclear power plants in which out-of-plane fluid phenomena dominate. Moreover, from the remote probe inspections when the replacement steam generator (RSG) is cold and unpressurized, I consider it impossible to reliably predict the effectiveness of the many thousands of AVB contact points for when the tube bundle is in a hot, pressurized operational state, (v) The combination of the omission of the in-plane AVB restraints, the unique in-plane activity levels of the SONGS RSGs, together the very demanding interpretation of the remote probe data from the cold and depressurized tube inspection, render forecasting the wear of the tubes and many thousands of restraint components when in hot and pressurized service very challenging indeed.”

4DAB Safety Team even goes this far to state, “Blaming MHI for computer modeling mistakes is another SCE excuse because: (1) SCE should have checked the result of MHI for computer modeling before accepting the steam generators, (2) According to the NRC report dated 11/09/2012, “The licensee and Mitsubishi continued to evaluate this unresolved item and no final conclusions were reached at the time of the inspection. The NRC is continuing to perform independent reviews of existing information.”  

5 In the SONGS 2001 Power Uprate Application approved by NRC (ADAMS Ascension Number ML010950020), SCE stated, “The NRC has amended its regulations to allow holders of operating licenses for nuclear power plants to reduce the assumed reactor power level used in evaluations of emergency core cooling system (ECCS) performance (reference 8.1). This amendment provides licensees the option to apply a reduced uncertainty for ECCS evaluation. This action allows SONGS Units 2 and 3 to pursue an approximate 1.42% cost beneficial power uprate without compromising the margin of safety of the facility.  At 100% power operation, steam generator pressures typically vary between 800 psia and 815 psia, compared to the original nominal design operating pressure of 900 psia. The lower steam generator pressure is the result of a recent reduction in the normal range of RCS operating temperatures. The uprate will result in a slight increase in steam generator pressure from current nominal 100% RTP operating conditions. Based on a projected increase of 2.3% in the secondary side fluid velocity, normal operation flow induced vibration analysis is impacted by the velocity increase. Current analysis considered that tubes with more than one consecutive inactive eggcrate were staked and plugged, and two nonconsecutive inactive eggcrates are acceptable. The Stability Ratio (SR) is defined as: SR = Veff/Vcr , where Veff= effective velocity, Vcr = critical velocity; and Values of SR < 1 are considered acceptable. The maximum stability ratios calculated are: SR = 0.64 (one eggcrate uncredited), SR = 0.66 (alternate eggcrates uncredited). Ignoring any changes in the fluid density resulting from the modification, no change in Vcr is expected. As an approximation, the modified Veff is assumed to increase by 2.3%, i.e., the same as the fluid velocity. The modified maximum SR will be 0.66 x 1.023 = 0.675 < 1 (i.e., acceptable). Therefore, the existing steam generator eggcrate evaluation will not be impacted by the uprate.” If the OSGs were operating at 815 psia from 2001 as stated in SONGS Power uprate application approved by NRC in 2001, fluid interstitial velocities would be closer to 29 feet/sec, based on data interpolated from Tables 8-2 and 8-3, rather than 22.90 feet/sec as shown in Table 8-3 for SONGS Original Combustion Engineering Steam Generators.  That being the case, flow-induced random vibrations would have accelerated the tube-to-support wear, increased the tube plugging rate and shortened the remaining useful life of SONGS Original Combustion Engineering Steam Generators (OSGs). FEI probably did not happen in the OSGs because of the fixed bar vertical and bat wing support systems, higher tube clearances, and no tube lanes in the central portion of the tube bundle due to a stay cylinder (which are not present in the RSGs).

ABBREVIATIONS AND ACRONYMS

·          ACRS: NRC’s Advisory Committee on Reactor Safeguards
·          ADAMS: NRC’s Agencywide Documents Access and Management System
·          AIT: NRC’s Augmented Inspection Team
·          AREVA: Nuclear engineering firm owned by French Atomic Energy Commission
·          AVB: Anti Vibration Bar
·          CAL: Confirmatory Action Letter
·          CFR: Code of Federal Regulations
·          CPUC: California Public Utilities Commission
·          DBA: Design Basis Accident
·          ECT: Eddy Current Testing
·          ECCS: Emergency Core Cooling System
·          EDF: French nuclear parts manufacturing company, also owns transmission lines in France, etc.
·          EPRI: Electric Power Research Institute
·          FEI: Fluid Elastic Instability
·          FIRV: Flow-Induced Random Vibrations
·          FSAR: Final Safety Analysis Report
·          FSM: Fluid elastic Stability Margin
·          FWLB: Feed-Water Line Break
·          GDC: General Design Criteria
·          GSI: Generic Safety Issue
·          ID: Inner Diameter
·          INES: International Nuclear Events Scale
·          LOCA: Loss Of Coolant Accident
·          MHI: Mitsubishi Heavy Industry
·          MSIV: Main Steam (line) Isolation Valve
·          MSLB: Main Steam Line Break
·          MWt: Mega-Watts Thermal
·          NOPD: Normal Operating Pressure Differential
·          NPE: Journal of Nuclear Plant Engineering
·          NPP: Nuclear Power Plant
·          NRC: Nuclear Regulatory Commission
·          NRR: NRC’s Office of Nuclear Reactor Regulations
·          OA: Operational Assessment
·          OD: Outer Diameter
·          OSGs: SONGS Original Combustion Engineering Steam Generators
·          RSGs: SONGS Replacement Steam Generators built by MHI
This press release will be posted on the web at this linkDAB Safety Team Documents.
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The DAB Safety Team: Don, Ace and a BATTERY of safety-conscious San Onofre insiders plus industry experts from around the world who wish to remain anonymous.  These volunteers assist the DAB Safety Team by sharing knowledge, opinions and insight but are not responsible for the contents of the DAB Safety Team's reports.  We continue to work together as a Safety Team to prepare additional DAB Safety Team Documents, which explain in detail why a SONGS restart is unsafe at any power level without a Full/Thorough/Transparent NRC 50.90 License Amendment and Evidentiary Public Hearings.  For more information from The DAB Safety Team, please visit the link above.
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Copyright February 6, 2013 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 Attorney