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IAEA-CN-155-050 MITIGATION OF DEGREDATION OF HIGH ENERGY SECONDARY CYCLE PIPING DUE TO FLOW ASSISTED CORROSION (FAC) AND LIFE MANAGEMENT OF HIGH ENERGY PIPING IN INDIAN NUCLEAR POWER PLANTS T.M. Moolayil Nuclear Power Corporation of India Ltd., India Email address of main author: tmathew@npcil.co.in Abstract Ensuring safe, reliable operation of secondary cycle system in Nuclear Power Plants (NPPs) is very important not only from power generation point of view but also from the industrial safety concerns. Secondary cycle comprises of various high energy systems such as main steam system, re-heat system, boiler feed water system, auxiliary feed water system, condensate system, boiler blow down system, separator drain system, reheater drain system, heater drain system, steam drains system etc. Failure of any pipes and fittings pertaining to high energy systems piping can result in complex challenges to the operating staff and plant.Most of the secondary cycle systems are housed in the turbine building and it is routine for the workers to enter this building for daily checks and other purposes. It is important to not only prevent the radiation hazards but also prevent industrial accidents at Nuclear Power Plants.On 9th February 2006 in Kakrapar Atomic Power Station unit-2 (KAPS-2), 220 MWe pressurized heavy water reactor (PHWR) in India, a pipe segment in the 10% feed water line to steam generator (SG-4) immediately downstream of flow element ruptured releasing steam in boiler room. This failure was assessed to be because of flow assisted corrosion (FAC). Subsequent to above incident, lot of efforts had been put to mitigate degradation of high energy secondary cycle piping due to FAC and to prevent similar incidence of piping failures in high energy systems of Indian NPPs. This paper brings out various measures adopted to mitigate flow assisted corrosion (FAC) related degradation and life management of high energy system piping of secondary cycle systems in Indian NPPs. 1. Introduction The phenomena of wall thinning in carbon steel piping due to flow assisted corrosion (FAC) had resulted in rupture of both single phase and two phase high energy systems piping of secondary cycle in Nuclear Power Plants worldwide. Based on the experience of FAC and studies conducted by many authors, carbon steel material is now being considered susceptible to FAC. A Study was done to review whether high energy system piping secondary cycle of Indian Pressurized Heavy Water Reactors are likely to have similar kind of degradation. Based on the available informations it is noted that FAC phenomena can not be totally eliminated, but it has to be managed by regular inspection, repair/replacement of degraded piping components, improvements in piping layout design and usage of better FAC resistant material. 2. Incident pipe line rupture at KAPS-2 220 MWe PHWR (India)

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IAEA-CN-155-050 On 9th February 2006 in Kakrapar Atomic Power Station unit-2, a pipe segment in the 10% feed water line to steam generator (SG-4) immediately downstream of flow element ruptured releasing steam in boiler room. The process details at full power operation and other specification of the ruptured pipe segment is given below. Process fluid --- Feed water (liquid) Normal operating temperature --- 171 C Design pressure --- 72 kg/cm Normal flow & Velocity --- 31 Tones /hr & 2.33 m/sec Material --- Carbon steel SA 106 Gr.B Size and thickness --- 80 NB and 7.62 mm (nominal wall thickness) Dissolved oxygen(feed water) --- < 5 ppb pH(feed water) --- 8.8 to 9.5 (maintained average 9.2) The inspection done on this pipe during 2004 showed considerable reduction in wall thickness. Minimum measured thickness observed at the immediate upstream and down stream of the rupture locations were 1.46 mm and 1.63 mm respectively.Minimum thickness required for continuous operation was 2.89 mm.The investigation indicated that the failure was due to FAC. Ruptured location of 10 % feed water line to S.G and schematic is given in the fig.1 and 2 respectively.

FIG. 1. Ruptured 10% feed water line to SG-4 at KAPS-2

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FIG. 2. Schematic of 10 % feed water line to S.G 3. Brief description of the event KAPS-2 was operating at 155 MWe. At 17:00 hrs of 09/02/2006, Steam Generator (SG-4) level low alarm appeared. Level was tried to be maintained by opening standby control valve (CV). Levels of all the boilers were found to be coming down. For SG-1, SG-2 and SG-3 low level alarm appeared. Fire alarm from fire detector 6054 also appeared due to presence of steam in boiler room. Further set back got initiated on SG-1 level very low at 17:12 hrs. Turbine was tripped manually at 17:14 hrs when Turbo Generator (TG) generation reduced to 26 MWe. De-aerator very low level alarmed. Immediately reactor was tripped manually by actuating primary shutdown system (PSS) at 17:17 hrs. Main boiler feed pumps (MBFPs) got tripped on auto on de-aerator level very low and auxiliary boiler feed pump (ABFP) got started on auto. Field personnel reported heavy steam leak in boiler room. Primary heat transport system (PHTS) was cooled down, depressurized and shut down cooling pump was started. Boiler room entry could be made at 22:00 hrs. Break in 10 % feed line to SG-4 in boiler room at downstream of flow element (FE-90) was observed. 11 nos. of blow out panels in Boiler room were found ruptured. 4. A brief about Flow Assisted Corrosion (FAC) FAC is a carbon steel pipe system is characterized by the simultaneous dissolution of iron from the iron oxide-fluid interface and formation of an iron oxide film at the oxide-metal interface. Flow provides a vital role in providing a sink of dissolution[3]. One of the major risks associated with FAC is that it may result in abrupt rupture of the piping causing serious safety concern to plant equipment and personnel. It can occur in single

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IAEA-CN-155-050 phase or two-phase regions. A thin layer of porous oxide mostly magnetite (Fe3O4) forms on the inside surface of carbon steel feed water piping and piping components when exposed to de-oxygenated water in the temperature range of about 95 to 2600C (2000 to 5000F)[3]. Generally this layer protects the underlying piping from the corrosive environment and limits further corrosion. The flow assisted corrosion is an extension of the generalized carbon steel corrosion process in stagnant water. A corrosion process causes wall thinning of carbon steel piping exposed to wet steam this process is called two phase FAC[3]. If the piping is exposed to dry or super heated steam, no FAC takes places. A liquid phase must be present for the FAC damage to occur. As reported corroded surfaces produced by single phase FAC have a different appearance than those formed by two phase FAC. When single-phase FAC rate for a larger diameter piping is high, the corroded surface is characterized by over lapping horse shoe pits that give an orange peal appearance[3]. The corroded surface of a large diameter piping exposed to two-phase flow has a well known tiger stripping appearance[3]. As reported, following factors are known to have influence on FAC. Material susceptibility Phase of steam Piping layout & resulting local flow conditions and turbulence (such as near valves or

nozzles, down stream of orifices, closely spaced elbows, bends etc) Velocity System temperature Operating conditions Water chemistry and pH

FAC is observed when specific combinations of material, water chemistry (including dissolved oxygen, ferrous ion concentration, metallic impurities in water, pH), and hydrodynamic conditions coexist. 5. Analysis of KAPS-2 pipeline rupture The rupture 10% feed water line of KAPS-2 on 9th February 2006 was an alarming incidence which called for a still more in depth study of the failure and to suggest further steps to strengthen FAC management program. Failure analysis of the ruptured pipe segment was done by chemical analysis visual examination, stereo microscopic examination, scanning electron microscopic examination, surface examination, micro-structural examination, hardness measurement, XRD analysis, thickness mapping etc. The thickness of the pipe line had reduced from the original 7.62mm to a minimum of 0.4 mm at the location of failure[1]. On the pipeline part of the failed component, the thickness had reduced to a longer distance of at least 15 cm. The thickness was 2.2 mm at a distance of 7 cm from the fractured surface. The chemical composition of various elements analyzed weight percentage from the failed sample are carbon 0.22%, manganese 0.61%, phosphorous 0.019%, sulfur 0.008% and silicon 0.33%. The composition of unspecified elements are chromium 0.032%, copper 0.009%, molybdenum 0.001%, nickel nil and vanadium 0.003%[1]. The fractured surface showed a clear ductile failure with no indication of cleavage facets, which indicates an overload failure[1]. The analysis carried out on the broken part of carbon

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IAEA-CN-155-050 steel pipe segment reported that failure was due to pipe wall thinning on account of FAC. The inside surface of the ruptured pipe showed horseshoe pits giving an orange peel appearance that is a characteristic of single phase FAC[1]. The water chemistry of the station was also obtained and submitted to Department of Atomic Energy (DAE) advisory committee on steam and water chemistry for verification. All the parameters were within the limits. Almost all stations were carrying out ultrasonic (UT) thickness measurement of vulnerable locations based on their experience. A systematic periodic monitoring program for UT thickness measurement of all FAC vulnerable components pertaining to high energy systems of Secondary Cycle existed in KAPS-1 and KAPS-2 at the time of pipe rupture. KAPS -1 (220 MWe PHWR) had completed one cycle of examination as per the periodic monitoring program during shut down in 2005 and KAPS-2 had carried out examination partly during shut down in 2004.The ruptured location i.e. downstream of flow element was included in the program as it was vulnerable to FAC. Inspection was done at this location during 2004 and showed considerable reduction in wall thickness. But replacement/repair or re-examination was not done at this location afterwards. 6. Remedial measures taken after kaps-2 pipeline rupture After KAPS-2 pipeline rupture some more additional vulnerable components (around

380) pertaining to various high energy systems of secondary cycle of KAPS-1 & KAPS-2 were inspected by UT thickness measurement in addition to the most vulnerable locations examined as per the existing periodic monitoring program. This was done to asses overall healthiness of the plant before start up. Both the units were started after carrying out inspection at around 850 components in each unit and analyzing the inspected data. All the degraded components were removed from the system and replaced with new components. It was thought prudent to enhance the scope of examination including all FAC potential

components/locations in various high energy lines of secondary cycle for all operating stations to generate base line data and to assess the overall healthiness. The feedback and information was disseminated to all projects and stations. Guidelines for repair/replacement/successive examination based on balance life and

procedure for U.T examination, weld overlay, grid size criteria for examination and format for recording inspection data etc have been prepared and issued to all stations and projects. Study was conducted to review materials for pipes, fittings and other components used

in secondary cycle systems, identify suitable materials and to choose the most suitable material which can resist FAC. Decision was taken to replace the carbon steel material (which is now being reported to more susceptible for FAC) with low alloy steel ASTM-SA-335 Gr. P22 (2% Cr, 1% Mo) in case of FAC prone systems/lines/locations for all the stations and projects. It was also decided to use pipe & fittings with one scheduler higher than the required schedule while replacing the existing carbon steel material with low alloy steel at FAC prone locations.

7. Findings of U.T examination carried at different operating stations 7.1. Identification of commonly affected locations

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IAEA-CN-155-050 After reviewing the results of UT thickness examination carried out as per the periodic monitoring program of most vulnerable components for operating stations, the systems /lines/portion of piping which are commonly vulnerable to FAC were identified. The most commonly affected areas at almost all stations are indicated in Table-1. Table 1. List of commonly affected areas due to FAC

Serial number

Most commonly affected high energy pipe lines / locations due to FAC at different operating stations

a

10 % feed water line down stream of control valves

b 90 % feed water line down stream of control valve c Down Steam of control valve of live steam re-heater drain, bled steam re-

heater drain, separator drain in the normal path and alternate path d Extraction -7 line e Extraction - 6 line f Steam drain system down stream of restriction orifices (ROs) g Heater / MSR (moisture separator re-heater) vents down stream of

restriction orifices (ROs) h Heater drain system down stream of Control Valves (CVs) i Boiler blow down system down stream of control valves near boiler blow

down tank 7.2. Other findings of examination of secondary cycle components Degradation is noticed in many of the secondary cycle systems and on components such

as elbows, reducers, pipe etc.T Thickness reduction is noticed in boiler blow down system, separator drain system, re-

heater drain system etc where the bulk velocity is lower than normal recommended allowed velocities. Degradation is noticed in the Secondary Cycle Components in the temperature range of

90C to 250C. Average wear rate of 150 to 200 microns is noticed in some of the commonly

vulnerable systems / lines. Analysis has been done on the data collected from various units where each unit carried out comprehensive UT thickness inspection of around 3000 to 3500 components as per initial examination program (described later).It shows high corrosion at following systems/ locations which may not be due to one factor but a combination of FAC influencing factors. Main feed water lines near HPheater-6 and its downstream line to S.G 90% feed water control valve stations 10% feed water control valve stations Downstream of boiler feed pump/ auxiliary boiler feed pump discharge nozzles Boiler Blow Down lines Reheater drain line in alternate path to flash tanks

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IAEA-CN-155-050 8. Long term measures taken to mitigate degradation and life management of high energy secondary cycle piping 8.1. FAC monitoring program Objective of the monitoring program issued for all operating stations and projects are as follows. The Program has been developed in two phases. It should be long term monitoring program Identifying which systems are susceptible to FAC and sample selection of these systems

for inspection based on engineering judgment under experience, previous inspection etc. Inspecting components selected for inspection Analyzing inspection data to determine FAC wear rates and balance life Guidelines of future inspection times based on inspection results Repairing or replacing piping components determined or predicted to wear below the

minimum thickness required

8.1.1. Phase 1 Initial examination program The purpose of initial examination program is to collect one time baseline data for the maximum number of components pertaining to high energy system piping of secondary cycle at all plants by UT thickness measurements and assess balance life of the inspected components. These data will help in assessing the condition of secondary cycle piping components affected by FAC. Bases for selection of components for this examination are as follows. All pipes & fittings upstream as well as downstream up to a distance of 1.5 meters of

restriction orifices, flow elements, control valves, bypass valves, motorized valves, non-return valves, manual valves and steam traps All piping components such as reducers, expanders, bends, elbows, tees and branch

connections in high energy system piping Main nozzles of equipments, pumps and branch pipe up to a distance of 1.5 meters

8.1.2. Phase 2 Periodic monitoring program This program was developed to periodically examine the most vulnerable components pertaining to high energy system piping of secondary cycle from the consideration of FAC for each operating station. This examination shall be carried once in six years to assess the healthiness of components and to assess the wear pattern / rate. The bases for identifying FAC vulnerable components are as follows. Areas where local flow disturbances are expected Areas where wetness is high Areas where velocities are high Areas where two phase flow are expected Areas having industrial failure history on other NPPs

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IAEA-CN-155-050 Components pertaining to all high energy systems such as main steam system, re-heater system, boiler feed water system, boiler blow down system, moisture separator and re-heater drain system, heater drain system, steam drain system condensate system, extraction steam system, auxiliary steam system, auxiliary feed water system etc are included in the periodic inspection program. 8.2. System for recording inspection data and review A comprehensive data management sheet (format for recording data) has already been worked out and forwarded to all stations and projects to bring out uniformity in the reporting of inspection data pertaining to Secondary Cycle Piping. This sheet facilitate in recording present inspection data, previous inspection data if any, system, line number, component detail, material used, dimensional details, process parameters, previous inspection history etc. FAC related UT thickness inspection data of secondary cycle piping components will be entered in the format for recording issued to all stations and electronic form (soft copies) of same will be forwarded to Head quarters to facilitate quick assessment and balance life estimation. After analysis recommendations regarding replacement and balance life of each component will be forwarded to respective stations for implementation. 8.3. Basis for assessment of balance life To arrive at the corrosion/wear rate for the components examined, the prevailing measured minimum pipe wall thickness is subtracted from the reference initial thickness and divided by number of hot operating years. In absence of the pre-service inspection or in-service inspection data in case of very first inspection, nominal wall thickness (NWT) is taken as the reference initial thickness.However during any subsequent inspection, minimum measured thickness of previous inspection will taken as reference initial thickness. Minimum wall thickness of the pipe for design pressure is worked out by using formula as per design code.This minimum wall thickness is subtracted from the prevailing minimum wall thickness and divided by corrosion /wear rate to arrive the balance life of the inspected component. Evaluation of balance life is done once the prevailing measured minimum pine wall thickness is less than 0.875 NWT only. 8.4. Guidelines and Procedures Guidelines and procedures issued to all stations and projects as part of FAC management program are given below. To provide step by step method to be followed regarding initial examination, first

examination and successive examination of components selected for inspection Criteria for replacement/ repair of degraded components Criteria for grid size marking on the components to be inspected procedure for U.T thickness measurement procedure for weld deposit Steps to identify the suitable material for replacing the degraded components and

generating its base line data Guidelines for inspection of balance items which are not included in the periodic

inspection program but pertaining to secondary cycle system

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IAEA-CN-155-050 In Indian PHWRs biennial shut down(BSD) is of operating station is once in 2 years. The Guidelines say that for the components having balance life up to 2 years in any examination shall be replaced in the same shut down itself. Components where balance life is found in more than 2 years but up to 4 years may be replaced/repaired in the same shut down if no shut down is planned before next BSD or shall be examined before completion of 50 % of balance life. For the components, having balance life more than 4 years, reassessment will be carried out before 50 % balance life is over at an appropriate biennial shut down (BSD). Flow chart of the activities associated with examination, balance life estimation, repair / replacement / successive examination is given in the figure 3 below. Guidelines of criteria for grid sizing of identified components for UT examination have been prepared referring code case ASME-N480. Square grid of the components selected for examination is categorized in three different groups. For components of sizes 80mm to 100mm, 150mm to 500mm and 500mm to 1000mm square grid sizes are 30mm, 50mm and 100mm respectively. For higher size component higher is the grid size and circumferential grids are started from toe of the reference weld. If at any point thinning is observed, thickness gauging is continued in the same direction until the nominal thickness readings are obtained. Most of the degraded components which are not suitable for the service are replaced after assessment of life. However as an alternative to replacement restoration of thickness of high energy carbon steel pipe and fiitings is accepted in case thinning is localized and required replacement material is not available. However most of the degraded components have been replaced with fresh new components.

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FIG. 3.Activity flow chart for replacement/repair/successive examinatin 9. Material

Existing carbon steel pipes and pipe fittings are to be replaced with better FAC resistant materials at FAC prone portion of piping / lines of various high energy systems of secondary cycle. It is observed that carbon steel is highly susceptible to FAC and low alloy steel-2.25%Cr, 1%Mo (SA-335 Gr.P-22 for pipes and SA-234 Gr.WP-22 / SA-182 Gr. F-22 for pipe fittings) is most suitable FAC resistant material for secondary cycle piping. 10. Water chemistry aspects The matter on FAC has been referred to advisory committee on steam and water chemistry along with related data for further review of water chemistry to reduce FAC. Water chemistry as per recommendation of advisory committee on steam and water chemistry is being maintained. In feed water pH recommended is 8.8 to 9.5 and dissolved oxygen is less than 5

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IAEA-CN-155-050 ppb. The concentration of iron, ethanolamine, pH and conductivity to be measured in different steam, condensate lines, drain lines and feed water lines wherever sampling is possible to establish the base line data. 11. FAC management action plan for operating stations To implement UT thickness gauging monitoring program, including that for base line

data, for high-energy system piping of secondary cycle for all operating stations as mentioned above. To replace progressively the existing carbon steel pipe and fittings of the lines / portion

of piping of high energy systems which are prone for FAC with low alloy steel SA-335 Gr.P22 (for pipes) and SA-234 Gr.WP22/SA-182 Gr.F22 (for fittings) being more FAC-resistant material. To follow water chemistry as per recommendations of advisory committee on steam and

water chemistry and are already pursuing review of this matter referred to them. 12. FAC management action plan for projects under construction To generate baseline data of all installed piping components pertaining to high-energy

system piping of secondary cycle by UT thickness measurement before start up of plant. Such components include pipes & fittings upstream as well as down stream up to a distance of 1.5 meters of restriction orifices, flow elements, control valves, by pass valves, motorized valves, non-return valves, manual valves and steam traps. Base line data is also generated for piping components such as reducers, expanders, bends, elbows, tees and branch connections, equipment nozzles and piping close to equipment nozzles. To examine all identified vulnerable components as per the periodic monitoring

program from consideration of FAC by UT thickness measurement within 12 to 24 months after first start up of plant and assess balance life of the components. Thereafter periodic monitoring program will be repeated after every six years. To replace the existing carbon steel pipe and fittings of the lines / portion of piping of

high energy systems which are prone for FAC with low alloy steel SA-335 Gr.P22 (for pipes) and SA-234 Gr.WP22/SA-182 Gr.F22 (for fittings). Action has been already initiated for procurement of these materials. 13. FAC mitigation plan for future plants To use better FAC resistant material instead of carbon steel in FAC prone lines / portion

of piping of high energy systems. To provide higher corrosion allowance for pipes and pipe fitting in FAC prone lines /

portion of piping of high energy systems. Excess material, over and above that required for pressure integrity and structural and mechanical strength, can be provided. This excess material is allowed to waste away over the design life of the piping system.

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IAEA-CN-155-050 Proper velocity assumptions will be considered wherever felt necessary while sizing the

piping system for future projects. To develop piping layout to minimize flow disturbances. In the future projects all

efforts will be made to design the piping system geometry so as to minimize turbulent flow, direct pipe wall impingement, vortex flows which are the perceptible causes to increase FAC To implement UT thickness gauging periodic monitoring program of piping

components. 14. Conclusion As discussed above overall FAC management program of secondary cycle high energy piping and piping components is being achieved in Indian Nuclear Power Plants through continuous examination and monitoring of components in all stations, its residual life analysis, following uniform guide lines for repair / replacement and performing successive examinations. Actions are also taken for replacing the pipes and fittings at FAC prone lines/ portion of piping of high energy systems with a better FAC resistant material (i.e. low alloy steel),of one schedule higher thickness than required, in case of stations and projects. For projects under construction base line data of the large number of installed components is also being generated for future reference. Water chemistry in various lines of secondary cycle is also maintained as per recommendations of advisory committee on steam and water chemistry. For future projects efforts are being initiated to minimize the effect of FAC influencing factors through improved pipe layout, better FAC resistant materials, higher corrosion allowance, and proper velocity assumptions during line sizing etc. All above actions are aiming mitigation of degradation of high energy piping and life management of secondary cycle piping due to Flow Assisted Corrosion (FAC).

REFERENCES

[1] Failure analysis Report of 10% feed water line at KAPS-2 performed by material

science division of Bhabha Atomic Research Centre [2] ASME code case N480, Section XI, Division 1 Examination requirements for

pipe wall thinning due to single phase Erosion and corrosion [3] NUREG/CR5632 Incorporating aging effects into probabilistic risk assessment

A feasibility study utilizing reliability physics models prepared by C.L Smith, V.N Shah, T.Kao, G.Apostolakis