THE EFFECT OF POST-IRRADIATION ANNEALING ON VVER-440 RPV MATERIALS MECHANOLOCAL PROPERTIES AND NANO-STRUCTURE UNDER RE-IRRADIATION

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1 Proceedings of the ASME 2009 Pressure Vessels and S2009 Piping July Division 19-23, 2009, Conference San Francisco, PVP2009 July 26-30, 2009, Prague, Czech Republic PVP THE EFFECT OF POST-IRRADIATION ANNEALING ON VVER-440 RPV MATERIALS MECHANOLOCAL PROPERTIES AND NANO-STRUCTURE UNDER RE-IRRADIATION S. Rogozkin SSC RF ITEP A.Chernobaeva RRC urchatov Institute A. Aleev SSC RF ITEP A. Nikitin SSC RF ITEP A. Zaluzhnyi SSC RF ITEP D. Erak RRC urchatov Institute Ya. Shtrombakh RRC urchatov Institute O. Zabusov RRC urchatov Institute L. Debarberis JRC Petten, The Netherlands A. Zeman IAEA Vienna, Austria ABSTRACT The present work provides the analyses of embrittlement behavior and atom probe tomography study of nano-structure evolution of VVER-440 RPV materials under irradiation and re-irradiation. Specimens from VVER-440 weld with high level of cupper (0.16 wt.%) and phosphorus ( wt.%) were irradiated in surveillance channels of Rovno Nuclear Power plant unit 1 (Ro-1). The embrittlement behavior has been assessed by transition temperature shift. INTRODUCTION Several VVER-440 in Russia and Europe units are under operation after annealing of reactor pressure vessels (RPVs). Safety operation of these units requires reliable forecast of mechanical properties behavior under re-irradiation for the weld 4, located in front of the core. That is the element of construction, which limits radiation lifetime of the whole RPV. At the present prediction of radiation embrittlement of RPV steels under re-irradiation is carried out using «lateral shift» scheme. Estimation of mechanical properties behavior under re-irradiation using «lateral shift» is characterized by over conservatism in some cases [1 4]. ]. So, the development of new re-embrittlement models is actual problem. This is the aim of international project "Primavera". "Primavera" is an international project directed on development of re-irradiation embrittlement model. RRC «Кurchatov Institute» (Russia), JRC (The Netherlands), Fortum, VTT (Finland) and ISM (Bulgaria) participate in this project. The mechanical properties study in unirradiated, irradiated, and re-irradiated states were earlier reported [5, 6]. The present work provides the analyses of results of mechanical properties and tomographic atom probe (TAP) study of nano-structure evolution of VVER-440 RPV materials under irradiation and re-irradiation. TAP study was conducted in Institute for Theoretical and Experimental Physics (SSC RF ITEP, Russia). Special attention was paid to the influence of copper and phosphorous on radiation embrittlement during irradiation and re-irradiation. Two weld materials with the same copper but different phosphorous contents (0.027 and wt.%) were chosen to be studied. That allowed us to estimate the effect of phosphorous both under irradiation and re-irradiation. In accordance with earlier published data [7 10] the contents of copper in the solid solution before irradiation and before reirradiation are considerably different in welds with high level of copper (0.16 wt.%). The comparison of irradiated and reirradiated states allows to estimate the effect of copper on radiation embrittlement. 1 Copyright 2009 by ASME

2 MATERIALS The weld 501, produced in accordance with the standard technology for the first generation of VVER-440 by Izhora plant, was used in this study. It had been studied in several projects [5, 6, 11]. The average content of phosphorous was wt.%. The range of phosphorus contents in this weld is wt.%. The gradient of phosphorus content in radial (relative of RPV) direction is typical for the weld with high level of phosphorus. Such results were obtained in several studies: trepans from Novovoronezh NPP unit 2 RPV, surveillance specimens Ro-1 and trepans from Greifswald NPP-1 RPV [12]. The chemical composition of weld 501 was studied in several laboratories, and the distribution of phosphorous in the layers of the weld was published in [5, 6, 11, 13]. The materials studied in this work are the different layers of weld 501. Their chemical compositions differ only in phosphorous contents. All the other element contents were the same within the measurement accuracy [5]. The chemical compositions of materials studied are shown in table 1. TABLE 1. BUL COMPOSITIONS OF WELD MATERIALS WITH LOW (LP) AND HIGH (HP) PHOSPHOROUS CONTENTS С Si Mn P S Cr Ni Mo Cu V LP(wt%) HP(wt%) LP(at%) HP(at%) EXPERIMENT At the first stage, all the samples were irradiated in surveillance channels of Rovno NPP unit 2 (Ro-2) up to a fluence of см -2 (Е>0.5 МэV) 1,2. After irradiation, they were annealed in accordance with standard regime for VVER-440/230 RPVs (475 о С, 100 hours). Then they were reirradiated. Some of specimens were irradiated in surveillance channels of Ro-2. At the same time the other samples were irradiated in surveillance channels of in Ro-1. The fluence rate in surveillance channels of Ro-2 and Ro-1 was and cm -2 с -1, respectively. The irradiation program is shown in the Fig. 1. Mechanical properties of the materials in states 1 8 (Fig. 1) were studied. The change of the properties was assessed by ductile-to-brittle transition temperature shift ( ΔT ): Δ T = T T0, (1) 1 The fluence is given for neutrons with Е>0.5 МэV 2 Some extra samples were irradiated in channels for surveillance specimens in Ro-1 to get point 2 in Fig.1 for intermediate fluence. FIG. 1. THE EXPERIMENT DESIGN ON THE DEPENDENCE OF DUCTILE-TO-BRITTLE TRANSITION TEMPERATURE SHIFT VERSUS NEUTRON FLUENCE where T is the transition temperature after irradiation, after irradiation and annealing, and after re-irradiation. T 0 is the transition temperature in unirradiated state. Values of T and T 0 were determined by testing of standard Charpy specimens using the technique described in Russian normative RD EO [14]. The yield stress was determined using fivefold 3 mm smooth tensile specimens. The fine structure (composition of matrix, the presence of nanoclusters or precipitates and their composition) of the weld materials was studied using TAP. In this paper TAP-investigations of states 1, 3, 4 and 6 (blue dashed circles in Fig.1) of the weld with high phosphorous content are presented. TABLE 2. THE LIST OF STATES UNDER INVESTIGATION STATE (FLUENCE, СМ -2 ) NUMBER OF AP SPECIMENS WT% * CHARPY TENSIL E 1 UNIRRADIATED IRR**, R-1 (12.2) IRR, R-2 (59.5) IRR, R-2(57.1)+AN IRR R-2+AN+IRR, R-1 (11.6) IRR R-2+AN+IRR, R-1 (25.0) IRR R-2+AN+IRR, R-2 (59.5) UNIRRADIATED IRR, R-1 (12.2) IRR, R -2 (59.5) IRR, R-2 (57.1)+AN IRR, R-2+AN+IRR R-1 (12.7) IRR, R-2+AN+IRR R-1 (25.8) IRR, R-2+AN+ IRR R-2 (52.0) IRR, R-2+AN+IRR R-2 (61.9) 10 2 * state number, Fig. 1 ** irr irradiation, an-annealing, R-1 Rovno-1, R-2 Rovno-2 LP (Р ) НР (Р ) 2 Copyright 2009 by ASME

3 A A B B FIG. 2. THE VALUES OF ΔT IN IRRADIATED, ANNEALED AND RE-IRRADIATED WELDS VERSUS FLUENCE (A LP, B HP) The results of the study of these materials in unirradiated state are presented in [5]. First results on mechanical properties behaviour under irradiation and subsequent re-irradiation after recovering annealing were published in [6]. This paper presents new test data and APT results. MECHANICAL PROPERTIES The values of ΔT for all the states studied are shown in the Fig. 2. The absolute values of under re-irradiation are not higher then T under primary irradiation for comparable fluences. Thus the annealing allows to extend the radiation dose more then in two times to exceed a fix value of T. It is easy to see in Fig.2 that the rate of T rise ( dt df ) for the weld with phosphorous content wt.% is different under primary irradiation and re-irradiation. The value of ( dt df ) for high phosphorus weld (0.038 wt.%) it is practically the same under primary irradiation and reirradiation. To compare ΔT values under primary irradiation and reirradiation, they were plotted at the same coordinates in T FIG. 3. COMPARISON OF Δ FOR WELDS LP (А) AND НР (B) UNDER PRIMARY IRRADIATION AND RE- IRRADIATION following way: Δ T values for primary irradiation were determined according to (1); Δ T for re-irradiation were determined according to Δ T = T ( re-irradiated ) T ( annealed ). (2) Power function was used to describe embrittlement under primary irradiation and re-irradiation: C (2) Δ T = C(1) F. (3) Parameters С(1) and С(2) were estimated by non-linear least-squares method. The results obtained are shown in Fig. 3 and in table 3. The analysis of the coefficients presented in the table 3 shows that the fluence exponent is different for primary irradiation and re-irradiation. This is especially evident in the material with low phosphorous content. This seems to indicate the change in the embrittlement mechanism under reirradiation. The higher phosphorous content, the smaller difference in embrittlement under primary and re-irradiation at comparable fluences. T 3 Copyright 2009 by ASME

4 TABLE 3. THE EQUATIONS FOR CURVES Δ T = C(1) F C (2) Material State C (2 Δ T = C(1) F LP Primary irradiation 0.28 Δ T = 26.6 F HP Re-irradiation 0.75 Δ T = 0.94 F Primary irradiation Δ T = 51.0 F 0.24 Re-irradiation 0.34 Δ T = 26.6 F For the weld with phosphorous content wt.% irradiated up to fluence of cm -2, the difference in ΔT values under primary and re-irradiation is 58 о С. Higher phosphorous content wt.% decreases it up to 26 о С. Thus, annealing is the most effective for welds with the lowest phosphorous content (in this case for the weld with phosphorous content wt.%), because it not only decreases the absolute value T, but also slows down ( dt df ) under re-irradiation. Annealing of the weld with higher phosphorous content only decreases the T but does not affect significantly its following rise under re-irradiation. To illustrate the relative effect of phosphorous content on ) the transition temperature shift rise, Fig.4 shows ΔT values for comparable fluences. As it is clear from Fig. 4, the effect of the phosphorous content increase is stronger, when fluence is higher. When fluences are comparable, the effect of the phosphorous content is stronger under re-irradiation. Data published in [7 10] show the copper content depletion due to irradiation. Annealing of irradiated materials at temperature 475 o C does not lead to increase of copper content. It means that copper content in the solid solution of RPV materials in state irradiation + annealing (before reirradiation) is considerably lower, then in unirradiated state. Comparison the same weld in irradiation and re-irradiation state allows separating the effect of copper on irradiation embrittlement. To show the relative effect of copper on embrittlement, Δ T values for each material after primary and re-irradiation of comparable fluences are presented in Fig. 5. There is no clear dependence of copper effect on transition temperature shift on fluence as for phosphorus. It means that transition temperature rise deepens more on phosphorus content then on copper content. The data in the Fig.5 shows that depends on fluence of FIG. 5. DIFFERENCE IN ΔT UNDER PRIMARY IRRADIATION AND RE-IRRADIATION effect of copper is not so evident then for phosphorus. At the same copper content cupper effect on ΔT is lower when the phosphorus content is higher. FIG. 4. DIFFERENCE IN ΔT VALUES: Δ T = ΔT ( HP) Δ T ( LP) TOMOGRAPHIC ATOM PROBE (TAP) STUDY In this paper the first results on TAP investigations of weld material with high phosphorous (HP) content are presented. States 1, 3, 4 and 6 (see Fig 1, table 2) were characterized by Energy Compensated Optical Tomographic Atom Probe (ECOTAP) installed at ITEP. Atom probe investigations were performed at high vacuum < Torr. Specimen temperature was varied from 40 up to 80 ; a pulse repetition rate had the maximum value of 1.6 khz. Pulse fraction was within 15-23% and was changed correspondingly to the specimen temperature. Mass resolution was measured as ratio of the mass of the peak to its full width at half maximum intensity FWHM ( M ΔΜ 50% ) and was greater than 700 and for some peaks reached Dependence of pulse evaporation efficiency on pulse fraction and specimen temperature for Cr and Cu was observed, other elements did not show any 4 Copyright 2009 by ASME

5 dependence on these parameters within used ranges. Optimal parameters were determined as 45 and 19% pulse fraction. The entire procedure for a needle-shaped specimen preparation for TAP investigations consisted of two main stages C FIG. 6. ATOM MAPS FOR IRRADIATED STATE: A) DATA SET CONTAINING P-CLUSTERS AND CARBIDES, B) DATA SET CONTAINING Cu-CLUSTERS, C) DATA SET CONTAINING DISC CARBIDE A including standard electropolishing technique. The first stage was billets preparation from a bulk material using electroerosion cutting in water. This technique does not induce strains; therefore specimen s structure remains unaffected, as opposed to mechanical cutting. In this stage billets were produced with sizes approximately mm 3. The billets were made thinner using the method of anodic electropolishing (anodic etching) in electrolyte. The diameters of specimen tip varied from 50 nm to 100 nm. The prepared specimens were tested by transmission electron microscopy (TEM). Although the result of TAP study is the three-dimensional distribution of chemical elements only some of solutes that play the definite role in forming or detecting peculiarities are shown in figures. However, atom maps were used only for preliminary detection of peculiarities while their parameters (size, element concentration, element distribution) were calculated by means of maximum separation method [15, 16]. This method allows to distinguish ultrafine precipitates, clusters from the solutes in the matrix and to estimate their sizes, composition and number density. It is based on the assumption that the distance between solute atoms in an enriched precipitate is significantly smaller than that in the surrounding matrix. Therefore, the solute atoms that belong to a solute-enriched precipitate may be distinguished from those in the matrix based on a maximum separation distance, r max. In order to leave out smaller random agglomerations it is used a minimum size cutoff limit n min. In this work characterization of nanoscale clusters was carried out with the values of 0.6 nm for r max and 4 for n min. The size of the solute-enriched peculiarities was estimated in terms of the Guinier radius r g, which was determined from positions of the solute atoms in each precipitate [15]. Unirradiated state. Due to high brittleness of the material in this state during atom probe characterization only three data B 5 Copyright 2009 by ASME

6 sets with more than events have been obtained. Data processing has revealed inhomogeneous phosphorus distribution in the investigated volumes. A few areas were found to be highly enriched in P. Phosphorus concentration in these volumes excesses average over the bulk material more than by 5 times. At the same time there were areas where size of 2-4 nm) are presented. Considerable enrichment in Si, Mn and partly in Cu has been detected. Estimated number density of these clusters in this volume is ~10 18 cm -3. Composition of each cluster is shown in Fig. 7. P-clusters 1 and 2 (Fig. 7) are situated in vicinity of carbide clusters (Fig. 6a) and therefore enriched in vanadium and carbon. It should be noted that average P concentration in this data set including all clusters is at% that is slightly higher than that for the bulk value (0.068 at%), which indicates that initially this area was enriched in P. This inhomogeneity in P distribution was similar to what we had come across in the unirradiated state. Probably the area initially had been oversaturated with P which led to formation of clusters mainly enriched in phosphorus. Carbide clusters. In Fig. 6a, 6c several carbide clusters can be seen. It should be taken into account that in Fig. 6c the observed satellite carbide cluster does not resemble the ones detected in other volumes; probably, it is influence of the nearby disc carbide. Carbide clusters composition is shown in the Table 4. The estimated size of these clusters is ~4-5 nm and their number density for this volume is about cm -3. TABLE 4. ELEMENT DISTRIBUTION IN CARBIDE CLUSTERS AND DIS CARBIDES FIG. 7. HISTOGRAM OF ELEMENT DISTRIBUTION IN P-CLUSTERS phosphorous presence was less than in the bulk material. However, phosphorus enriched areas could not be considered as a cluster or a precipitate due to their sizes (~ 50nm) and nearly homogeneous distribution of solute elements. It is also should be noted that concentration of V and C changes from one TAP data set to another as these elements form a variety of carbide particles. Such like inhomogeneities were also detected in the other states. Irradiated state. High phosphorous weld irradiated up to cm -2 (state # 3, see table 2) was characterized by atom probe and three data sets with more than events were obtained. Data processing revealed that under irradiation matrix depletion in some chemical elements took place due to radiation-induced segregation on various imperfects in the initial structure as well as precipitate formation. Four types of nanoscale structure components have been found: а) fine clusters mainly enriched in phosphorus atoms (P-clusters); b) nanoclusters enriched in copper atoms (Cu-clusters); c) small V-C enriched clusters (carbide clusters); d) V-C disc carbides. Concentration of solutes in all types of clusters was considerably smaller than that of iron. Only in the core of disc carbides Fe-atoms were not presented. 3D reconstructions of the investigated volume containing phosphorus and carbide clusters are presented in Fig.6. Each element is displayed separately taking into account that the volumes shown for each data set are the same. P-clusters. In Fig. 6a the volume that includes P-clusters (areas that are highly enriched in phosphorus and with typical at% Cr V Mn C Si P Cu Ni Mo N Carbide cluster 2,6 8,0 1,2 2,3 1,2 0,6 0,2 0,1 1,2 0,6 2,4 7,5 1,4 2,0 2,1 0,1 0,2 0,1 0,8 1,0 Disk carbide 6,0 42,8 0,4 19,6 3,5 0,3 0,5 0,0 5,4 3,6 Disk carbide (re-irradiated) 5,6 30,4 0,98 19,3 3,0 0,7 1,1 0,15 4,8 1,6 Cu-clusters. In Fig. 6b the volume containing Cu enriched clusters is shown. These clusters are 2-3 nm in diameter with the number density near cm -3. These clusters are also enriched in P, Si or Mn. Element concentrations are shown in Table 6. In this volume P concentration is slightly lower than in TABLE 5. AVERAGE ELEMENT CONCENTRATIONS IN INVESTIGATED VOLUMES CONTAINING Cu-CLUSTERS at% Cr V Mn Si P Cu Ni Mo 1,8 0,0 1,2 0,7 0,0 Averaged* ,8 0,0 1,2 0,7 0,0 Matrix** * average element concentration in the volume (Fig. 6b) ** element concentration without Cu-clusters the bulk (see tables 1 and 5), which justify low enrichment of clusters by P. Therefore, no phosphorous enriched clusters were formed in this state. It should be noted that copper enriched clusters are smaller and denser than P-clusters observed in the other volume (Fig, 6a). 0,1 0 0,0 6 0,1 1 0,1 1 0,2 3 0,2 3 6 Copyright 2009 by ASME

7 Tomographic atom probe results on element concentrations in Cu-enriched precipitates are shown in table 6. A significant variation in sizes and copper content can be seen. Average copper concentration in such clusters defined by maximum separation method varies from 6% to 12%. Detailed TABLE 6. CLUSTER COMPOSITION IN IRRADIATED AND at% IRRADIATED+ANNEALED+ RE-IRRADIATED STATES Irradiated irr. + ann.+ + re-irr. Mn 2,2 4,4 2,1 2,7 0,0 0,0 0,7 4,5 2,2 1,6 1,0 4,2 Si 2,2 0,7 0,7 1,4 5,1 0,0 3,5 4,5 2,7 1,6 1,5 0,0 P 0,0 0,4 0,7 1,4 0,0 0,0 1,4 0,0 0,3 1,6 3,4 1,4 Cu 8,7 9,6 12,3 12,3 12,8 12,5 6,9 11,4 15,0 9,7 6,6 7,3 r g, Å 4,4 8,9 7,3 7,6 5,2 4,6 9,4 5,3 12,5 7,6 9,0 6,3 A investigation had showed that some Cu-clusters consisted of a dense core and a nearby area considerably enriched in copper Fig 8a. However, for all other Cu-clusters there is no core to be distinguished (Fig 8b). Presence of enriched Cu center resulted in higher Cu concentration values calculated for these objects 10-15%. Virtually Cu-cluster can be divided into two parts: the first part is the core which can be described as area with high Cu level, in our case up to 40% and the second part a cloud-like area around the core with Cu concentration about 3-5%. On Fig. 8 radial concentrations for both types are plotted as a function of distance from the mass center of the Cu-cluster, whereas Cuconcentration is calculated in a spherical volume 3 Å thick. Disc carbide. In the third data set (Fig. 6c) disc carbide and the satellite carbide cluster can be seen. Element concentrations of these peculiarities were calculated. The disc carbide is not only much denser than P-Cu clusters and even carbide clusters, but its core is totally depleted in iron (Fig. 9). However, the shape of disc carbide and its satellite is similar both are flat (elongated in 2 dimensions). As the disc carbide bigger than the investigated volume it is impossible to measure exact size. However, its thickness could be estimated and it varies 2-3 nm due to its tapered shape. Considerable presence of Mo and Cr in this cluster justifies low concentrations of these elements in the matrix. However, Mo is segregated on the interface between carbide core and surrounding Fe-matrix (Fig. 9), and Cr enriches the core of carbide. B FIG. 9. ATOM MAPS OF THE DIS CARBIDE DETECTED IN IRADIATED STATE (FIG. 6C) Phosphorus concentration in the volume (0.103 at.%) is several times higher than the value in the bulk material (see table 1). Areas with higher density of P atoms can be seen, however, they can not be described as P clusters. These areas can be an initial stage in cluster formation or atmospheres near C-carbides. FIG. 8. RADIAL CONCENTRATION DISTRIBUTION* OF TWO DIFFERENT COPPER ENRICHED PRECIPITATES IN THE WELD AFTER IRRADIATION TO A FLUENCE cm -2. A DENSE PARTICLE WITH CORE, B PARTICLE WITHOUT DEFINITE CORE * spherical volumes of 3 nm thickness were used to calculate concentrations Irradiated and Annealed. While behaviour of material in this state (state #4, table 2) is similar to the unirradiated one, which resulted in high breakage rate during TAP characterization, only two data sets with more than events were obtained. Moreover, in the processed data no clusters were found. Probably, during annealing P-Cu enriched clusters induced by an initial irradiation dissolve, however the behavior of elements is different. In the previous work it was shown that annealing of weld leads both to dissolving of small 7 Copyright 2009 by ASME

8 TABLE 7. ELEMENT CONCENTRATION OF INVESTIGATED VOLUME (FIG. 9A) (RE-IRRADIATED STATE #6) at% Cr V Mn C Si P Cu Ni Mo N Average* 1,60 0,60 0,75 0,37 0,633 0,16 0,071 0,082 0,28 0,055 Matrix** 1,52 0,11 0,72 0,09 0,59 0,11 0,037 0,080 0,21 0,026 * average element concentration in the volume ** element concentration without Cu- and carbide clusters radiation induced clusters and to formation of rear, relatively large Cu precipitates mainly at dislocations and other structural features [9, 10]. The number density of such peculiarities was lower than for radiation induced by initial irradiation. Re-irradiated state. Re-irradiated state #6 (see table 2) with high P concentration were investigated. For both temperatures, volumes with high and low phosphorus concentration were revealed, which agrees with the results on highly inhomogeneous P distribution in the weld material described above. In the state #6, that was irradiated up to cm -2 (see table 2), a disc carbide as well as Cu clusters were found. Atom maps of volumes are presented in Fig. 10. In the first data set (Fig. 10a) enormously high V, C and P concentrations were detected (see table 7). The disc carbide Fig. 11 is similar to one we have observed in the irradiated state see Fig. 9. Its thickness varies from 1.0 to 3.5 nm, but its lateral size is impossible to determine as carbide spreads beyond investigated volume. The core of the disc carbide is formed mainly by vanadium, carbon and chromium atoms. On the other hand, phosphorus, copper and iron is not presented in the carbide center. interface, however, as the carbide is not as dense as in irradiated state, its core is too thin. FIG. 11. ATOM MAPS OF DIS CARBIDE IN RE-IRRADIATED STATE #6 The observed Cu clusters are similar to those in the irradiated state see Fig. 6b. The element concentrations are shown in tab. 6 and 7. The typical cluster size is 2-3 nm with number density around cm -2. Cu atoms are closer to the center of the cluster and the nearby area is considerably depleted in phosphorus, its concentration in matrix is 0.11 at% which is still higher than in the bulk at% (table 1). In the other TAP data set (Fig. 10b) only traces of P and Cu were found, hence no structural features were seen. A B FIG. 10. ATOM MAPS OF RE-IRRADIATED MATERIAL CONTAINING DISC CARBIDE AND P-Cu CLUSTERS (STATE #7) Moreover, P and Cu atoms were found around it and formed something like an atmosphere. Mo atoms also segregate to the DISCUSSION OF TAP RESULTS VVER-440 welds have been already investigated by means of tomographic atom probe [17, 18]. In those studies samples from the weld of reactor pressure vessel were trepanned while the reactor was stopped for recovering annealing after nearly 20 years operation. Neutron flux was considerably less than in present work and its average value was cm -2 s -1. Atom probe investigations had revealed high number density of small Cu-enriched clusters with sizes of ~2 nm in diameter. Cu concentration was estimated to be 20% for clusters in matrix and 35% for ones located on dislocations [19]. Detected in this work clusters differ from ones that were found in trepans [17-19]. In this research in the investigated material two types of Cu-enriched clusters were observed, namely, one containing a small dense center (with diameter ~ 1.5 nm and Cu concentration 20%) and a cloud of copper atoms around it (Cu concentration is about 3-5%), the other being a conglomeration of copper atoms without compact center. This difference can be result of different dose rates. Due to shorter irradiation period compactization process of nucleated clusters 8 Copyright 2009 by ASME

9 was not completed. And indeed, during 20 years of reactor operation Cu-cluster compactization and growth due to depletion of nearby regions might be quite probable. Such difference in formed clusters will definitely have influence on material properties after subsequent annealing and reirradiation stages. It is known that during annealing Cu clusters dissolve forming larger precipitates [19], however, it is hardly expected that dissolution of such radically different clusters will run in similar ways during annealing. That in turn results in different properties of annealed material as well as mechanical. As a result, secondary nucleation of Cu-clusters took place under re-irradiation after annealing. Formation of similar clusters under re-irradiation was also observed in A533B steel [19]. It is necessary to note that the composition of Cu enriched clusters depends on both dose rate and mainly composition of irradiated material. This effect was shown on model Fe-Cu alloys [20]. High phosphorous concentration in the investigated material (0.038 wt%) led not only to copper enriched cluster formation but also to segregation of phosphorous into relatively small clusters. Such behaviour had not been observed in specimens cut from the templets, where P content was less (0.029 wt%) [9]. CONCLUSION The study two materials with the same level of copper and different level of phosphorus in different states (irradiated, annealed and re-irradiated) have been done. Both materials are different layers from weld 501, produced in accordance with the standard technology for the first generation of VVER ) Transition temperature absolute values are lower after reirradiation than after primary irradiation for comparable neutron dose. 2) Annealing allows to extent the neutron dose more then in two times to receive some fixed transition temperature value. 3) Strengthening and embrittlement rates are lower under reirradiation. The lower is the phosphorous content, the stronger is this effect. 4) Under primary irradiation of the HP weld formation of Cu enriched clusters (with size near 2 nm, number density ~ сm -3 ), P enriched clusters (number density ~ сm -3 ) as well as disk carbide and carbide clusters takes place. 5) New nucleation of Cu-enriched clusters was observed under re-irradiation. ACNOWLEDGMENTS The authors acknowledge the support of the international project "Primavera" (European commission), funding this research under the contracts IE.B and IE.B Research at the ITEP was also sponsored by State Corporation Rosatom. REFERENCES 1 Yu. Nikolaev, A. ryukov, Using the floating curve model for assessment of WWER-440 steel embrittlement due to re-irradiation., Proceeding of IAEA specialist meeting on irradiation embrittlement and mitigation, 1997, Vladimir, Russia. 2 Ya. Shtrombakh, Examination of WWER-440 RPV steel re-irradiation behavior using materials from operating units., Proceeding of IAEA specialist meeting on irradiation embrittlement and mitigation, 1999, Madrid, Spain. 3 P. Platonov, Ya. Shtrombah, A. ryukov, B. Gurovich, Yu. Кorolev and J. Shmidtk, Results on research of templates from ozloduy-1 reactor pressure vessel. Nuclear Engineering and Design 191 (1999) M. Valo, Ya. Shtrombakh, A. ryukov, S. Vodenicharov, J. ohopaa, Tentative re-irradiation behavior of WWER- 440 welds after annealing, Specialists meeting on irradiation embrittlement and mitigation proceeding, U, A. Chernobaeva, J. Shtrombah, A. rjukov, D. Erak, P. Platonov, J. Nikolaev, E. rasikov, L. Debarberis, Yu. ohopaa, M. Valo, S. Vodenicharov and T. amenove, Material characterization and selection for the international research project PRIMAVERA, International Journal of Pressure Vessels and Piping, Volume 84, Issue 3, March 2007, Pages A. Chernobaeva, D. Erak, P. 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Zabusov., Atomic scale observation of the microstructure of a VVER 440 Steel to understand properties of irradiated, annealed or re-irradiated materials., Proceeding of IAEA specialist meeting on irradiation embrittlement and mitigation, 2004, ristal Goos, Russia. 10 P. Pareige B. Radiguet, A. Suvorov, M. ozodaev, E. rasikov, O. Zabusov, J.P. Massoud «Threedimensional atom probe study of irradiated, annealed 9 Copyright 2009 by ASME

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