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Results on research of templates from Kozloduy-1 reactor pressure vessel
Nuclear Engineering and Design 191 (1999) 313 – 325 www.elsevier.com/locate/nucengdes
Results on research of templates from Kozloduy-1 reactor pressure vessel P. Platonov a,*, Ja. Shtrombakh a, A. Kryukov a, B. Gurovich a, Ju. Korolev a, J. Shmidt b a
Reactor Technologies and Materials Institute, Russian Research Centre, Kuchato6 Square, 123182 Moscow, Russian Federation b SIEMENS AG KWU, W-8520 Erlangen, Germany Received 27 January 1998; received in revised form 1 April 1999; accepted 29 April 1999
Abstract The studies on the specimens manufactured from the templates cut out from the weld 4 of Kozloduy NPP Unit 1 reactor vessel have been conducted. The data on chemical composition of the weld metal have been obtained. Neutron fluence, mechanical properties, ductile to brittle transition temperature (DBTT) using mini Charpy samples have been determined. The phosphorus and copper content averaged over all templates is 0.046 and 0.1 wt.%, respectively. The fluence amounted up to 5 × 1018 n cm − 2 within 15 – 18 fuel cycles, and about 5 × 1019 n cm − 2 for the whole period of operation. These values agree well with calculated data. DBTT was determined after irradiation (Tk) to evaluate the vessel metal state at the present moment, then after heat treatment at the temperature of 475°C to simulate the vessel metal state after thermal annealing (Tan), and after heat treatment at 560°C to simulate the metal state in the initial state (Tk0). As a result of the tests the following values were obtained: Tk, +91.5°C; Tan, + 63°C; and Tk0, 54°C. The values of Tk and Tan obtained by measurements were found to be considerably lower than those predicted in accordance with the conservative method accepted in Russia (177°C for Tk and 100°C for Tan). Thus, the obtained results allowed to make a conclusion that it is not necessary to anneal Kozloduy NPP Unit 1 reactor vessel for the second time. The fractographic and electron-microscopic research allowed to draw some conclusions on the embrittlement mechanism. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Kozloduy unit 1 reactor vessel; Embrittlement mechanism; Chemical composition
1. Introduction Kozloduy NPP Unit 1 was put into operation in 1974. The pressure vessel of this reactor belonged to the first generation of non-plated * Corresponding author. Tel.: +7-095-196-9941; fax: + 7095-196-1701.
WWER-440/230 and had an increased content of phosphorus. Calculations indicated that the life time of the vessel must have been exhausted quite fast. In 1989 as a preventive mitigation measure the annealing of the vessel in the zone near weld 4 was performed by technology developed in Russia. As a result the vessel properties were recov-
0029-5493/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S 0 0 2 9 - 5 4 9 3 ( 9 9 ) 0 0 1 1 9 - 3
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Table 1 The data of analysis of chemical composition carried out for the templates cut out from weld N4, NPP ‘Kozloduy-1’ (wt.%) Template number
C
Si
Mn
S
P
Cr
Ni
Mo
V
Cu
1 1 2 2 3 3 4 4 5 5 6 6 6 6
0.048 0.052 0.055 0.050 0.046 0.042 0.050 0.042 0.050 0.047 0.048 0.052 0.047 0.045
0.55 0.49 0.50 0.49 0.37 0.35 0.55 0.44 0.56 0.52 0.42 0.58 0.55 0.52
1.19 1.08 1.17 1.09 1.21 1.11 1.20 1.11 1.25 1.08 1.25 1.16 1.21 1.17
0.012 0.013 0.014 0.013 0.013 0.015 0.011 0.012 0.013 0.011 0.013 0.014 0.013 0.013
0.047 0.042 0.048 0.044 0.049 0.046 0.048 0.044 0.051 0.042 0.050 0.048 0.048 0.045
1.56 1.71 1.59 1.70 1.64 1.60 1.65 1.64 1.66 1.73 1.66 1.73 1.59 1.56
0.29 0.30 0.29 0.26 0.22 0.12 0.29 0.17 0.33 0.30 0.25 0.32 0.31 0.30
0.41 0.43 0.41 0.42 0.42 0.40 0.42 0.40 0.43 0.42 0.42 0.43 0.41 0.39
0.09 0.14 0.12 0.12 0.10 0.06 0.14 0.07 0.14 0.14 0.09 0.14 0.13 0.13
0.10 0.10 0.10 0.10 0.10 0.10 0.11 0.10 0.11 0.10 0.11 0.11 0.10 0.10
(internal) (external) (internal) (external) (internal) (external) (internal) (external) (internal) (external) (internal) (external) (internal) (external)
ered. However, because of high pollution by phosphorus by 1996 the necessity of reannealing had arisen. This was connected with the fact that the calculations indicated that the RPV material could have reached its critical condition already by 1996. The high phosphorus content was verified by direct measurements as well as by calculations. However the methods of calculation of DBTT under irradiation after the RPV annealing accepted at that time were too conservative. Therefore, it could be proposed that the real DBTT shift was less than it was expected due to calculations. This assumption was confirmed by the results of the earlier tests of the templates cut out from NVNPP-3 and 4 RPV metal and Kozloduy NPP Unit 2 metal, which had demonstrated that the real DBTT shift had been considerably lower than that obtained by calculations. In connection with this it was proposed that the templates should be cut out from the weld 4 on the inside of the vessel to determine the real properties of the metal. In September 1996 the templates were cut out and delivered to the Reactor Technologies and Materials Institute of the RRC ‘Kurchatov Institute’. In line with the program the following points of research were performed: 1. Determination of the chemical composition of the template metal.
2. Determination of the fast neutron fluence, which exerted an effect of fluence on the vessel metal under reirradiation. 3. Manufacture of the specimens from the templates and preparation of them for mechanical tests in the states after irradiation, after annealing at 475°C for 150 h and after annealing at 560°C for 2 h. 4. Hardness measurements and indentation diagram to evaluate the yield stress of the metal. 5. Macro- and microstructural studies and fractographic investigations. 6. Measurement of DBTT of irradiated specimens to determine Tk for the present state of the vessel. Measurement of residual DBTT after annealing at 475°C to simulate the vessel state after annealing. Measurement of residual DBTT after annealing at 560°C with the aim of Tk0 recovery in accordance with the methods accepted in Russia.
2. Determination of the chemical composition of the template metal The chemical composition was determined on the inside and the outside of the templates by the optical emission spectrometer ‘BAIRD SPECTROVAC FSQ’. The results, as an average of the values obtained by several measurements, are
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formed in the Metal Science Institute of the Bulgarian Academy of Science (to be published in this issue), making it possible to take these values for calculation of the embrittlement kinetics.
3. Estimation of the fast neutron fluence
Fig. 1. The scheme of neutron dosimeter probe cutting. Table 2 Comparison between calculated and experimental neutron fluences for weld N4 in 17th and 18th campaigns (the angle 13°) Campaign
17 18
Neutron fluence with E\0.5 MeV (n cm−2) Calculation
Experiment
1.452×1018 0.882×1018
(1.469 0.17)×1018 (0.8769 0.105)×1018
given in Table 1. Table 1 shows that the results of the measurements of the phosphorus content are close to each other and this is also observed for the copper content, making it possible to get the averaged data — 0.046 and 0.1 for phosphorus and copper, respectively. The obtained results agree well with the data of measurements per-
The fast neutron fluence, which exerted an effect on the inner surface of the vessel is determined by the calculation–experimental method Brodkin et al., 1996 on the samples taken from the corners of the templates, as it is shown in Fig. 1. The application of the reaction 54Fe(n,p)54Mn allowed determination of the fluence of the last 17 and 18 fuel circle reliably enough and made it possible to compare the results with the calculated values. The values are compared in Table 2, from which it follows that the calculated data well agree with the experiment. This makes it possible to use the calculated values for determination of the fluence for the whole time of operation. Data on calculated and experimental values of fluence for all templates are given in Table 3.
4. Macro- and microstructure Macro- and microstructure was studied with the optical microscope ‘Telatom’, using the procedure of preparation of polished samples, usual for such cases. The structure consists in alternating areas of equiaxial and columnar grains with fantype orientation in a heat flux direction. Fig. 2 presents the typical structure. It is this character of the structure, which can explain the peculiari-
Fig. 2. Macrostructure of the weld metal.
316
Table 3 The values of specific activities, densities, and fast neutron fluences with E\0.5 MeV for the templates from weld N4, NPP ‘Kozloduy-1’. Sample
54
Mn specific activity (106 Bq g−1)
54
Mn average activity (106 Bq g−1)
Average density of the neutron flux (1010 n cm−2s−1)
Neutron fluence, campaigns Neutron fluence, campaigns numbers 15–18 (1018 n cm−2) numbers 1–18, (1019 n cm−2)
1
a b c d
10.8 0.99 1.18 1.08
1.08
4.50 90.54
4.25 9 0.51
3.84
2
a b c d
1.23 1.09 1.25 1.12
1.17
4.88 90.58
4.619 0.55
3.89
3
a b c d
1.18 1.08 1.25 1.16
1.18
4.93 90.59
4.66 9 0.56
3.90
4
a b c d
1.30 1.17 1.38 1.20
1.26
5.2690.63
4.979 0.60
3.95
5
a b c d
1.31 1.16 1.34 1.18
1.25
5.21 90.62
4.92 9 0.59
3.94
6
a b c d
1.33 1.17 1.31 1.13
1.24
5.189 0.62
4.8990.58
3.94
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Template number
Specimen number
Template number
Condition
Test temperature (°C)
Absorbed energy (J)
Point on the curve KVC-T
In the centre of the fracture
Quota (%)
Grain type
Average size (mm)
Grain type
Average size (mm)
Columnar grains
Equaxed grains
Ductile belts
EA EA EA ÞCG ÞCG ÞCG
150 150 150 900 1200 950
EA ÚCG EA ÞCG ÞCG ÞCG
150 350 150 900 1200 950
– 30 – 100 75 30
100 35 55 – – –
– 35 45 – 25 70
ÚCG
600
EA
200
15
85
–
750 200 150 200 250
EA EA EA EA EA
150 200 150 200 250
15 – – – –
40 35 100 65 35
45 65 – 35 65
33 31 36 46 42 45
3 3 3 4 4 4
Reirradiated
−25 50 250 −125 40 225
0.77 8086 16.23 0.26 5.01 13.83
15
1
Annealed 475°C 150 h
−50
0.51
14 16 25 21 26
1 1 2 2 2
50 200 −125 40 150
8.47 15 0.38 7.32 14.84
LS DBT US
ÚCG EA EA EA EA
19
1
0.38
LS
EA
450
ÚCG
200
20
80
–
18 27 48 49 39
1 2 4 4 3
Annealed 560°C 2 −110 h 0 0 12.5 200 150
5.4 6.04 9.75 17.62 16.36
DBT DBT DBT US US
CG EA ÞCG
CG
CG
200 250 1100 200 200
EA EA ÞCG/EA
CG
CG
220 250 1000/400 200 200
30 – 45 40 30
65 75 10 – –
5 25 45 60 70
a
LS DBT US LS DBT US
Under the notch
DBT
LS, lower shelf; DBT, ductile to brittle transition region; US, upper shelf; EA, equiaxed; CG, column grains; Þ, perpendicular to the notch; , parallel to the notch; Ú, with the angle to the notch.
P. Platono6 et al. / Nuclear Engineering and Design 191 (1999) 313–325
Table 4 Fracture grain structure of mini-Charpy specimens (Kozloduy, Unit 1)a
317
318
P. Platono6 et al. / Nuclear Engineering and Design 191 (1999) 313–325
Table 5 Average mechanical properties of metal of weld metal 4 determined by kinetic hardness method State
Template number
Hardness HB (kgf/mm2)
Rp0.2, (MPa)
Reirradiation
3 4
233 222
594 576
After annealing at 475°C 150 h
1 2
211 213
549 546
After annealing at 560°C 2 h
1 2 3 4
207 213 205 203
536 547 537 528
ties of the fracture character of the specimens and the results of the impact tests depending on the position and orientation of a notch axis with respect to columnar grains (Table 4).
475°C for 150 h and after irradiation and annealing at 560°C for 2 h (Table 5).
7. Impact bend tests 5. Specimens manufacture Nine specimens 5× 5 ×27.5 mm in size with a V-type notch intended for impact bend tests were manufactured of every template. Drawings of the specimens and the technology of their manufacture are described earlier Kryukov et al., 1996. Only some peculiarities should be noted here, namely, position of the notch was chosen depending on position of weld boundaries, their displacement and macrostructural character.
6. Hardness measurement The kinetic method1 was used for hardness measurement. Using an indentation curve, this method not only allows the Brinell hardness number to be obtained but also allows the yield stress (Rp0.2) necessary for choosing the criterion for DBTT determination to be estimated. The measurements were performed for three states: after irradiation, after irradiation and annealing at 1 Hardness measurements have been performed by M. Bakirov (VNIIAES). Average values of mechanical properties of the weld 4 metal obtained by the kinetic hardness method are presented in Table 5.
Impact bend tests were performed to determine the critical temperature (Tk) at the present state of the vessel metal, the critical temperature (Tka) after recovery annealing and to evaluate the initial state (Tk0) of the weld 4 metal, that is, in the state after irradiation (State 1), after irradiation and annealing at 475°C for 150 h (State 2) and after irradiation and annealing at 560°C for 2 h (State 3). In tests the impact testing machine RKP-300 with potential energy of 80 J and pendulum speed at the instant of impact 3.87 m/s in accordance with the standards used. The results of testing the specimens in the states after irradiation and after annealing at 475 and 560°C are presented in Figs. 3–5 accordingly. To obtain every impact strength curve the specimens of two templates were used in the tests. Attention is drawn to the fact of ‘splitting’ of the curve, obtained by testing the specimens manufactured from templates N3 and 4 in irradiated state. The fact of splitting may be explained by the difference in orientation of columnar grains with respect to the notch axis. But even if the indicated difference is taken into account, scatter of the test results is not large. As it follows from Fig. 5, annealing smoothes this difference. Figs. 3–5 represent criterional levels, by which the critical brittleness temperature was determined
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Fig. 3. The temperature dependencies of the absorbed energy for metal of weld N4 RPV NPP ‘Kozloduy-1’, re-irradiated.
Fig. 4. The temperature dependencies of the absorbed energy for metal of weld N4 RPV NPP ‘Kozloduy-1’, re-irradiated and annealed at 475°C, 150 h.
in accordance with Russian standards. The values of the critical temperature determined in accordance with these criteria are given in Table 6. To obtain the values of the critical brittleness temperature for the standard Charpy specimens,
correlative relationships obtained earlier (Kryukov et al., 1996) were used. According to them the critical temperature is 50°C higher for Charpy specimens, then for mini Charpy. To determine the initial critical temperature the proce-
320
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Fig. 5. The temperature dependencies of the absorbed energy for metal of weld N4 RPV NPP ‘Kozloduy-1’, re-irradiated and annealed at 560°C, 2 h. Table 6 Criteria values of DBTT for sub-size specimens (5×5×27.5 mm) of Kozloduy NPP unit 1 weld metal 4 Template number
3 4 3 and 4 1 and 2 1.3 and 2.4
Criteria temperature (°C) 5J
6J
+13 +34.5 +20 +13 −5.5
+25 +58 +33.5
7.5 J
8.5 J
9J
Db=0.35mm
+58 +84 +71.5
+41 +11
+52 +78 +65 +52 +17
+38 +66.5 +50 +23 +3
Table 7 Integral data on the metal of weld N 4, NPP ‘Kozloduy-1’ State of the metal
Irradiation+annealing 475°C+ irradiation Irradiation+annealing 475°C+irradiation+annealing 475°C Irradiation+annealing 475°C+irradiation+annealing 560°C
dure accepted in Russia was used: as the initial critical temperature the mean of the values obtained for the states after annealing at 560 and 475°C was taken.
Concentration (wt.%) P
Cu
0.046
0.10
Tk0 (°C)
F (×1018) (n cm-2)
Tk (°C)
+54
4.82 4.43 4.63
+91.5 +63 +44.5
Thus, the results obtained allow determination of all characteristics necessary for prediction of embrittlement after annealing. These data are given in Table 7.
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8. Fractographic and electron-microscope research
8.1. Fractography
Fig. 6. Fracture surfaces in specimen N16. Ductile type of fracture.
Fig. 7. Fracture surfaces in specimen N33. The area of brittle fracture with cleavage and quasi-cleavage.
Research into the fracture surface of mini Charpy specimens was performed with the scanning electron microscope SXR-50 (CAMECA), France. In Table 8 the results of this research are given. The attention is drawn to the fact that intercrystalline fracture was noticed in all states of the metal except for annealing at 560°C. This is the characteristic feature of the Kozloduy-1 vessel metal, because up to the present intercrystalline fracture in the upper shelf, that is, ductile intercrystalline fracture, has not been noticed during tests of WWER-440 steel. This is likely to be connected with the fact that the phosphorus content in the Kozloduy-1 weld metal is much higher than that is typical for the welds of other reactors. This results in increased segregation in the grain boundaries and also in the interface of precipitates, separated out along the grain boundaries. Electron-microscopic research gives considerable support to this assumption. As it follows from Table 8 the share of ductile intercrystalline fracture increases with the temperature of the tests, while the share of cleavage, quasicleavage decreases. Annealing at 475°C for 150 h practically does not influence the share of brittle intercrystalline component, but decreases the share of ductile intercrystalline fracture at comparable temperatures. Annealing at 560°C for 2 h results in considerable decrease of both intercrystalline and ductile intercrystalline fracture, and also increases the share of ductile fracture to some extent. Figs. 6–8 presents the most typical results of fractography of ductile cleavage and intercrystalline ductile fracture. In Fig. 8 the grain boundaries are well seen in typical ductile fracture.
8.2. Electron-microscope research Fig. 8. Fracture surfaces in specimen N16. The area with ductile intergranular type of fracture.
Transition electron-microscope research were conducted with the electron microscope
322
Specimen number
Template number Fluence, after reirradiation (×1018) (n cm-2)
33 31 36 46 42 45
3 3 3 4 4 4
4.66
15
1
4.25
14 16 25 21 26
1 1 2 2 2
4.61
19
1
4.25
18 27 48 49 39
1 2 4 4 3
4.25 4.61 4.97 4.97 4.66
a
Condition
Reirradiated
4.97
Annealed 475°C 150 h
Annealed 560°C 2 h
Test temperature (°C)
Tk (°C)
Amax (J)
0.77 8.86 16.23 0.26 5.01 13.83
16.23
−50
0.51
15.98
50 200 −125 40 150
27
8.47 15 0.38 7.32 14.84
−110
1
0.38
−25 50 250 −125 40 225
0 0 12.5 200 150
LS, lower shelf; DBT, ductile to brittle transition region; US, upper shelf.
28
Absorbed energy (J)
54
5.4 6.04 9.75 17.62 16.36
13.83
17.62
Point on the curve KVT
Quota of different fracture modes (%)
Intergranular
Ductile Intergranular
Ductile
Cleavage
Quasi-cleavage
10 10 – 10 5 5
– 10 15 – 10 15
5 55 85 – 35 75
10 10 – 5 5
75 15 – 85 45 5
5
–
5
20
70
DBT US LS DBT US
10 – 10 10 10
5 10 – 5 10
50 90 – 45 80
10 – 15 15 Traces
25 – 75 25 Traces
LS
Traces
–
–
10
90
DBT DBT DBT US US
5 5 5 – –
Traces Traces 5 5 5
35 35 55 95 95
20 20 5 – –
40 40 30 – –
LS DBT US LS DBT US LS
P. Platono6 et al. / Nuclear Engineering and Design 191 (1999) 313–325
Table 8 Summary data of fractographic analysis results for investigated mini-Charpy specimens (Kozloduy, Unit 1)a
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Fig. 9. Image of dislocation structure of irradiated sample.
Fig. 10. Decoration of the grain boundaries with the precipitates, disk shape precipitates (dark field image × 100 000) in reirradiated specimen N46.
Fig. 11. Dark-field image of the rounded precipitates ( × 100 000).
TEMSCAN-200CX with accelerating voltage of 200 kV. The research was performed on the fragments of mini-Charpy specimens in all three states.
323
Electron-microscope investigations of the specimens in irradiated state have indicated the following: 1. The structure contains radiation defects of the ‘black dots’ type, which were identified as dislocation loops (Fig. 9). The medium size of the loops is 4–5 nm and the density is 0.9– 1.0× 1015 cm − 3. Annealing at 475°C for 150 h removes these defects totally. 2. Along with radiation defects, disk-shaped precipitates were detected (Fig. 10). They are similar to those detected earlier, during research into the metal of NVNPP-2 reactor vessel Gurovich et al., 1997. The medium size of precipitates is 11.5 nm; thickness 1–2 nm; and density 2–3 ×1015 cm − 3. These precipitates are located in the planes {100}, and as a result they may be seen in two interperpendicular orientations. The density and the average size of the precipitates vary only slightly under annealing of Kozloduy-1 weld metal, as distinct from the data obtained for the metal of NVNPP-2, where under annealing at 475°C the density of disk-shaped precipitates decreased by an order of magnitude. 3. The grain boundaries were found to be decorated with precipitates of different types, diskshaped among them (Fig. 10). This is fact, which can explain the appearance of specific ductile grain-boundary fracture, if formation of phosphorus segregations is supposed at the boundaries of precipitates. 4. The most interesting stage of TEM research became the fact of detecting rounded precipitates by electron microscope. They were detected for the first time. Medium size, 2–3 nm, density 5–7×1017 cm − 3, Fig. 11. Annealing both at 475°C for 150 h and at 560°C for 2 h lead to sharp decrease of the density of precipitates and some increase of their size. The density of these precipitates is considerably higher than that of the other structure elements, and this suggests that these precipitates make the greatest contribution to the metal embrittlement. The characteristics of all structure elements detected in the irradiated weld metal are represented in Table 9.
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Table 9 Density and dimensions of radiation defects and precipitates of Kozloduy-1 Condition
nloops (×1015) (cm-3)
Bd\loops (nm)
ndisk (×1015) (cm-3)
d%disk (nm)
nrounded (×1015) (cm-3)
Bd\lrounded (nm)
Reirradiated F= 4.97×1018 (n cm-2) Reirradiated + annealed 475°C 150 h Reirradiated +annealed 560°C 2h Irradiateda F= 6.5×1019 (n cm−2) Irradiateda +annealed 475°C 150 h Irradiateda + annealed 560°C 2h Unirradiateda
0.9–1.0
4.0–5.0
2.0–3.0
11.5
500–700
2.0–3.0
–
–
1.0–1.5
11.5
20–30
3.0–4.0
–
–
0.8–0.9
11.5
15–20
3.0–4.0
7.0–8.0
5.0
50–60
10.5
–
–
–
–
3.5–4.0
13.5
–
–
–
–
0.7–0.9
20.5
–
–
–
–
0.5–0.6
20.4
–
–
a
Data for the weld metal of NVNPP-2 (P content =0.035 wt.%).
9. Discussion It is common knowledge that the embrittlement kinetics of the WWER-440 vessel steel can be described by two methods TFr =Tk0 + DTres +AF(Fr)1/3
(1)
TFr =Tk0 + (DT 3res +A 3FFr)1/3
(2)
where TFr is the critical brittleness temperature, Tk0 is the initial critical temperature, DTres is the residual shift after annealing, AF is the embrittlement coefficient, 800 (P%+0.07 Cu%). The first method — called the conservative — is currently accepted as the main in Russian Standards, but in some cases the second method — the ‘lateral shift’ scheme — is used (see Fig. 12). In the cases, when the real properties of the vessel remain unknown, the value of 20°C is taken for the quantity DTres, if the phosphorus content does not exceed 0.04 wt.%, and 40°C is taken, if the phosphorus content is greater.
Using the data of Tables 1 and 6 it is possible to calculate the expected values of the brittleness temperature of the weld material and to compare them with experimental results. Such comparison is given in Fig. 12, from which it follows that the magnitude of TFr of the weld metal measured by experiments is considerably less than the expected value, calculated by the conservative method and by the ‘lateral shift’ method as well. Of course, this difference may be reduced to a minimum, if all possible errors are taken into account, including the error connected with determination of the correlative relationship between standard specimens and mini-Charpy. However, such a tendency is detected in other research work carried out on the metal of the operating reactor vessels (NVNPP Units 3 and 4, Kozloduy NPP Unit 2) Kryukov et al., 1996; Korolev et al., 1998. Therefore, it may be proposed that in fact there is a great margin between allowable critical brittleness temperature and real. The final conclusion about the vessel metal state and further kinetics of em-
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325
Fig. 12. Transition temperature as a function of neutron fluence for ‘Kozloduy-l’ weld metal 4.
brittlement would be possible only after additional experiments. Among them: research into embrittlement of specially prepared welds with the high phosphorus content under reirradiation and additional irradiation of the templates from Kozloduy1 vessel2. However, up to now it has already become clear that reannealing of the pressure vessel is not necessary and even with the conservative calculation method the design basis life time of the reactor vessel may be substantiated.
sible for radiation hardening and embrittlement of the vessel steel under study have been detected with the electron microscope. 4. The data on mechanical properties and critical brittleness temperature demonstrate that the state of the weld 4 metal is considerably better than may be expected on the basis of the conservative calculation method accepted as standard and even ‘lateral shift’ method. Consequently reannealing of the vessel is unnecessary.
10. Conclusion
References
1. Thus, a number of studies on the templates of weld 4 of Kozloduy-1 reactor vessel has been performed. These studies includes: determination of the chemical composition, fast neutron fluence, research into macro- and microstructure, electron microscopy, determination of mechanical properties and critical brittleness temperature. 2. The data on chemical composition and fast neutron fluence well agree with the data obtained at Bulgarian research institutes. 3. The structural particles, which may be respon-
Brodkin E.D., Egorov A.L., Vikhrov V.I., Zaritsky S.M., 1996. The determination of the VVER-1000 surveillance neutron fluence using the 54Mn activity measurements and tort neutron spectra calculations. Proceedings of the 1996 Topical Meeting Radiation protection and shielding. North Falmouth, MA, April 1996, vol. 1. Published by American Nuclear Society, La Grande Park, IL 60526, USA. Gurovich, B.A., Kuleshova, E.A., Nikolaev, Yu.A., Shtrombach, Ya.I., 1997. Assessment of relative contributions from different mechanisms to radiation embrittlement of reactor pressure vessel steels. J. Nucl. Mat. 246, 91 – 120. Korolev, Yu.N., Kryukov, A.M., Nikolaev, Yu.A., Platonov, P.A., Shtromlakh, Ya.I., Langer, R., Leitze, C., Rieg, C.-Y., 1998. Assessment of Irradiation response of WWER-440 welds using samples taken from Novo-Voronezh Unit 3 and 4 reactor pressure vessels. Nucl. Eng. Des.185, 309 –317. Kryukov, A., Platonov, P., Shtromlakh, Ya., Nikolaev, V., Klausnitzer, E., Leitz, C., Rieg, C.-Y., 1996. Investigations of samples taken from Kozloduy Unit 2 reactor pressure vessel. Nucl. Eng. Des. 160, 59 – 76.
2 At present the first of two experiments have come to an end and they have substantiated the possibility of the application of the ‘lateral shift’ method to Kozloduy-1; additional irradiation of templates is being conducted.
Results on research of templates from Kozloduy-1 reactor pressure vessel P. Platonov a,*, Ja. Shtrombakh a, A. Kryukov a, B. Gurovich a, Ju. Korolev a, J. Shmidt b a
Reactor Technologies and Materials Institute, Russian Research Centre, Kuchato6 Square, 123182 Moscow, Russian Federation b SIEMENS AG KWU, W-8520 Erlangen, Germany Received 27 January 1998; received in revised form 1 April 1999; accepted 29 April 1999
Abstract The studies on the specimens manufactured from the templates cut out from the weld 4 of Kozloduy NPP Unit 1 reactor vessel have been conducted. The data on chemical composition of the weld metal have been obtained. Neutron fluence, mechanical properties, ductile to brittle transition temperature (DBTT) using mini Charpy samples have been determined. The phosphorus and copper content averaged over all templates is 0.046 and 0.1 wt.%, respectively. The fluence amounted up to 5 × 1018 n cm − 2 within 15 – 18 fuel cycles, and about 5 × 1019 n cm − 2 for the whole period of operation. These values agree well with calculated data. DBTT was determined after irradiation (Tk) to evaluate the vessel metal state at the present moment, then after heat treatment at the temperature of 475°C to simulate the vessel metal state after thermal annealing (Tan), and after heat treatment at 560°C to simulate the metal state in the initial state (Tk0). As a result of the tests the following values were obtained: Tk, +91.5°C; Tan, + 63°C; and Tk0, 54°C. The values of Tk and Tan obtained by measurements were found to be considerably lower than those predicted in accordance with the conservative method accepted in Russia (177°C for Tk and 100°C for Tan). Thus, the obtained results allowed to make a conclusion that it is not necessary to anneal Kozloduy NPP Unit 1 reactor vessel for the second time. The fractographic and electron-microscopic research allowed to draw some conclusions on the embrittlement mechanism. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Kozloduy unit 1 reactor vessel; Embrittlement mechanism; Chemical composition
1. Introduction Kozloduy NPP Unit 1 was put into operation in 1974. The pressure vessel of this reactor belonged to the first generation of non-plated * Corresponding author. Tel.: +7-095-196-9941; fax: + 7095-196-1701.
WWER-440/230 and had an increased content of phosphorus. Calculations indicated that the life time of the vessel must have been exhausted quite fast. In 1989 as a preventive mitigation measure the annealing of the vessel in the zone near weld 4 was performed by technology developed in Russia. As a result the vessel properties were recov-
0029-5493/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S 0 0 2 9 - 5 4 9 3 ( 9 9 ) 0 0 1 1 9 - 3
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314
Table 1 The data of analysis of chemical composition carried out for the templates cut out from weld N4, NPP ‘Kozloduy-1’ (wt.%) Template number
C
Si
Mn
S
P
Cr
Ni
Mo
V
Cu
1 1 2 2 3 3 4 4 5 5 6 6 6 6
0.048 0.052 0.055 0.050 0.046 0.042 0.050 0.042 0.050 0.047 0.048 0.052 0.047 0.045
0.55 0.49 0.50 0.49 0.37 0.35 0.55 0.44 0.56 0.52 0.42 0.58 0.55 0.52
1.19 1.08 1.17 1.09 1.21 1.11 1.20 1.11 1.25 1.08 1.25 1.16 1.21 1.17
0.012 0.013 0.014 0.013 0.013 0.015 0.011 0.012 0.013 0.011 0.013 0.014 0.013 0.013
0.047 0.042 0.048 0.044 0.049 0.046 0.048 0.044 0.051 0.042 0.050 0.048 0.048 0.045
1.56 1.71 1.59 1.70 1.64 1.60 1.65 1.64 1.66 1.73 1.66 1.73 1.59 1.56
0.29 0.30 0.29 0.26 0.22 0.12 0.29 0.17 0.33 0.30 0.25 0.32 0.31 0.30
0.41 0.43 0.41 0.42 0.42 0.40 0.42 0.40 0.43 0.42 0.42 0.43 0.41 0.39
0.09 0.14 0.12 0.12 0.10 0.06 0.14 0.07 0.14 0.14 0.09 0.14 0.13 0.13
0.10 0.10 0.10 0.10 0.10 0.10 0.11 0.10 0.11 0.10 0.11 0.11 0.10 0.10
(internal) (external) (internal) (external) (internal) (external) (internal) (external) (internal) (external) (internal) (external) (internal) (external)
ered. However, because of high pollution by phosphorus by 1996 the necessity of reannealing had arisen. This was connected with the fact that the calculations indicated that the RPV material could have reached its critical condition already by 1996. The high phosphorus content was verified by direct measurements as well as by calculations. However the methods of calculation of DBTT under irradiation after the RPV annealing accepted at that time were too conservative. Therefore, it could be proposed that the real DBTT shift was less than it was expected due to calculations. This assumption was confirmed by the results of the earlier tests of the templates cut out from NVNPP-3 and 4 RPV metal and Kozloduy NPP Unit 2 metal, which had demonstrated that the real DBTT shift had been considerably lower than that obtained by calculations. In connection with this it was proposed that the templates should be cut out from the weld 4 on the inside of the vessel to determine the real properties of the metal. In September 1996 the templates were cut out and delivered to the Reactor Technologies and Materials Institute of the RRC ‘Kurchatov Institute’. In line with the program the following points of research were performed: 1. Determination of the chemical composition of the template metal.
2. Determination of the fast neutron fluence, which exerted an effect of fluence on the vessel metal under reirradiation. 3. Manufacture of the specimens from the templates and preparation of them for mechanical tests in the states after irradiation, after annealing at 475°C for 150 h and after annealing at 560°C for 2 h. 4. Hardness measurements and indentation diagram to evaluate the yield stress of the metal. 5. Macro- and microstructural studies and fractographic investigations. 6. Measurement of DBTT of irradiated specimens to determine Tk for the present state of the vessel. Measurement of residual DBTT after annealing at 475°C to simulate the vessel state after annealing. Measurement of residual DBTT after annealing at 560°C with the aim of Tk0 recovery in accordance with the methods accepted in Russia.
2. Determination of the chemical composition of the template metal The chemical composition was determined on the inside and the outside of the templates by the optical emission spectrometer ‘BAIRD SPECTROVAC FSQ’. The results, as an average of the values obtained by several measurements, are
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315
formed in the Metal Science Institute of the Bulgarian Academy of Science (to be published in this issue), making it possible to take these values for calculation of the embrittlement kinetics.
3. Estimation of the fast neutron fluence
Fig. 1. The scheme of neutron dosimeter probe cutting. Table 2 Comparison between calculated and experimental neutron fluences for weld N4 in 17th and 18th campaigns (the angle 13°) Campaign
17 18
Neutron fluence with E\0.5 MeV (n cm−2) Calculation
Experiment
1.452×1018 0.882×1018
(1.469 0.17)×1018 (0.8769 0.105)×1018
given in Table 1. Table 1 shows that the results of the measurements of the phosphorus content are close to each other and this is also observed for the copper content, making it possible to get the averaged data — 0.046 and 0.1 for phosphorus and copper, respectively. The obtained results agree well with the data of measurements per-
The fast neutron fluence, which exerted an effect on the inner surface of the vessel is determined by the calculation–experimental method Brodkin et al., 1996 on the samples taken from the corners of the templates, as it is shown in Fig. 1. The application of the reaction 54Fe(n,p)54Mn allowed determination of the fluence of the last 17 and 18 fuel circle reliably enough and made it possible to compare the results with the calculated values. The values are compared in Table 2, from which it follows that the calculated data well agree with the experiment. This makes it possible to use the calculated values for determination of the fluence for the whole time of operation. Data on calculated and experimental values of fluence for all templates are given in Table 3.
4. Macro- and microstructure Macro- and microstructure was studied with the optical microscope ‘Telatom’, using the procedure of preparation of polished samples, usual for such cases. The structure consists in alternating areas of equiaxial and columnar grains with fantype orientation in a heat flux direction. Fig. 2 presents the typical structure. It is this character of the structure, which can explain the peculiari-
Fig. 2. Macrostructure of the weld metal.
316
Table 3 The values of specific activities, densities, and fast neutron fluences with E\0.5 MeV for the templates from weld N4, NPP ‘Kozloduy-1’. Sample
54
Mn specific activity (106 Bq g−1)
54
Mn average activity (106 Bq g−1)
Average density of the neutron flux (1010 n cm−2s−1)
Neutron fluence, campaigns Neutron fluence, campaigns numbers 15–18 (1018 n cm−2) numbers 1–18, (1019 n cm−2)
1
a b c d
10.8 0.99 1.18 1.08
1.08
4.50 90.54
4.25 9 0.51
3.84
2
a b c d
1.23 1.09 1.25 1.12
1.17
4.88 90.58
4.619 0.55
3.89
3
a b c d
1.18 1.08 1.25 1.16
1.18
4.93 90.59
4.66 9 0.56
3.90
4
a b c d
1.30 1.17 1.38 1.20
1.26
5.2690.63
4.979 0.60
3.95
5
a b c d
1.31 1.16 1.34 1.18
1.25
5.21 90.62
4.92 9 0.59
3.94
6
a b c d
1.33 1.17 1.31 1.13
1.24
5.189 0.62
4.8990.58
3.94
P. Platono6 et al. / Nuclear Engineering and Design 191 (1999) 313–325
Template number
Specimen number
Template number
Condition
Test temperature (°C)
Absorbed energy (J)
Point on the curve KVC-T
In the centre of the fracture
Quota (%)
Grain type
Average size (mm)
Grain type
Average size (mm)
Columnar grains
Equaxed grains
Ductile belts
EA EA EA ÞCG ÞCG ÞCG
150 150 150 900 1200 950
EA ÚCG EA ÞCG ÞCG ÞCG
150 350 150 900 1200 950
– 30 – 100 75 30
100 35 55 – – –
– 35 45 – 25 70
ÚCG
600
EA
200
15
85
–
750 200 150 200 250
EA EA EA EA EA
150 200 150 200 250
15 – – – –
40 35 100 65 35
45 65 – 35 65
33 31 36 46 42 45
3 3 3 4 4 4
Reirradiated
−25 50 250 −125 40 225
0.77 8086 16.23 0.26 5.01 13.83
15
1
Annealed 475°C 150 h
−50
0.51
14 16 25 21 26
1 1 2 2 2
50 200 −125 40 150
8.47 15 0.38 7.32 14.84
LS DBT US
ÚCG EA EA EA EA
19
1
0.38
LS
EA
450
ÚCG
200
20
80
–
18 27 48 49 39
1 2 4 4 3
Annealed 560°C 2 −110 h 0 0 12.5 200 150
5.4 6.04 9.75 17.62 16.36
DBT DBT DBT US US
CG EA ÞCG
CG
CG
200 250 1100 200 200
EA EA ÞCG/EA
CG
CG
220 250 1000/400 200 200
30 – 45 40 30
65 75 10 – –
5 25 45 60 70
a
LS DBT US LS DBT US
Under the notch
DBT
LS, lower shelf; DBT, ductile to brittle transition region; US, upper shelf; EA, equiaxed; CG, column grains; Þ, perpendicular to the notch; , parallel to the notch; Ú, with the angle to the notch.
P. Platono6 et al. / Nuclear Engineering and Design 191 (1999) 313–325
Table 4 Fracture grain structure of mini-Charpy specimens (Kozloduy, Unit 1)a
317
318
P. Platono6 et al. / Nuclear Engineering and Design 191 (1999) 313–325
Table 5 Average mechanical properties of metal of weld metal 4 determined by kinetic hardness method State
Template number
Hardness HB (kgf/mm2)
Rp0.2, (MPa)
Reirradiation
3 4
233 222
594 576
After annealing at 475°C 150 h
1 2
211 213
549 546
After annealing at 560°C 2 h
1 2 3 4
207 213 205 203
536 547 537 528
ties of the fracture character of the specimens and the results of the impact tests depending on the position and orientation of a notch axis with respect to columnar grains (Table 4).
475°C for 150 h and after irradiation and annealing at 560°C for 2 h (Table 5).
7. Impact bend tests 5. Specimens manufacture Nine specimens 5× 5 ×27.5 mm in size with a V-type notch intended for impact bend tests were manufactured of every template. Drawings of the specimens and the technology of their manufacture are described earlier Kryukov et al., 1996. Only some peculiarities should be noted here, namely, position of the notch was chosen depending on position of weld boundaries, their displacement and macrostructural character.
6. Hardness measurement The kinetic method1 was used for hardness measurement. Using an indentation curve, this method not only allows the Brinell hardness number to be obtained but also allows the yield stress (Rp0.2) necessary for choosing the criterion for DBTT determination to be estimated. The measurements were performed for three states: after irradiation, after irradiation and annealing at 1 Hardness measurements have been performed by M. Bakirov (VNIIAES). Average values of mechanical properties of the weld 4 metal obtained by the kinetic hardness method are presented in Table 5.
Impact bend tests were performed to determine the critical temperature (Tk) at the present state of the vessel metal, the critical temperature (Tka) after recovery annealing and to evaluate the initial state (Tk0) of the weld 4 metal, that is, in the state after irradiation (State 1), after irradiation and annealing at 475°C for 150 h (State 2) and after irradiation and annealing at 560°C for 2 h (State 3). In tests the impact testing machine RKP-300 with potential energy of 80 J and pendulum speed at the instant of impact 3.87 m/s in accordance with the standards used. The results of testing the specimens in the states after irradiation and after annealing at 475 and 560°C are presented in Figs. 3–5 accordingly. To obtain every impact strength curve the specimens of two templates were used in the tests. Attention is drawn to the fact of ‘splitting’ of the curve, obtained by testing the specimens manufactured from templates N3 and 4 in irradiated state. The fact of splitting may be explained by the difference in orientation of columnar grains with respect to the notch axis. But even if the indicated difference is taken into account, scatter of the test results is not large. As it follows from Fig. 5, annealing smoothes this difference. Figs. 3–5 represent criterional levels, by which the critical brittleness temperature was determined
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319
Fig. 3. The temperature dependencies of the absorbed energy for metal of weld N4 RPV NPP ‘Kozloduy-1’, re-irradiated.
Fig. 4. The temperature dependencies of the absorbed energy for metal of weld N4 RPV NPP ‘Kozloduy-1’, re-irradiated and annealed at 475°C, 150 h.
in accordance with Russian standards. The values of the critical temperature determined in accordance with these criteria are given in Table 6. To obtain the values of the critical brittleness temperature for the standard Charpy specimens,
correlative relationships obtained earlier (Kryukov et al., 1996) were used. According to them the critical temperature is 50°C higher for Charpy specimens, then for mini Charpy. To determine the initial critical temperature the proce-
320
P. Platono6 et al. / Nuclear Engineering and Design 191 (1999) 313–325
Fig. 5. The temperature dependencies of the absorbed energy for metal of weld N4 RPV NPP ‘Kozloduy-1’, re-irradiated and annealed at 560°C, 2 h. Table 6 Criteria values of DBTT for sub-size specimens (5×5×27.5 mm) of Kozloduy NPP unit 1 weld metal 4 Template number
3 4 3 and 4 1 and 2 1.3 and 2.4
Criteria temperature (°C) 5J
6J
+13 +34.5 +20 +13 −5.5
+25 +58 +33.5
7.5 J
8.5 J
9J
Db=0.35mm
+58 +84 +71.5
+41 +11
+52 +78 +65 +52 +17
+38 +66.5 +50 +23 +3
Table 7 Integral data on the metal of weld N 4, NPP ‘Kozloduy-1’ State of the metal
Irradiation+annealing 475°C+ irradiation Irradiation+annealing 475°C+irradiation+annealing 475°C Irradiation+annealing 475°C+irradiation+annealing 560°C
dure accepted in Russia was used: as the initial critical temperature the mean of the values obtained for the states after annealing at 560 and 475°C was taken.
Concentration (wt.%) P
Cu
0.046
0.10
Tk0 (°C)
F (×1018) (n cm-2)
Tk (°C)
+54
4.82 4.43 4.63
+91.5 +63 +44.5
Thus, the results obtained allow determination of all characteristics necessary for prediction of embrittlement after annealing. These data are given in Table 7.
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321
8. Fractographic and electron-microscope research
8.1. Fractography
Fig. 6. Fracture surfaces in specimen N16. Ductile type of fracture.
Fig. 7. Fracture surfaces in specimen N33. The area of brittle fracture with cleavage and quasi-cleavage.
Research into the fracture surface of mini Charpy specimens was performed with the scanning electron microscope SXR-50 (CAMECA), France. In Table 8 the results of this research are given. The attention is drawn to the fact that intercrystalline fracture was noticed in all states of the metal except for annealing at 560°C. This is the characteristic feature of the Kozloduy-1 vessel metal, because up to the present intercrystalline fracture in the upper shelf, that is, ductile intercrystalline fracture, has not been noticed during tests of WWER-440 steel. This is likely to be connected with the fact that the phosphorus content in the Kozloduy-1 weld metal is much higher than that is typical for the welds of other reactors. This results in increased segregation in the grain boundaries and also in the interface of precipitates, separated out along the grain boundaries. Electron-microscopic research gives considerable support to this assumption. As it follows from Table 8 the share of ductile intercrystalline fracture increases with the temperature of the tests, while the share of cleavage, quasicleavage decreases. Annealing at 475°C for 150 h practically does not influence the share of brittle intercrystalline component, but decreases the share of ductile intercrystalline fracture at comparable temperatures. Annealing at 560°C for 2 h results in considerable decrease of both intercrystalline and ductile intercrystalline fracture, and also increases the share of ductile fracture to some extent. Figs. 6–8 presents the most typical results of fractography of ductile cleavage and intercrystalline ductile fracture. In Fig. 8 the grain boundaries are well seen in typical ductile fracture.
8.2. Electron-microscope research Fig. 8. Fracture surfaces in specimen N16. The area with ductile intergranular type of fracture.
Transition electron-microscope research were conducted with the electron microscope
322
Specimen number
Template number Fluence, after reirradiation (×1018) (n cm-2)
33 31 36 46 42 45
3 3 3 4 4 4
4.66
15
1
4.25
14 16 25 21 26
1 1 2 2 2
4.61
19
1
4.25
18 27 48 49 39
1 2 4 4 3
4.25 4.61 4.97 4.97 4.66
a
Condition
Reirradiated
4.97
Annealed 475°C 150 h
Annealed 560°C 2 h
Test temperature (°C)
Tk (°C)
Amax (J)
0.77 8.86 16.23 0.26 5.01 13.83
16.23
−50
0.51
15.98
50 200 −125 40 150
27
8.47 15 0.38 7.32 14.84
−110
1
0.38
−25 50 250 −125 40 225
0 0 12.5 200 150
LS, lower shelf; DBT, ductile to brittle transition region; US, upper shelf.
28
Absorbed energy (J)
54
5.4 6.04 9.75 17.62 16.36
13.83
17.62
Point on the curve KVT
Quota of different fracture modes (%)
Intergranular
Ductile Intergranular
Ductile
Cleavage
Quasi-cleavage
10 10 – 10 5 5
– 10 15 – 10 15
5 55 85 – 35 75
10 10 – 5 5
75 15 – 85 45 5
5
–
5
20
70
DBT US LS DBT US
10 – 10 10 10
5 10 – 5 10
50 90 – 45 80
10 – 15 15 Traces
25 – 75 25 Traces
LS
Traces
–
–
10
90
DBT DBT DBT US US
5 5 5 – –
Traces Traces 5 5 5
35 35 55 95 95
20 20 5 – –
40 40 30 – –
LS DBT US LS DBT US LS
P. Platono6 et al. / Nuclear Engineering and Design 191 (1999) 313–325
Table 8 Summary data of fractographic analysis results for investigated mini-Charpy specimens (Kozloduy, Unit 1)a
P. Platono6 et al. / Nuclear Engineering and Design 191 (1999) 313–325
Fig. 9. Image of dislocation structure of irradiated sample.
Fig. 10. Decoration of the grain boundaries with the precipitates, disk shape precipitates (dark field image × 100 000) in reirradiated specimen N46.
Fig. 11. Dark-field image of the rounded precipitates ( × 100 000).
TEMSCAN-200CX with accelerating voltage of 200 kV. The research was performed on the fragments of mini-Charpy specimens in all three states.
323
Electron-microscope investigations of the specimens in irradiated state have indicated the following: 1. The structure contains radiation defects of the ‘black dots’ type, which were identified as dislocation loops (Fig. 9). The medium size of the loops is 4–5 nm and the density is 0.9– 1.0× 1015 cm − 3. Annealing at 475°C for 150 h removes these defects totally. 2. Along with radiation defects, disk-shaped precipitates were detected (Fig. 10). They are similar to those detected earlier, during research into the metal of NVNPP-2 reactor vessel Gurovich et al., 1997. The medium size of precipitates is 11.5 nm; thickness 1–2 nm; and density 2–3 ×1015 cm − 3. These precipitates are located in the planes {100}, and as a result they may be seen in two interperpendicular orientations. The density and the average size of the precipitates vary only slightly under annealing of Kozloduy-1 weld metal, as distinct from the data obtained for the metal of NVNPP-2, where under annealing at 475°C the density of disk-shaped precipitates decreased by an order of magnitude. 3. The grain boundaries were found to be decorated with precipitates of different types, diskshaped among them (Fig. 10). This is fact, which can explain the appearance of specific ductile grain-boundary fracture, if formation of phosphorus segregations is supposed at the boundaries of precipitates. 4. The most interesting stage of TEM research became the fact of detecting rounded precipitates by electron microscope. They were detected for the first time. Medium size, 2–3 nm, density 5–7×1017 cm − 3, Fig. 11. Annealing both at 475°C for 150 h and at 560°C for 2 h lead to sharp decrease of the density of precipitates and some increase of their size. The density of these precipitates is considerably higher than that of the other structure elements, and this suggests that these precipitates make the greatest contribution to the metal embrittlement. The characteristics of all structure elements detected in the irradiated weld metal are represented in Table 9.
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Table 9 Density and dimensions of radiation defects and precipitates of Kozloduy-1 Condition
nloops (×1015) (cm-3)
Bd\loops (nm)
ndisk (×1015) (cm-3)
d%disk (nm)
nrounded (×1015) (cm-3)
Bd\lrounded (nm)
Reirradiated F= 4.97×1018 (n cm-2) Reirradiated + annealed 475°C 150 h Reirradiated +annealed 560°C 2h Irradiateda F= 6.5×1019 (n cm−2) Irradiateda +annealed 475°C 150 h Irradiateda + annealed 560°C 2h Unirradiateda
0.9–1.0
4.0–5.0
2.0–3.0
11.5
500–700
2.0–3.0
–
–
1.0–1.5
11.5
20–30
3.0–4.0
–
–
0.8–0.9
11.5
15–20
3.0–4.0
7.0–8.0
5.0
50–60
10.5
–
–
–
–
3.5–4.0
13.5
–
–
–
–
0.7–0.9
20.5
–
–
–
–
0.5–0.6
20.4
–
–
a
Data for the weld metal of NVNPP-2 (P content =0.035 wt.%).
9. Discussion It is common knowledge that the embrittlement kinetics of the WWER-440 vessel steel can be described by two methods TFr =Tk0 + DTres +AF(Fr)1/3
(1)
TFr =Tk0 + (DT 3res +A 3FFr)1/3
(2)
where TFr is the critical brittleness temperature, Tk0 is the initial critical temperature, DTres is the residual shift after annealing, AF is the embrittlement coefficient, 800 (P%+0.07 Cu%). The first method — called the conservative — is currently accepted as the main in Russian Standards, but in some cases the second method — the ‘lateral shift’ scheme — is used (see Fig. 12). In the cases, when the real properties of the vessel remain unknown, the value of 20°C is taken for the quantity DTres, if the phosphorus content does not exceed 0.04 wt.%, and 40°C is taken, if the phosphorus content is greater.
Using the data of Tables 1 and 6 it is possible to calculate the expected values of the brittleness temperature of the weld material and to compare them with experimental results. Such comparison is given in Fig. 12, from which it follows that the magnitude of TFr of the weld metal measured by experiments is considerably less than the expected value, calculated by the conservative method and by the ‘lateral shift’ method as well. Of course, this difference may be reduced to a minimum, if all possible errors are taken into account, including the error connected with determination of the correlative relationship between standard specimens and mini-Charpy. However, such a tendency is detected in other research work carried out on the metal of the operating reactor vessels (NVNPP Units 3 and 4, Kozloduy NPP Unit 2) Kryukov et al., 1996; Korolev et al., 1998. Therefore, it may be proposed that in fact there is a great margin between allowable critical brittleness temperature and real. The final conclusion about the vessel metal state and further kinetics of em-
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325
Fig. 12. Transition temperature as a function of neutron fluence for ‘Kozloduy-l’ weld metal 4.
brittlement would be possible only after additional experiments. Among them: research into embrittlement of specially prepared welds with the high phosphorus content under reirradiation and additional irradiation of the templates from Kozloduy1 vessel2. However, up to now it has already become clear that reannealing of the pressure vessel is not necessary and even with the conservative calculation method the design basis life time of the reactor vessel may be substantiated.
sible for radiation hardening and embrittlement of the vessel steel under study have been detected with the electron microscope. 4. The data on mechanical properties and critical brittleness temperature demonstrate that the state of the weld 4 metal is considerably better than may be expected on the basis of the conservative calculation method accepted as standard and even ‘lateral shift’ method. Consequently reannealing of the vessel is unnecessary.
10. Conclusion
References
1. Thus, a number of studies on the templates of weld 4 of Kozloduy-1 reactor vessel has been performed. These studies includes: determination of the chemical composition, fast neutron fluence, research into macro- and microstructure, electron microscopy, determination of mechanical properties and critical brittleness temperature. 2. The data on chemical composition and fast neutron fluence well agree with the data obtained at Bulgarian research institutes. 3. The structural particles, which may be respon-
Brodkin E.D., Egorov A.L., Vikhrov V.I., Zaritsky S.M., 1996. The determination of the VVER-1000 surveillance neutron fluence using the 54Mn activity measurements and tort neutron spectra calculations. Proceedings of the 1996 Topical Meeting Radiation protection and shielding. North Falmouth, MA, April 1996, vol. 1. Published by American Nuclear Society, La Grande Park, IL 60526, USA. Gurovich, B.A., Kuleshova, E.A., Nikolaev, Yu.A., Shtrombach, Ya.I., 1997. Assessment of relative contributions from different mechanisms to radiation embrittlement of reactor pressure vessel steels. J. Nucl. Mat. 246, 91 – 120. Korolev, Yu.N., Kryukov, A.M., Nikolaev, Yu.A., Platonov, P.A., Shtromlakh, Ya.I., Langer, R., Leitze, C., Rieg, C.-Y., 1998. Assessment of Irradiation response of WWER-440 welds using samples taken from Novo-Voronezh Unit 3 and 4 reactor pressure vessels. Nucl. Eng. Des.185, 309 –317. Kryukov, A., Platonov, P., Shtromlakh, Ya., Nikolaev, V., Klausnitzer, E., Leitz, C., Rieg, C.-Y., 1996. Investigations of samples taken from Kozloduy Unit 2 reactor pressure vessel. Nucl. Eng. Des. 160, 59 – 76.
2 At present the first of two experiments have come to an end and they have substantiated the possibility of the application of the ‘lateral shift’ method to Kozloduy-1; additional irradiation of templates is being conducted.