Julie Flambard

Florence Carrette

Carole Monchy-Leroy

Eric Andrieu

Lydia Laffont
email: lydia.laffont@ensiacet.fr

Influence of the transient conditions on release of corrosion products and oxidation of alloy 690 tubes during pressurized water reactor restart after steam generators replacement

Keywords:

Introduction

ABSTR AC T

The radioactivity of the Reactor Coolant System (RCS) of Pressurized Water Reactors (PWRs) mainly cornes from the release of corrosion products of Steam Generator (SG) tubes made of Ni-base alloys. In order to reduce this activity and thus the radiation exposure of PWR operators during maintenance operations, it is necessary ta minimize the release. That requires prior understanding of the various mechanisms involved. EDF R&D constructed a loop, BOREAL, to specifically measure rates of release of SG tubes in various conditions of primary environment. Tubes were usually tested at high temperature, under con stant conditions of primary chemistry. So, it is necessary to carefully investigate the impact of transient conditions during a PWR restart after SG replacement. Tests were performed on the industrial material with curvature, roughness, defects and heterogeneities, regularly observed on this type of component. Characterisations of the inner surface were done on as-received and corroded specimens of SG tubes and were correlated with the obtained release kinetics.

The native oxide layer is formed of a very thin layer (1-2 nm) of oxidised matrix, without specific en richment. During the restart, the most critical step for the release phenomenon is revealed from 170 °( to 297 °C. The majority of the metal is indeed released into the lluid during this step. The characterisa tions after release tests have made it possible to propose oxidation and release mechanism during a PWR restart after SG replacement. Up to 170 °C, a thin layer of amorphous chromium oxide is formed by selec tive dissolution of iron and nickel. When the temperature rises, this chromium oxide layer is not stable enough to be protective and the diffusion phenomena are activated. At 325 °C, the oxide does not exhibit any particular enrichment and corresponds to an oxidised metal layer; an equilibrium is established and the rate of release reaches a pseudo-stationary regime.

An important part of the activity and the contamination of the Reactor Coolant System (RCS) of Pressurized Water Reactors (PWRs) mainly originates from the activation of corrosion prod ucts released by the very large surface area of Steam Generator (SG) tubes made of nickel base alloys (alloy 600 -75% Ni, 15% Cr, 10% Fe, alloy 690-60% Ni, 30% Cr, 10% Fe). The released iso tope 58 Ni is activated into the isotope 58 Co that contaminates ail the surfaces of the RCS. ln order to reduce this radioactivity and hence the radiation exposure during the maintenance and associ-ated operation costs, it is necessary to control and minimize the release. To achieve this goal, it takes in advance an adequate un derstanding of the various mechanisms involved. The oxide film formed on the nickel base alloys plays a key role in nickel release, as well as the PWR primary environment and the material charac teristics that can also influence the oxide growth [1,2]. The oxide film formed under PWR conditions, at high temperature in nom inal primary water (-1000 ppm Boron, -2 ppm Lithium, -20-30 cm 3 /kg NTP Hydrogen), is usually described as a duplex oxide layer with a compact inner layer enriched in chromium and a dis continuous outer layer that contains mainly, according to the tests facility, nickel ferrite, nickel chromite and nickel hydroxide [START_REF] Sennour | A detailed TEM and SEM study of Ni-base alloys oxide scales formed in primary con ditions of pressurized water reactor[END_REF][START_REF] Lafont | Analysis and TEM examinations of corrosion scales grown on Alloy 690 exposed to pressurized water at 325 °C: Corrosion scales on Alloy 690 grown in pressurized water[END_REF][START_REF] Carrette | Analysis and TEM examinations of corrosion scales grown on alloy 690 exposed to PWR environ ment[END_REF]. Therefore, the chromium rich layer is considered to be the main factor of corrosion resistance of these alloys. Sorne authors suggest that, at high temperature, nickel base alloy behaviour regarding the oxidation and the release depends on the first stage of oxidation [START_REF] Machet | XPS and SfM study of the growth and struc ture of passive films in high temperature water on a nickel-base alloy[END_REF],

Severa! models were proposed to explain the mechanisms of oxidation and/or release of corrosion products in the primary en vironment. These models assume the duplex microstructure of the growing corrosion scale and also a continuous, uniform and ho mogeneous layer of oxides over the entire surface of the materiaL Furthermore, they consider additional assumptions about transport phenomena within the scale [START_REF] Castle | The role of diffusion in the oxidation of mild steel in high temperature aquous solutions[END_REF][8][9], dissolution of the scale [9,10], pore blocking and precipitation [8,[START_REF] Lister | The mechanism and kinetics of corro sion product release from stainless steel in lithiated high temperature water[END_REF], For example, the mode! of proposed by Gardey [ 11 ], essentially from electropolished wafers, combines the previous models of Robertson [9], Evans [8] and Lis ter [START_REF] Lister | The mechanism and kinetics of corro sion product release from stainless steel in lithiated high temperature water[END_REF] and considers the growth of a corrosion scale consisting of three different layers, the compact innermost layer acting as a diffusion barrier while the external layer is porous, The presence of pores in the oxide, which does not seem to be yet experimen tally validated, is also mentioned, The mode! of Carrette [START_REF] Carrette | Analysis and TEM examinations of corrosion scales grown on alloy 690 exposed to PWR environ ment[END_REF][START_REF] Carrette | kinetics of corrosion products release from Nickel-base alloys corroding in primary water conditions: a new modelling of release[END_REF] is based on characterisations of oxide layers, and underlying material, essentially formed on polished, electropolished and cold-worked wafers and a few SG tubes in primary medium at 325 °C, on cor rosion/release tests carried out on the same previous samples in primary medium à 325 °C and with the use of tracers (D 2 0 and Xe) [START_REF] Carrette | Analysis and TEM examinations of corrosion scales grown on alloy 690 exposed to PWR environ ment[END_REF], lt considers phenomena of diffusion, space charges and disso lution, The work of Machet [START_REF] Machet | XPS and SfM study of the growth and struc ture of passive films in high temperature water on a nickel-base alloy[END_REF] on the oxide layers formed during the first oxidation times on 600 and 690 mirror polished alloys ( wafers and flattened SG tubes), excludes a growth of parabolic type, associated with a purely diffusional mechanism, Moreover, ail these models are based on tests conducted at high temperature, around 300 -325 °C, they do not integrate what might happen at lower temperatures, Finally, the majority of these models were es tablished on tests carried out on mode! samples (mirror polished or even electropolished wafers, mirror polished flattened SG tubes) and none of these models makes it possible to account for the in fluence of the parameters of materials or linked to the medium, on the kinetics of corrosion and release, EDF R&D has designed and built a specific loop, named BO REAL, to continuously measure release rates of SG tubes in vari ous primary water conditions of PWRs [START_REF] Guinard | BOREAL: a new facility for studying the release of steam generator tubes[END_REF], The BOREAL loop re produces the chemical, thermal and hydraulic conditions of the PWR primary water, Iwo types of parameters have an impact on the release phenomenon: the thermochemical conditions of pri mary water [START_REF] Lafont | Analysis and TEM examinations of corrosion scales grown on Alloy 690 exposed to pressurized water at 325 °C: Corrosion scales on Alloy 690 grown in pressurized water[END_REF][START_REF] Lister | The mechanism and kinetics of corro sion product release from stainless steel in lithiated high temperature water[END_REF][START_REF] Guinard | BOREAL: a new facility for studying the release of steam generator tubes[END_REF][START_REF] Carrette | Impact of boron content of cor rosion product release rate in PWRs[END_REF] and the internai surface state of SG tubes [START_REF] Guinarct | Influence of initial surface condition on the release of nickel alloys in the primary circuit of PWRs[END_REF][START_REF] Guinard | Investigations for reducing the release of steam generator materials: experiments and modelling[END_REF][START_REF] Carrette | Impact of the surface state of steam generator tubes on the release of corrosion Products in pressurized water reactors[END_REF], Release tests were usually perforrned on alloy 690 tubes ( that replace alloy 600 tubes) at high temperature (285 -325 °C), under constant conditions of primary chemistry, For a better un derstanding of the release phenomenon, it is necessary to carefully investigate the impact of transient phases like start-up conditions of a reactor, During a restart of reactor, physico-chemical condi tions significantly vary: pressure and temperature rise, boron and lithium concentrations change, hydrogen content increases, the pri mary fluid switches from oxidizing to reducing conditions, So, re lease tests were carried out with a temperature and chemistry pro gram close to an actual reactor start-up on an industrial surface of SG tubes with their curvature, roughness, defects, and hetero geneities, regularly observed on this type of component and not on mode! samples, Considering a reactor restart after SG replace ment, release tests were made on new tubes that is to say on non oxidised tubes, The objective of this paper is to present the impacts of temper ature and chemistry transients, close to an actual reactor restart, on the oxidation and release of SG tubes in alloy 690, The aim of the study is to understand what happens in terms of release and oxidation of SG tubes during a restart after SG replacement, with the constraints of restart imposed by operations of reactors and operators, As a first step, release tests were performed in the BO REAL loop in order to simulate the transient phases of a reactor restart after SG replacement and to study the influence of chem istry and temperature on the release kinetics, ln a second step, em phasis was put on the characterisation of the oxides formed dur ing the corrosion tests, compared to the as-received tube surface state, These studies were performed using Scanning Electron Mi croscopy (SEM), Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) and Transmission Electron Microscopy (TEM) as well as Scanning Transmission Electron Microscopy (STEM) coupled with Energy Dispersive Spectroscopy (EDS), These observations are cor related with release kinetics and provide a better understanding of the various implied mechanisms in the oxidation and release of corrosion products of SG tubes in alloy 690,

Materials and experimental methods

2,1, Test specimens

Ali tests were run with tube sections from a single steam gener ator tube made of alloy 690, The length of these specimens is 250 mm, The SG tube is of recent manufacturing, its outer diameter is 19.05 mm and its thickness is 1.09 mm, lt was provided by Valinox Nucléaire and its heat number is HR282703, The bulk composition of the SG tube is given in Table 1.

The raw material undergoes initial hot transformations to ob tain tube blanks, These blanks pass by a first transformation by cold rolling, followed by an intermediate recrystallization heat treatment under hydrogen at very high temperature, Then a sec ond cold rolling transformation is carried out to obtain the steam generator tubes with a diameter of 19.05 mm and a thickness of 1.09 mm. After a sequence of cleaning and rinsing, the tubes un dergo a final annealing heat treatment under hydrogen at 1060 -1100 °( with a severe cooling between 900 -500 °C to avoid pre cipitation of intragranular carbides. After a straitghtening operation to obtain the linearity criteria necessary for the assembly of the tubes in the steam generators, an additional heat treatment under vacuum at 715 °C is carried out.

After manufacturing, no additional surface treatment was per formed on the SG tube and its inner surface; tube sections used for the tests have an industrial inner surface state. Only before the tests, tube sections are cleaned under ultrasound with acetone, ethanol then ultrapure water.

Corrosion tests

The corrosion tests were carried out in the BOREAL loop [START_REF] Guinard | BOREAL: a new facility for studying the release of steam generator tubes[END_REF] simulating primary water conditions. This equipment is a re circulating loop that reproduces chemical and thermo-hydraulical conditions of the primary fluid circulating inside SG tubing of PWRs. This facility is composed of plastics in the cold section and of zirconium or titanium alloys in the hot section, to avoid interfer ence with the release of corrosion products of the SG tube tested. The use of Ultra Thin Layer Activation (UTLA) coupled with a gam mametric detection allows to obtain, by continuous measurement, the kinetic of the release [START_REF] Guinard | Ultra thin layer activation by recoil implantation of radioactive heavy ions : applicability in wear and cor rosion studies[END_REF]. The release kinetic is expressed as the thickness of released metal as a function of the corrosion time and the sensitivity of detection is Jess than two nanometers in terms of thickness Joss.

The tests reproduce as accurately as possible the temperature and chemistry (B, Li, pH, 0 2 , H 2 ) transients during a reactor restart. At the beginning, the aerated fluid is at room temperature (-25 °C) without lithium and hydrogen but with boron (2500 ppm), the pH is of 4.6. The fluid is then deaerated from 60 °C to decrease the content of oxygen until 5 ppb at 170 °C. The oxygen content is in fact Jess than 10 ppb from 80 °C. As part of the release tests in the BOREAL loop, this done by nitrogen injection. At 297 °C, the boron Tobie 1 Bulk composition of steam generator tubes made of alloy 690 in wt. %

Wt, % 59,23 29,63 10,08 0, 17 0,29 0,20 0,30 0,02 0,03 < 0,008 0,006 0,0002 concentration is reduced (2100 ppm), lithium (2,2 ppm) and hy drogen (5 -20 cm 3 /kg (NTP)) are injected, The calculated pH of the lluid at this temperature is of 6,6, At 306 °C, the boron con centration is reduced (1600 ppm), lithium and hydrogen staying at the same concentrations (2,2 ppm, 5 -20 cm 3 /kg (NTP)), the cal culated pH of the lluid at this temperature is of 6,9, The last sec tion of the test is performed in nominal operating primary water at 325 °C with a calculated pH of 73 (B = 1200 ppm, Li = 2,2 ppm, H 2 = 25 -35 cm 3 /kg (NTP)), From 25 °C to 325 °C, there are several transients that vary from a few hours to a few days,

23, Characterisations

The internai surface state of the tube and the oxides formed after the corrosion tests were observed by SEM, The SEM observa-tions were carried out on a ZEISS Sigma microscope, at 5 kV using various magnifications and modes, The difficulty of the study is the preparation and the observation of an industrial surface, with de fects, roughness, angle of curvature, Due to these difficulties, the cross-sections were prepared using ion polishing system (Ilion 2 GATAN), To protect the oxide, the sample was coated with gold be fore the cross-section, Samples for TEM observations were prepared using a FEI Dual Bearn HELIOS nanolab 600, TEM foils were further thinned and cleaned in a Precision Ion Polishing System (PIPS Il) until the de sired thickness was reached, Doing so, the artefacts induced during the lamellae extraction and thinning in the FIB were erased [START_REF] Legras | Optimisation ofTEM prepa ration in metallic materials using low voltage ions[END_REF], TEM analyses were made to investigate the oxide scale formed (thickness, morphological and chemical composition), TEM charac terisations of the internai surface tube and the oxides were re- The oxide composition and the underlying material were also determined by using ToF-SlMS, ToF-SIMS data were acquired us ing a ToF-SIMS V spectrometer (Ion TOF), The sputtering was per formed using a 2 keV (80 nA) Cs+ ion beam, rastered over an area of 300 x 300 µm 2 , A 25 keV Bi+ analysis beam was used to scan an area of 30 x 30 µm 2 , The oxide sputtering rate was established assuming a constant rate, whatever the oxide composition (as the oxide density is always very similar), the resulting value is 0321 ± 0,037 nm/s, Following the intensity profile versus sputter time of specific oxide anions enable to evaluate the composition of the oxide layer whereas the intensity profile of Ni 2 -allows to determine the thick ness of the layer [START_REF] Mazenc | Caractérisation par TOF-SIMS des couches de passivation des tubes de générateur de vapeur en Alliage 690 pour l'industrie nucléaire : apport à la compréhension des mécanismes[END_REF], Ni 2 -is representative of the metallic matrix and as soon as the profile of this ion reaches a constant value, one can assume that the base metal is reached, Table 2 summarizes the link between the representative oxide anions analysed by ToF-SIMS and the corresponding oxides,

BSE

Resultses

3,1, Initial characterizations of the as-received SG tube

In order to characterize the topography, different segments of the SG tube were tested with the as-received surface condition, The observations by SEM and EDS analyses of the internai surface shows the significant presence of inclusions of Alumina in the in ternai surface, Titanium traces, partially oxidised, are also detected, mainly at grain boundaries of the extreme surface, Average grain size was obtained thanks to EBSD mapping and data treatments using the OIM analysis software (Fig, 3 ), The im posed misorientation for the grain calculation was of live degrees allowing to represent at best the grain population, The average grain size of the 690 alloy steam generator tubes is about 50 µm in the thickness of the tube, As previously observed on 690 alloy SG tubes [START_REF] Carrette | Characterisation of oxides formed on the in ternai surface of steam generator tubes in alloy 690 corroded in the primary environment of pressurised water reactors[END_REF], the structure is heterogeneous, there are small and large grain populations on ail the tube thickness, it still respects French guidelines (5 < conventional index of grain size G < 9), Data on 600 and 690 alloys demonstrated that, across the thick ness of the material, a structure homogeneous with small grains or large grains or heterogeneous in terms of grain size has no influence on corrosion and release rates [11,[START_REF] Guinard | Investigations for reducing the release of steam generator materials: experiments and modelling[END_REF]21,[START_REF] Carrette | Characterisation of oxides formed on the in ternai surface of steam generator tubes in alloy 690 corroded in the primary environment of pressurised water reactors[END_REF], Neverthe less, these areas with very small grains but also with large grains at the extreme internai surface imply the opportunity of differ ent behaviours concerning corrosion and release phenomena [START_REF] Carrette | Characterisation of oxides formed on the in ternai surface of steam generator tubes in alloy 690 corroded in the primary environment of pressurised water reactors[END_REF],

When very small grains of 1 -2 µm diameter on a thickness of 2 µm maximum are present at the extreme surface, the release rate seems to be divided by a factor 2 [START_REF] Carrette | Characterisation of oxides formed on the in ternai surface of steam generator tubes in alloy 690 corroded in the primary environment of pressurised water reactors[END_REF], The beneficial effect of this type of microstructure would corne from increased diffusion of chromium to the surface via grain boundaries to form internai protective layer, This kind of microstructure at the extreme surface of SG tubes remains nevertheless very sporadic and it is preferable not recommended to avoid stress corrosion cracking, To describe the inner surface state before oxidation, TEM char acterisations were performed, The Fig, 4 shows HMDF STEM im age of the internai surface of the tube where grain boundaries are present and STEM EDX mapping of the chemical elements of interest, The presence of aluminium oxide is visible up to 3 µm whereas titanium precipitates, in the form of TiN are also observed up to 6 µm, The presence of aluminium and titanium were already noted on the inner surface and in the bulk of SG tubes [START_REF] Carrette | Characterisation of oxides formed on the in ternai surface of steam generator tubes in alloy 690 corroded in the primary environment of pressurised water reactors[END_REF][START_REF] Meng | The role ofîiN inclusions in stress corrosion crack initiation for Alloy 690TI in high-temperature and high-pressure water[END_REF], The between them along grain boundaries (Fig, Sa) are highlighted, The intergranular chromium carbides, formed during the manufac turing process, are significantly developed by thermal treatment These chromium carbides, which protect Alloy 690 from stress cor rosion cracking, are identified as Cr 23 C 6 using selected area diffrac tion pattern (SAED) [START_REF] Bowman | The crystal structure of Cr23C6[END_REF] and have a cube-cube relationship with the matrix [25],

The Fig, 6 presents a STEM HMDF image combined with an EDX profile from the surface of the oxide to the matrix, The na-tive oxide layer does not seem to present a particular enrichment except for oxygen, lt would correspond to a thin layer of oxidised matrix, The thickness of this native oxide layer is around 2 nm, In addition to this very thin oxide layer, small nodules are regularly observed along the surface, The Fig, 7 presents STEM HMDF im age of the extreme surface of the tube combined with EDX analy sis done on these small nodules (EDX boxes having the same size ), EDX quantification (Fig, 7) reveals an area with an enrichment in chromium or just the oxidised matrix, ToF-SIMS analyses were performed to estimate the thickness and the chemical composition of the native oxide, Fig, 8 shows the ToF-SIMS profiles obtained on the initial surface state of the tube, The Ni 2 -signal reaches its maximum intensity after "' 8 s of sputtering corresponding to a native oxide layer thickness of"' 2-3 nm, A detailed examination of the different signais in the profiles provides more information, The inner extreme surface is charac terised by intense hydroxides signais, Then the oxide signais, that reach their maximum intensity around 4 -5 seconds of sputtering, demonstrate that the native oxide corresponds to oxidised matrix, without any enrichment of chemical element The presence of Alu mina and titanium oxide at the surface is confirmed, Beyond the oxide/metal interface, we can notice the slow decrease of MO 2 - and Mo-signais (M being a metallic cation) and the slow increase of the Ni 2 -signais, This effect may result from the very low rough ness of the inner surface, due to the manufacturing of the SG tube,

The tube studied does not seem to have a gradient in chemical composition, However, a microstructure gradient is demonstrated with a heterogeneous grain size in the thickness of the tube and the presence of numerous inclusions and precipitates, The surface appears to be very disturbed, the grain boundaries are very well defined and steps, revealing the dense planes are visible at high magnifications, The oxygen-rich layer present in the initial state is very thin, 1 to 2 nm and no enrichment in a particular chemical element is highlighted, One of the objectives of this study was to simulate as closely as possible the stages of a PWR restart on a new SG tube us-ing the BOREAL loop and thus, to obtain the kinetics of release associated,

32, Release kinetics

The release kinetic of the 690 alloy SG tube during transient phases is presented in Fig, 9a, For remember, at the beginning, the aerated lluid is at room temperature without lithium and hydro gen but with boron (2500 ppm), Nitrogen is injected from 60 °C to decrease the content of oxygen until 5 ppb at 170 °C At 297 °(, the boron concentration is reduced (2100 ppm), lithium (2,2 ppm) and hydrogen (5 -20 cm 3 /kg (NTP)) are injected, During the first sec tions of the restart, about 300 hours at low temperature (25 °C to 130 °(), the release is very low, From 50 °C, a slight release around 1 nm is noticeable on the kinetics, This is no very significant, how ever, this release indicates that the phenomenon begins to acti vate with this increase of temperature, Between 170 °C and 297 °C, a significant climb of the release is observed, The last section of the test, performed in nominal operating primary water (325 °C, B = 1200 ppm, Li= 2,2 ppm, H 2 = 25 -35 cm 3 /kg (NTP)), shows a low release rate that decreases over time, rellecting the formation of the protective oxide layer, The oxide layer seems to be protec tive and stable, 11 nm of metal are released at the end of the test and a pseudo-stationary regime is established with a constant and low release rate of 2,4,10-3 nm/h (calculated between 600 and 800 hours of testing), To improve the understanding of the release and oxidation phe nomena around the critical temperature, a test was achieved until the step at 170 °C ( during 200 hours ), Fig, 9b ( orange curve) in dicates that at this step, the released quantity is lower than the released quantity when the temperature rises 297 °C The final re lease is around 2 -3 nm and the release rate calculated over the last 150 hours is 7,3,10-3 nm/h, When the test is continued until 325 °C (blue curve ), the kinetic is similar at the previous test, a sig nificant climb of the release is observed between 170 °C and 297 °C Tuen, in nominal operating primary water, the release rate is low and decreases over time, 10 nm of metal are released at the end of the test and a pseudo-stationary regime is established with a con stant and low release rate of 2,4,10-3 nm/h, The duration of the step at 170 °C does not have an impact on the final release, To better understand the mechanisms implied in the release phenomenon, the inner surface of the SG tube sections have to be characterised after these transients tests,

33, Characterisation of the internai surface of the tubes after transients tests

The Figure 10 presents SEM images of the oxide on the in ner surface of the SG tube corroded during transients tests un til 170 °(, This test was stopped just before the temperature rise 297 °C, which means just before the significant climb of the re lease, The objective of these observations of oxides formed before the critical step, in term of release, is to better understand the evolution of the oxide formed during various stages of the reactor restart The oxide appears to be relatively fine, the grain bound aries are always well defined, as observed on the as-received sur face state, The steps revealing the dense planes seem to be erased, less visible than on the as-received state, Very small crystallites are appeared, they are quite homogeneously distributed on the surface, The Fig, 11 presents SEM images of the inner surface of the SG tube corroded in conditions of restart of reactor until 325 °C The grain boundaries are well defined and seem to be more pro nounced and steps are again well defined, The oxide layer is com posed of sticks and needles shaped elements mainly observed at grain boundaries and on disturbed areas, At high magnification, the surface appears covered by very small nanometer sized crys tallites, The small thickness of oxides and their heterogeneous as pect do not allow some analyses by EDX, In cross-section, limited intergranular oxide penetrations and small oxidised zones are also observed in Fig, 12 After these SEM observations, the oxide layers are studied by STEM EDX after preparation of the samples by FIB in cross-section, This allows liner characterisations and chemical analysis of the ox ide layer at the nano-scale, The average thicknesses of oxide layers are determined by several measurements carried out at different locations,

The Fig, 13 presents STEM bright field image of the extreme in ner surface of the oxidised tube as well as chemical analyses car ried out on the zones marked by rectangles, numbered from 1 to 4, The EDX analysis evidence that small crystallites (10 to 20 nm) observed on the surface appear to be highly enriched in chromium (rectangles 1, 2 and 3), The EDX linescan, presented in Fig, 14, con- To better understand the formation of the oxide at the extreme surface on precipitates already present at initial state, chemical mapping was performed at the metal/oxide interface, The maps in Fig, 15 show the STEM EDX analysis around an Alumina inclusion initially present at the as-received state, Alumina inclusions are in deed always present at this temperature (170 °(), STEM EDX maps of iron and chromium highlight the thin layer of chromium oxide on the surface of the sample as well as around the inclusion of Alumina, The inclusion of Alumina is covered and surrounded by a thin layer of chromium oxide,

The STEM bright field image combined with EDX maps of the essential chemical elements shown in Fig, 16 show a precipitate of titanium oxide, which was already present at initial state, This pre cipitate appears to be coated with chromium oxide and is topped by an aggregate of small nanometric crystallites of chromium ox ide,

The Fig, 17 shows the montage of bright field TEM images of the internai surface of the steam generator tube corroded in condi tions of reactor restart until 325 °C The surface is disturbed and it is not perfectly fiat: annealing twins, formed during the heat treat ment at the end of manufacturing, are observed, The oxides are slightly different from those formed until 170 °( ( described above) because of the presence of needles and sticks that are clearly visi ble as well as very small crystallites at the extreme surface, Fig, 18 presents the STEM EDX maps on area with needles and sticks shape elements that can reach a size of 100 -150 nm and appear to be chromium and nickel oxide and/or hydroxide, A con tinuous layer of oxide is visible and does not seem to present any particular enrichment On the other hand, the small crystallites, which cover the surface, seem to be chromium rich, These oxide crystallites measure approximately between 5 -20 nm, Alumina is not observed on the extreme surface on this sample, which indi cates that it must be dissolved in the lluid at high temperature (between 170 °C and 300 °(), Fig, 19 shows STEM EDX maps fo cused on an intergranular oxide penetration, from a titanium ox ide precipitate in the present case, as already observed by F, Meng et al, [START_REF] Meng | The role ofîiN inclusions in stress corrosion crack initiation for Alloy 690TI in high-temperature and high-pressure water[END_REF], This titanium oxide is recovered by small crystallites of chromium oxides, In the intergranular penetration, the chromium oxides are present followed by metallic NL Fig, 20 shows an EDX linescan superimposed to the STEM HAADF image of the inner extreme surface of the tube represent ing the chemical evolution of the oxide layer, This indicates an ox idised matrix layer, about 2 -3 nm thick, and a chromium oxide crystallites at the extreme surface,

In addition, the extreme surface of the tube was also studied by STEM in bright field mode in Fig, 21, EDX chemical analysis were carried out on different areas of the oxide, which have the same dimension, numbered from 1 to 4, Nanometric crystallites are ob served from 2 to 6 nm size and are composed of chromium oxide as shown in the analysis of area L A rather continuous layer of ox- ides is identified with a thickness between 2 and 5 nm, This layer corresponds to the oxidised matrix without any particular enrich ment (EDX analyses of areas 2 and 3 ), The EDX analysis of area 4 corresponds to the chemical composition of the alloy 690,

The nature and chemical composition of the oxides were also studied by ToF-SIMS spectrometry, For both samples, ToF-SIMS profiles from the inner extreme surface to the material bulk were acquired, Fig, 22 and Fig, 23 show the ToF-SIMS profiles obtained af ter transient's test in the BOREAL loop, For the test until 170 °( (Fig, 22 ), the Ni 2 -signal reaches its maximum intensity after 23 seconds of sputtering corresponding to an oxide layer thickness of approximately 7 nm, A detailed examination of the different sig nais in the profiles provides more information, The extreme sur face is characterised by intense hydroxides signais between 0 and 4 seconds of sputtering, Then the oxide signais, that reach their maximum intensity around 6 seconds of sputtering, demonstrate that the oxide is rich in chromium with characteristic ions of chromium oxide (Cro-, ero 2 -J, The presence of Alumina and ti tanium oxide at the surface is always detected at this step of the transie nt

For the test until 325 °C (Fig, 23 ), the Ni 2 -signal reaches its maximum intensity after 150 seconds of sputtering corresponding to an oxide layer thickness of approximately 48 nm, The overesti mate of the thickness is due to roughness induced by sticks and needles, The extreme surface is characterised by intense hydrox ides signais between 0 and 20 seconds of sputtering, more pre cisely the chromium hydroxide thanks to CrO 3 H-signal, Therefore, this extreme surface area of the oxide layer is mainly composed of chromium hydroxide, In the internai part of this zone, the CrO 2signal increases strongly, corresponding to the chromium oxide present just below the hydroxide, The chromium oxide in contact with the fluid can be indeed in the form of hydroxide while the part which is not in direct contact is in the form of oxide, More deeply, between 20 and 40 seconds of sputtering, the characteristic signais of the chromium, nickel and titanium oxides increase, The profiles have the same shape, which seems ta indi cate the presence of layer of oxidised matrix oxide, without partic ular enrichment The presence of titanium oxide is always detected but not Alumina signal, The titanium signais can be explained by the significant presence of titanium at the extreme surface at the as-received state, which then can diffuse in the oxide layers during corrosion, By looking in more detail at the metal/oxide interface, the in terface is clearly characterised by a slow decrease of characteris tic signais of the oxides and a slow increase of the Ni 2 -signal, characteristic of the metaL These are attributed ta the formation of a rough interface resulting from the initial roughness of the sur face of the sample and/or preferential oxidation with, for exam ple, faster oxidation at the grain boundaries and disturbed areas, Therefore, these phenomena lead ta the overestimation of the ox ide thickness, However, the profiles are interesting and comple mentary ta the observations made by microscopy, These analyses are rather coherent with previous observations,

Discussion

4,1, Evolution of the inner surface during the reactor restart after SGR

The experimental results presented above allow studying the effect of transient phases on the oxidation and the release of SG tubes, Fig, 24 shows the schematic drawings of the oxide on the nickel-based alloy after a restart of reactor (after steam genera tors replacement), For simplicity, the surface of the tube is shown plane, but it is important ta remember that samples of tube are not planar, They are slightly rough with a heterogeneous structure (various inclusions, folds), At the as-received state, an oxygen enriched layer with a thick ness of 1 ta 2 nm is highlighted, This oxide layer has no enrich ment in a particular chemical element (Ni, Cr, Fe). The inner sur face of the tube also has many inclusions of Alumina, which are exogenous particles coming from Corundum used during the man- ufacturing process. These inclusions, varying in size from 10 ta 300 nm, are embedded on the surface of the tube and are observed up ta 2 -3 µm deep. Titanium precipitates, more specifically titanium nitrides, are also present up a depth of 5 -6 µm, these precipitates are in the potential form of titanium oxides when they are close ta the extreme surface.

At the end of the step at 170 °C, sa before the important rise of release during the BOREAL test, a thin layer of chromium ox ide, with a thickness of 2 -3 nm, is formed. The Alumina inclu sions, present on the surface, are also covered with a thin layer of chromium oxide, this oxide surrounds the inclusion. The same phe nomenon is observed around the titanium oxides at the extreme surface. Small crystallites (10 -20 nm) of chromium oxide are ob served on the extreme surface. The Alumina inclusions and the titanium oxide precipitates initially present are less embedded in the materiaL The precipitates located deeper in the material have not changed.

At the end of the test, once the full power conditions of reac tor at 325 °C are reached and maintained for 3 weeks, a layer of oxides of 2 -5 nm is observed. This layer does not show any par ticular enrichment in a chemical element of the alloy. The nano metric chromium oxide crystallites are still observed, as at the end of the step at 170 °C, it seems that their density is a little higher. Needles and sticks, with an average length of 100 -150 nm and a thickness of a few nanometers, caver the surface with higher density at grain boundaries and disturbed areas. These specially shaped oxides appear ta be composed of oxides and/or hydroxides of nickel and chromium. Alumina inclusions, which were in contact with primary water, are no longer observed. They are dissolved in the medium during the rise in temperature. Titanium oxide pre-

4,2, Thennodynamic approach

Before the release tests and the characterisation of the internai surface of the tube sections after tests, oxides that can form ther modynamically during major transients from 25 °C to 325 °C were searched from literature data, Redox potential values and pH val ues (in temperature) were calculated by ChemworksTools (version 4,3,0) code with MULTEQ database (version 9) Data do not always exist for the different interesting transients of the present study that is to say at 25 °C, 170 °C, 297 °C and 325 °C Calculations and sta bility diagrams of oxides which are likely to form during the expo sition of Alloy 690 in nominal primary water at high temperature (conditions very closed to those of the present release tests: 1000 ppm boron, 2 ppm lithium, -30 cm 3 /kg (NTP) hydrogen) were es tablished by Sennour et aL [START_REF] Sennour | A detailed TEM and SEM study of Ni-base alloys oxide scales formed in primary con ditions of pressurized water reactor[END_REF], They reveal that, thermodynami cally, formed oxides can be classified by stability rate in the follow ing order: Cr203 > FeCr2O4 > NiCr2O4 > NiFe2O4 > NiO, More over, potential-pH diagrams of the Ni-Cr-Fe ternary system created for different temperatures (25 °C, 100 °C, 200 °C and 300 °C) by Bev erskog et al, [26] give an indication of oxides likely to form dur-ing the different transients of temperature and chemistry though data are not fully representative of the real system, the hypothesis [Fe(aq)]=[Cr(aq)]=[Ni(aq)]=l0-6 moL being considered in calcula tions, Thus, thermodynamically stable species are summarised in Table 3,

The results of the different characterisations are not entirely in agreement with thermodynamic data, They mention the presence of iron as iron oxide whereas iron is not really detected in the analyses, Rather in ionic form, oxide layers are essentially com posed of chromium oxide/hydroxide, Although in ionic form at low temperatures, nickel is rather oxidised as oxide/hydroxide than metallic at high temperature in the present reducing conditions, The differences observed between the results of the characterisa tions and thermodynamic data indicate that the material/fluid sys tem is out of equilibrium, This is not surprising given the many temperature and chemistry changes from 25 °C to 325 °C with steps varying from a few hours, a few dozen hours to a few hundred hours for the last one à 325 °C Moreover, the predictions does not take into account in one hand the microstructural aspects of the inner surface state with the heterogeneities as Al and Ti detection, the role that they can play in the formation of the oxide scale ( and in the release ), the various interfaces that can be observed and in the other hand, the kinetic aspects of the different phenomena in volved ( dissolution, precipitation, release, diffusion ,, , ),

43, Experimental approach

The description of oxides formed on the 690 alloy in primary water is quite far from the descriptions usually available in the lit erature for 600 (Mill Annealed and Thermal Treated) and 690 al loys, The internai chromium enriched oxide layer at 325 °C is not observed, The origin of this observation is probably due to the test conditions, In particular, two major differences can be noted, The the conditions, in particular the conditions of circulation of the lluid inside the tube during the tests, ln this study, the BOREAL loop reproduces at the tube scale a turbulent hydraulic regime, close to the conditions observed inside steam generator tubes of the primary circuit of PWRs, The llow rate is clearly higher than that usually practiced in tests of the literature, ranging from static condition in an autoclave to loop tests with laminar llow regimes of the order of 20 L/h maximum, These two major differences can largely explain the differences observed between the results of this study and those of the literature, However, the results of this study are in agreement with those from a recent expertise on the internai surface of a 600TT alloy steam generator tube extracted from a nuclear power plant after 200 000 hours of service [START_REF] Panait | Microstructural investigations of a steam gen erator tube mdde after zinc injection[END_REF], The SEM and TEM observations show an oxide consisting of an external layer, of approximately 5 µm, discontinuous and very porous interpreted as an exogenous oxide coming from the primary medium (more or Jess rich in iron and nickel), This layer would not be directly due to the oxidation of the tube, Then, an oxidation of the base metal over approximately 2 µm, rich in nickel, chromium and iron, with zones more or Jess enriched in one of these elements, is observed, In addition, par tially oxidised particles of around 100 nanometers in the oxidised matrix layer are enriched in chromium, Particles enriched in tita nium are also detected, which must corne from the tube at the as received state as noted in the context of this study, The thicknesses of the oxide layers are clearly greater (factor 100) than those ob served in this study, but remain consistent with the durations of exposure in primary medium (200 times longer),

43, Oxidation and release mechanism

From the previous characterisations and the release kinetics, an oxidation and release mechanism during a restart of reactor af ter steam generators replacements can be proposed on Fig, 25, At the end of the step at 170 °C, the relief of grain boundaries and steps is softened, The development of a layer mainly consisting of chromium oxide covers the initial relief by selective dissolution of nickel and iron, The solubility of metallic nickel, nickel oxide and iron oxide is greater at these temperatures than at 325 °(, The dif fusion phenomena are in the minority because the temperatures of first steps are too low to significantly activate diffusion, Therefore, the extreme surface is enriched in chromium and oxidizes by dif fusion of OH-hydroxyls anions present in the fluid, The kinetics of relaxation is then controlled by the kinetic limitation of solubil isation of corrosion products, The temperature of primary medium is not sufficient to dissolve the aluminium until this step, However, these Alumina inclusions are less deeply embedded than at the as received state and surrounded of chromium oxide, During the rise of temperature between 170 °C and 297 °C, the release strongly increases, The chromium oxide layer formed up to the 170 °C step dissolves, it is not enough protective against ox idation and release, The Alumina inclusions also dissolve in the medium and contribute to the release measured experimentally in the test loop, Sorne areas such as grain boundaries, disturbed ar eas and stair nosing's contribute indeed mainly to release, With increasing temperature, the diffusion phenomena of metal cations migrating from the material to the fluid are activated, The OH hydroxyls anions will also diffuse more quickly at high tempera ture, which leads to the oxidation of the surface, hence the thin layer of oxidised matrix, Due to the disturbed microstructure of the as-received state, diffusion can also be promoted by short cir cuits (grain boundaries, dislocations, gaps),

The release kinetics present a pseudo-stationary regime at the end of the test, this implies that diffusion is the phenomenon lim iting the release, In addition, a state of equilibrium is established between the limit diffusion layer and the oxide, explaining the rel atively weak and constant kinetics of release at the end of the test

The presence of needles and sticks composed of ox ides/hydroxides of nickel and chromium, at the end of the test may be due to the precipitation of nickel and chromium in solution during cooling on germs already present at high temper ature, This formation would be explained by a phenomenon of dissolution/precipitation during the cooling of the lluid due to a locally supersaturation, With the turbulent hydraulic regime in the installation, it is unlikely that these elements will be able to resist at these hydraulic conditions, This mode! couples the phenomena of oxidation and release, out of equilibrium, and it is based on industrial reality (materials and test conditions),

Conclusions

Sections of steam generator tubes were corroded in primary water conditions representative of restart conditions of a pressur ized reactor after steam generators replacement Kinetics of release kinetics of corrosion products were obtained using the BOREAL fa cility available at EDF R&D, These kinetic results associated to the characterisation of the inner surface of the tubes before and af ter the corrosion tests permit to propose an oxidation and release mechanism during a restart of reactor, This mode! is based on the following experimental observations:

The oxide layer at the as-received state is formed of a very thin layer (1 -2 nm) of oxidised matrix, without specific enrichment During the restart, the most critical step for the release phe nomenon is revealed from 170'C to 297'C Up to 170'C, a thin layer of amorphous chromium oxide is formed by selective dissolution of iron and nickel, When the temperature rises, this chromium oxide layer is not stable enough to be protective and the diffusion phenomena are activated, At 325'C, the oxide does not exhibit any particular en richment and corresponds to an oxidised metal layer, an equi librium is established and the rate of release reaches a pseudo stationary regime, For a better understanding of the behaviour of steam generator tubes in terms of oxidation and release during a reactor restart af ter one operating cycle or several operating cycles, a program of release tests and characterisations is being developed, This pro gram would be also include a study on the behaviour of steam generator tube during the reactor shutdown.

Figure 1 .

 1 Figure 1. SEM images of 690 alloy steam generator tubes (inner surface) in as-received conditions in the secondary electrons mode (a, c, e) and in the backscattered electrons mode (b, d, f) respectively

Figure 2 .Figure 3 .

 23 Figure 2. Backscattered SEM image and SEM EDX mapping (SkV) of alloy 690 steam generator tubes (inner surface) in as-received conditions

Figure 4 .Figure 6

 46 Figure 4. HAADF STEM image associated with STEM EDX mapping of 690 alloy steam generator tube (inner surface) in as-received conditions

Fig, 5 Figure 7 ) 14 - 14 -Figure 9 .

 5714149 Figure 7. a) STEM HAADF image of the inner extreme surface of the steam generator tube on alloy 690 at the initial state and d) EDX analysis (at. %) of identified areas

Figure 10 .Figure 11 .

 1011 Figure 10. SEM images of oxides formed on the inner surface of steam generator tube made of alloy 690 corroded during transients test simulating restart of reactor until 170 °( (image b obtained at higher magnification than image a)

Figure 12 .

 12 Figure 12. SEM images ( cross-section) of intergranular oxide penetrations in the material on the inner surface of steam generator tube made of alloy 690 corroded during transients test simulating restart of reactor until 325 °C

Figure 13 .Figure 14 .

 1314 Figure 13. a) STEM image of the inner extreme surface of the steam generator tube on alloy 690 after transients test until 170 °( and b) EDX analysis (at. %) of identified areas (red rectangles from 1 to 4)

Figure 15 .

 15 Figure 15. STEM bright field image and STEM EDX mappings of the oxide grown near an inclusion of Alumina on the inner extreme surface of steam generator tube on alloy 690 after transients test until 170 °C

Figure 16 .Figure 17 .

 1617 Figure 16. SfEM bright field image and SfEM EDX mappings of the mdde grown near a precipitate of titanium on the inner extreme surface of steam generator tube on alloy 690 after transients test until 170 °(

Figure 20 .

 20 Figure 20. STEM HMDF image of the inner extreme surface of the steam generator tube on alloy 690 after transients test until 325 °C superimposed to EDX composi tional profiles

Figure 21

 21 Figure 21. a) STEM bright field image of the inner extreme surface of the steam generator tube on alloy 690 after transients test until 325 °( and d) EDX analysis ( at. %) of identified areas

Figure 22 .

 22 Figure 22. ToF-SIMS profiles of a) the specific oxide anions 18 0-, Cr2-, Nb-, OH-and 02-b) AIO2-, no-, Cro-, FeoH-, Nia-, NiQH-, TiQ2-, Cro2-, Cr03H-for the inner extreme surface of steam generator tubes made of alloy 690 corroded during transient's test until 170 °C cipitates are still present The precipitates located deeper in the material have not changed,

Figure 23 .

 23 Figure 23. ToF-SIMS profiles of a) the specific oxide anions 18 0-, Cr2 -, Nb-, OH-and 02 -b) AI02 -, no-, Cro-, FeOH-, NiO-, NiOH-, Ti02 -, Cr02 -, Cr03H-for the inner extreme surface of steam generator tubes made of alloy 690 corroded during transient's test until 325 °C

Figure 24 .

 24 Figure 24. Schema of the mdde evolution and underlying material from the as-received state ta the end of a simulate restart of reactor after steam generators replacement on inner surface of a steam generator tube on alloy 690

Figure 25 .

 25 Figure 25. Schema of the oxidation and release mechanism on the inner surface of steam generator tube on alloy 690 during a restart of reactor after steam generators replacement

Table 3 Synthesis of oxides/hydroxides thermodynamically stable during transients of a PWR restart for SG tubes in Ni-base alloys.

 3

	Thermochemical data of release tests
	Temperature ( °C)	25	50	80	130	170	297	306	325
	02 / H2 presence	8 ppm	o, 10 ppb 02		5 ppb	5 cm 3 /kg	H,	35 cm 3 /kg
	Eh (V/ENH)	+ 0,942				+0,614	-0,685		-0,834
	B/Li (ppm)	2500/0				2500/0	2100/2.2		1200/2.2
	pH (l"C)	4,6			4,7		6,6 -6,7		-7,3
		Thermodynamically stable oxides/hydroxides
		Temperature ( °C)		25	100	200	300
		Eh (V/ENH)		+0,942	+0,761	+0,551	-0,691 ➔-0, 726
		pH(T 0 C) Oxides/Hydroxides	4.6 Fe203	4.6 Fe203	4.7 Fe203	6.7 FeCr204
				HCr04-Ni 2 +	HCr04-Ni 2 +	HCr04-	FeCr204

Acknowledgments

Authors thank Valinox Nucléaire for supplying the SG tube with which the release tests were carried out and the CEMHTI labora tory of Orléans, France who performed the implantations of cobalt on SG tubes for the gammametric detection during release tests in the BOREAL loop, Authors thank the team "Etude-Corrosion" of EDF R&D who per formed the release tests in the BOREAL loop and thank as well the CIRIMAT, UMS Castaing at Toulouse, France, for the TEM character isations and Science & Surface at Ecully, France, for ToF-SIMS anal yses,

Data availability

The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study,

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Declaration of Competing Interest

Ali the authors have approved the current manuscript and con firm that it bas not been published elsewhere and that it is not under consideration for publication in any other journal,

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