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Assessment of irradiation response of WWER-440 welds using samples taken from Novovoronezh unit 3 and 4 reactor pressure vessels
Nuclear Engineering and Design 185 (1998) 309 – 317
Assessment of irradiation response of WWER-440 welds using samples taken from Novovoronezh unit 3 and 4 reactor pressure vessels Yu. N. Korolev a,*, A.M. Kryukov a, Yu. A. Nikolaev a, P.A. Platonov a, Ya. I. Shtrombakh a, R. Langer b, C. Leitz b, C.-Y. Reig c a
Russian Research Centre, Kurchato6 Institute, 123182 Moscow, Russia b SIEMENS AG KWU, W-8520 Erlangen, Germany c Electricite de France, France
Received 20 June 1996; received in revised form 7 May 1998; accepted 29 May 1998
Abstract The results of the study on Novovoronezh unit 3 and 4 (NV NPP-3 and 4) reactor pressure vessel (RPV) radiation embrittlement measured using subsize impact specimens (5 × 5× 27.5 mm3) fabricated from samples taken from the corresponding RPV walls are presented. The post-irradiation annealing effectiveness and the embrittlement kinetics of Novovoronezh unit 3 and 4 RPV welds under re-irradiation are discussed. Ductile-to-brittle transition temperatures (DBTT) obtained using standard Charpy (TT10 × 10) and subsize impact (TT5 × 5) specimens of trepans cut out from Novovoronezh unit 2 RPV are compared. A new relation between TT10 × 10 and TT5 × 5 has been developed. © 1998 Published by Elsevier Science S.A. All rights reserved.
1. Introduction At the present time no theory exists to combine and explain all the accumulated experimental data on the effect (and complicated interplay) of metallurgical factors (microstructure, chemical composition) and irradiation condition (irradiation temperature, fast neutron flux, fluence and spectrum) on radiation embrittlement kinetics of RPV materials. Therefore, surveillance specimen programs providing permanent monitoring of radia* Corresponding author. Tel.: +7 095 196 9612; fax: +7 095 196 1701; e-mail: [email protected]
tion embrittlement of RPV materials are necessary to afford high safety operation of NPP. Due to the lack of surveillance programs for the first generation of WWER-440/230 units, the only way to monitor RPV steel irradiation embrittlement of those NPP is cutting out small samples (templets) from RPV walls. In the second half of the 1980s, the actual radiation lifetime of a number of WWER-440 units appeared to be exhausted, and annealing of welds of the core of these RPV (the condition of these welds is one of the major factors defining the lifetime) is practically the unique remedy for it prolongation. Thus, cutting out templets is neces-
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sary for the assessment of the actual condition of annealed RPV materials and prediction of the radiation lifetime. It is important to use subsize impact specimens instead of the standard Charpy specimens for estimation of DBTT due to the following reasons. Calculations showed that it is admissible to cut out templets with the maximal dimensions of 7× 60×95 mm3 (with consequent repair) from RPV so that its safety remains. Templets of the sizes indicated above are not appropriate for machining standard Charpy-V specimens necessary for determining the irradiation embrittlement in RPV steels in compliance with the Russian Guide (Calculation Standards for Strength of Equipment, 1989). In the beginning of the 1990s, the cutting out of templets from NV NPP-2, 3 and 4 in Russia and also from ‘Kozloduy-2’ in Bulgaria was accomplished. Within the frame of the program TACIS91, which was carried out between 1993 and 1996, the contract between EC and EdF —FRAMATOME— SIEMENS involved the program on complex validation of the method applied for assessment of the state of pressure vessel materials before and after annealing. The principal part of experimental works on cutting out the trepans and templets as well as determining the mechanical properties were carried out by two subcontractors: RRC ‘Kurchatov Institute’ and CRISM ‘Prometey’. RRC KI dealt mainly with studies of weld metal, and CRISM ‘Prometey’ —of base metal. This paper presents the experimental data on investigation of the welds obtained in RRC KI.
2. Test materials and impact test technique The material of samples taken from units 2, 3 and 4 of NV NPP, and also RPV steels of some research programs were used. Altogether, 34 materials were investigated within the frame of the research programs (Kryukov et al., 1996) and seven materials within the frame of TACIS-91. The tests of subsize impact specimens (5× 5× 27.5 mm3) were performed using a standard instrumented impact machine with a potential
energy 80 J and a pendulum impact velocity at the moment of impact 3.84 m s − 1. For the tests considered, the impact machine was equipped with an instrumented tup in accordance with ISO. One of two functions was recorded during a test: load-time or load-displacement. In addition, the impact machine was equipped with an incremental transducer of resolution 4096 counts per 360° with digital read-out in 12-byte Grey code. The incremental transducer allows measuring absorbed energy providing a maximum error of 0.078 J (1024 counts) for stored potential energy of the pendulum 80 J.
3. Results and discussion Cutting out small samples (templets) from RPV walls of some operating WWER-440 units has been carried out since 1990. This enables us to confirm both reliability of the trend curves specified by Russian Guide for estimation of radiation embrittlement of WWER-440 welds and the effectiveness of post-irradiation annealing. The templets were cut out from the internal surfaces of RPV without cladding. The data obtained in the present study were used for comparison of actual radiation embrittlement of RPV materials at different stages of operation (as-irradiated, after post-irradiation annealing and re-irradiation) with the predicted ones.
3.1. Justification of using subsize impact specimens for assessment of radiation embrittlement of RPV steel The admissible depth of templets taken from RPV surface generated a need for using subsize impact specimens for the determination of the DBTT of RPV steel. The maximum cross section of the specimens machined from these templets was 5 ×5 mm2. Therefore, it was necessary to develop a correlation between DBTT obtained using subsize impact specimens and standard Charpy specimens required by the Russian Guide (Calculation Standards for Strength of Equipment, 1989) for estimating the radiation embrittlement. To solve this problem the RRC ‘Kurchatov
Y.N. Korole6 et al. / Nuclear Engineering and Design 185 (1998) 309–317
Institute’ and CRISM ‘Prometey’ carried out experiments between 1980 and 1988 in order to determine the dependency and their relation on irradiation. Charpy specimens of 5× 5 and 10 × 10 mm2 made of steel 15Kh2MFA and its welds with different contents of impurities were tested. A significant part of these specimens was subjected to irradiation at 270°C with a fluence of 1×1020 n · cm − 2. A small part of the irradiated specimens was annealed after irradiation at 475°C for 100 h, which is the standard annealing for WWER-440 RPV. Some results of that research were published in (Kryukov et al., 1996; Leitz et al., 1996). The choice of a criterion to determine the DBTT using subsize specimens was based on the following approach. Due to the fact that in impact tests the DBTT is determined in a transition region by a fixed value of absorbed energy, the selection of this reference energy for impact specimens of different scale has been ensured by the condition of the constant ratio of this reference level to energy of fully ductile fracture, i.e. to the level of upper shelf energy (USE) region. Thus, it was supposed that for V-notched impact specimens of any size the following ratio is to be constant (Amaev et al., 1993a): KCVref/USE =const,
311
different scale (Fig. 1) shows that the following relation between the values of DBTT for 10× 10 and 5× 5 mm2 impact specimens in both as-received and irradiated conditions is valid: TT10 × 10 = 42× 0.92 TT5 × 5, °C
(2)
10 × 10
Standard deviation (s) for TT in Eq. (2) is 19°C. If the value of tangent is rounded to 1, then the standard deviation increases to 20°C. The corresponding equation is as follows: TT10 × 10 = 44× TT5 × 5, °C
(3)
It is worth noticing that the line of Eq. (3) lies within the 95% confidence band for the regression line (TT10 × 10 versus TT5 × 5) plotted using all the data shown in Fig. 1. The first and second regression coefficients of Eq. (2) were defined with a standard deviation of 3 and 0.05, respectively. Thus the free term of Eq. (2) can be varied with a confidence probability of 95% from 36 to 48. Standard deviation for the free term of Eq. (3) is 2.5, thus, the upper boundary of 95% confidence band for this parameter variation is 49°C. The following equation has been specified in 1991 to the Russian Standard for evaluating TT10 × 10 of RPV steel based on experimental data obtained using subsize impact bend specimens:
(1)
where KCVref is the reference level of absorbed energy. Hence, the ratio between reference levels of absorbed energy for impact specimens of different scales is defined by the ratio between the upper shelf energies for these specimens. Statistical treatment of the experimental data demonstrated that the reference absorbed energy of 47 J for standard Charpy specimens corresponds to the one of 6 J for impact bend specimens with section of 5×5 mm2. Alongside with a reference level of absorbed energy, some value of specimen lateral expansion is applied as a criterion for determination of DBTT. That is why, some amount of work was done for experimental substantiation of this criterion for subsize specimens. It was established that for specimens with section 5×5 mm2 this reference value is 0.35 mm. The analysis of the data from Kryukov et al. (1996) obtained by testing impact specimens of
Fig. 1. Comparison of testing standard Charpy and subsize impact bend specimens.
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Fig. 2. Orientation of standard Charpy and subsize specimens count out of trepan.
TT10 × 10 = 50 + TT5 × 5, °C,
(4a)
For the estimation of the 95% confidence band for the correlation between test results of standard Charpy and subsize impact specimens, the following equation was recommended: TT10 × 10 = 50 + TT5 × 5 92s, °C
(4b)
where s is 21°C.
3.2. Validation of the correlation between standard Charpy and subsize impact bend specimens For some reasons, post-irradiation annealing of NV NPP-2 was economically unexpedient and this unit was taken out of service in 1990. Decommission of NV NPP-2 enabled large samples of RPV (trepans) to be cut out of the wall throughout its thickness and to fabricate standard Charpy as well as subsize impact specimens (Fig. 2). A few pairs of series, i.e. standard Charpy and subsize specimens were fabricated from internal, external and intermediate parts of trepans in irradiated and annealed at regular conditions (at 475°C for 150 h). Two series of specimens were annealed at 560° for 2 h that simulated the unirradiated condition of the steel. All the post-irradia-
tion annealings were performed under laboratory conditions. The main difference between the data used for elaborating Eqs. (4a) and (4b) and the results of testing NV NPP-2 trepans is the long-term irradiation of the latter. Moreover, since it was a neutron flux gradient through the RPV wall different layers of the templets were exposed to irradiation to different fluences. Using the test results of NV NPP-2 trepans expanded the existing data set by the data with relatively high DBTT values (TT10 × 10 for the previous research programs was varied within the ranges −80} 140°C and −50} 236°C for the new NV NPP-2 test results). Thus, using the trepan test results was useful for validating the previously developed correlation dependence (Eqs. (2)–(4) and (4b)). The following correlation was established for impact specimens of different scale fabricated from the NV NPP-2 trepans (Fig. 3(a)): TT10 × 10 = 54+ 1.07 TT5 × 5, °C
(5)
The standard deviation of TT10 × 10 in Eq. (5) is 19.5°C. The slope of trend curve for materials with high DBTT increased in comparison with the one for materials with lower DBTT values (Fig. 1). However, the mean line specified by the Rus-
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313
Fig. 3. Comparison of test results of standard Charpy and subsize impact bend specimens.
sian Guide (Eqs. (4a) and (4b)) still lies in the 95% confidence region for regression line (the standard deviation for the coefficients of Eq. (5) were found to be 4.5 and 0.055, respectively). Fig. 3(b) presents a combination of the data obtained within the of TACIS-91 and Russian research programs developed in the 1980s (Kryukov et al., 1996). Total correlation dependence obtained from these data is closer to Eqs. (4a) and (4b) than Eq. (5): TT10 × 10 = 47 + 1.04 TT5 × 5, °C
the present investigation was to develop conservative estimation of the TT10 × 10 versus TT5 × 5 dependence. For this purpose the boundary of 95% confidence region of the regression line (Eq. (6)) was linearized and approximated by two lines (Fig. 4). The corresponding equations are as follows:
(6)
10 × 10
in Eq. (6) is 20°C. Standard deviation of TT The confidence region defined by Eqs. (4a) and (4b) and in Fig. 3 is nearly coincident. Regression analysis of the correlation TT10 × 10 versus TT5 × 5 for DBTT determined using the output criterion specified by Russian Guide (Calculation Standards for Strength of Equipment, 1989) produced the following regression line: TT10 × 10 = 45 + 0.96 TT5 × 5, °C
(7)
The corresponding dependence by the Russian Standard (Eq. (4a)) is even conservative in comparison with Eq. (7). The presented evaluation was found to be in good agreement with previously elaborated dependencies (Leitz et al., 1996). One of the aims of
Fig. 4. Conservative approach for correlation of Charpy and subsize specimens.
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314
TT10 × 10 = 52 + TT5 × 5 50), °C TT
10 × 10
5×5
= 52 + 1.09 TT
(TT
(8a) 5×5
]0), °C (8b)
The available experimental results (Fig. 3) prove that the regression dependence between DBTT evaluated from testing standard Charpy and subsize impact specimens and the TT10 × 10 versus TT5 × 5 dependence Eq. (4a) adopted in the Russian Standard nearly coincide. Thus, the Russian Standard (Eq. (4a)) is quite satisfactory in spite of the clearly defined tendency to acceleration of TT10 × 10 versus TT5 × 5 dependence with increasing DBTT. This effect can probably be contributed to modification of the material condition caused by long-term irradiation. To validate the authenticity of the relation specified by the Russian Standard (Eqs. (4a) and (4b)) the variation of the DBTT through the NV NPP-2 weld before and after post-irradiation annealing was measured using standard Charpy (Fig. 5(a)) and subsize impact bend specimens (Fig. 5(b)). It can be seen that the DBTT measured using standard Charpy and subsize impact bend specimens are quite comparable. The approach developed (Eqs. (8a) and (8b)) produced better assessment of DBTT changes in comparison with the Russian Standard (Eq. (4a)).
3.3. The results obtained for templets cut from NVNPP-3 and 4 RPV
Fig. 5. Variation of DBTT throughout NVNPP-2 RPV wall (weld metal) before and after post-irradiation annealing.
Small templets were cut out from the internal surfaces of NVNPP-2, 3 and 4 RPV and also from unit 2 at NPP ‘Kozoloduy’ to evaluate the actual condition of RPV materials between 1990 and 1995. The templets were cut out from all units, with the exception of NVNPP-2 before and after post-irradiation annealing and also after reirradiating. This paper includes the results of a study of these materials carried out at the ‘Kurchatov Institute’. Some results of that investigation have already been published (Leitz et al., 1996). The DBTT values for the materials of the above mentioned RPVs were determined for the templets before and after annealing as well as after re-irradiating. In addition, these values were
determined after special heat treatments, simulating a condition close to the initial (unirradiated) condition of welds. The study of templets was primarily aimed at a comparison between the actual and predicted values of radiation embrittlement, post-irradiation annealing effectiveness, and embrittlement due to re-irradiation. The experimental and predicted DBTT values of RPV materials under investigation in unirradiated, as-irradiated, annealed and re-irradiated conditions are compared in Fig. 6(a,b,c). Eqs. (8a) and (8b) was used for an estimation of the DBTT. The actual degree of radiation embrittlement for the NVNPP-4 was likewise found to be height than the inaccuracy in determination of DBTT in
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Fig. 6. Comparison of the experimental and predicted data on irradiation and annealing response of RPV welds.
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Fig. 6. (Continued)
unirradiated condition — TT0 (the calculated and experimental values of DTTF was found to be practically the same, see Fig. 6(a)). For NVNPP4, the predicted and experimental values of Charpy DBTT shifts caused by irradiation almost coincide. An estimation of DBTT (Eq. (4a)) specified by the Russian Standard and the new approach (Eqs. (8a) and (8b)) provide nearly the same results. This proves the presently adopted method of assessing radiation embrittlement of RPV steels to be valid. In the contrary, the method of evaluation of TT0 should be essentially modified. The residual embrittlement in NVNNP3 and NVNPP-4 welds after post-irradiation annealing does not exceed 20°C (Fig. 6). The result coincides with the predicted degree of recovery of weld metal properties due to annealing (Amaev et al., 1993b).
4. Conclusions (1) A relation between the values of the ductileto-brittle transition temperature obtained using standard Charpy and subsize impact bend speci-
mens was developed. The results of the study of trepans cut out from NVNNP-2 weld No. 4 and the results of previous Russian research programs were involved in elaborating these dependencies. (2) The presently valid Russian Standard at the DBTT measurement using subsize impact specimens was reliably justified. The correlation dependence proposed by Russian Standard was found to lie in the 95% confidence band of the regression line. A satisfactory correlation between DBTT shifts estimated using standard Charpy and subsize impact specimens was observed for trepans cut out from NVNNP-2 weld. (3) In order to increase the conservativity of Russian Standard, an attempt was made to develop a new approach for the TT5 × 5 versus TT10 × 10 dependence. For that purpose the upper boundary of the 95% confidence band of the regression line was linerarized and approximated by two lines. (4) Using subsize impact specimens fabricated from templets of NVNNP-3 and 4 welds, the actual condition of RPV steel was estimated in as-irradiated, annealed and re-irradiated conditions.
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References Amaev, A.D., Badanin, V.I., Kryukov, A.M., Nikolaev, V.A., Rogov, M.F., Sokolov, M.A., 1993. Use of subsize specimens for determination of radiation embrittlement of operating reactor pressure vessels. In: Corwin, W.R., Haggag, F.M., Server, W.L., (Eds.), Small Specimen Techniques Applied to Nuclear Reactor Vessel Thermal Annealing and Plant Life Extension, ASTM STP 1204 (American Society for Testing and materials, Philadelphia), pp. 424 – 439. Bmaev, A.D., Kryukov, A.M., Sokolov, M.A., 1993. Recovery of the transition temperature of irradiated WWER-440 vessel metal by annealing. In: Steele, L.E. (Ed.), Radiation Embrittlement of Nuclear Reactor Pressure Vessel Steels,
.
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ASTM STP 1170 (American Society for Testing and Materials, Philadelphia), pp. 369 – 379. Calculation Standards for Strength of Equipment and Pipes of Nuclear Power Units, 1989. PNAE-G-7-002-86, Erergoatomizdat, Moscow. Kryukov, A., Platonov, P., Shtrombakh, Ya., Nikolaev, V., Klausnitzer, E., Leitz, C., Rieg, C.-Y., 1996. Investigation of samples taken from Kozloduy unit 2 reactor pressure vessel. Nucl. Eng. Des. 160 (1996), 59 – 76. Leitz, C, Langer, L., Platonov, P., Shtrombakh, Ya., Kryukov, A., Rybin, V., Nikolaev, V., Rieg, C.-Y., 1996. Learning from investigation programmes on WWER 440/ 230 RPVs, Proc. 4th Int. Conf. Mat. Sci. Problems in NPP Equipment, vol. 1., CRISM ‘Prometey’, St. Petesburg, pp. 165 – 181.
Assessment of irradiation response of WWER-440 welds using samples taken from Novovoronezh unit 3 and 4 reactor pressure vessels Yu. N. Korolev a,*, A.M. Kryukov a, Yu. A. Nikolaev a, P.A. Platonov a, Ya. I. Shtrombakh a, R. Langer b, C. Leitz b, C.-Y. Reig c a
Russian Research Centre, Kurchato6 Institute, 123182 Moscow, Russia b SIEMENS AG KWU, W-8520 Erlangen, Germany c Electricite de France, France
Received 20 June 1996; received in revised form 7 May 1998; accepted 29 May 1998
Abstract The results of the study on Novovoronezh unit 3 and 4 (NV NPP-3 and 4) reactor pressure vessel (RPV) radiation embrittlement measured using subsize impact specimens (5 × 5× 27.5 mm3) fabricated from samples taken from the corresponding RPV walls are presented. The post-irradiation annealing effectiveness and the embrittlement kinetics of Novovoronezh unit 3 and 4 RPV welds under re-irradiation are discussed. Ductile-to-brittle transition temperatures (DBTT) obtained using standard Charpy (TT10 × 10) and subsize impact (TT5 × 5) specimens of trepans cut out from Novovoronezh unit 2 RPV are compared. A new relation between TT10 × 10 and TT5 × 5 has been developed. © 1998 Published by Elsevier Science S.A. All rights reserved.
1. Introduction At the present time no theory exists to combine and explain all the accumulated experimental data on the effect (and complicated interplay) of metallurgical factors (microstructure, chemical composition) and irradiation condition (irradiation temperature, fast neutron flux, fluence and spectrum) on radiation embrittlement kinetics of RPV materials. Therefore, surveillance specimen programs providing permanent monitoring of radia* Corresponding author. Tel.: +7 095 196 9612; fax: +7 095 196 1701; e-mail: [email protected]
tion embrittlement of RPV materials are necessary to afford high safety operation of NPP. Due to the lack of surveillance programs for the first generation of WWER-440/230 units, the only way to monitor RPV steel irradiation embrittlement of those NPP is cutting out small samples (templets) from RPV walls. In the second half of the 1980s, the actual radiation lifetime of a number of WWER-440 units appeared to be exhausted, and annealing of welds of the core of these RPV (the condition of these welds is one of the major factors defining the lifetime) is practically the unique remedy for it prolongation. Thus, cutting out templets is neces-
0029-5493/98/$ - see front matter © 1998 Published by Elsevier Science S.A. All rights reserved. PII S0029-5493(98)00226-X
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sary for the assessment of the actual condition of annealed RPV materials and prediction of the radiation lifetime. It is important to use subsize impact specimens instead of the standard Charpy specimens for estimation of DBTT due to the following reasons. Calculations showed that it is admissible to cut out templets with the maximal dimensions of 7× 60×95 mm3 (with consequent repair) from RPV so that its safety remains. Templets of the sizes indicated above are not appropriate for machining standard Charpy-V specimens necessary for determining the irradiation embrittlement in RPV steels in compliance with the Russian Guide (Calculation Standards for Strength of Equipment, 1989). In the beginning of the 1990s, the cutting out of templets from NV NPP-2, 3 and 4 in Russia and also from ‘Kozloduy-2’ in Bulgaria was accomplished. Within the frame of the program TACIS91, which was carried out between 1993 and 1996, the contract between EC and EdF —FRAMATOME— SIEMENS involved the program on complex validation of the method applied for assessment of the state of pressure vessel materials before and after annealing. The principal part of experimental works on cutting out the trepans and templets as well as determining the mechanical properties were carried out by two subcontractors: RRC ‘Kurchatov Institute’ and CRISM ‘Prometey’. RRC KI dealt mainly with studies of weld metal, and CRISM ‘Prometey’ —of base metal. This paper presents the experimental data on investigation of the welds obtained in RRC KI.
2. Test materials and impact test technique The material of samples taken from units 2, 3 and 4 of NV NPP, and also RPV steels of some research programs were used. Altogether, 34 materials were investigated within the frame of the research programs (Kryukov et al., 1996) and seven materials within the frame of TACIS-91. The tests of subsize impact specimens (5× 5× 27.5 mm3) were performed using a standard instrumented impact machine with a potential
energy 80 J and a pendulum impact velocity at the moment of impact 3.84 m s − 1. For the tests considered, the impact machine was equipped with an instrumented tup in accordance with ISO. One of two functions was recorded during a test: load-time or load-displacement. In addition, the impact machine was equipped with an incremental transducer of resolution 4096 counts per 360° with digital read-out in 12-byte Grey code. The incremental transducer allows measuring absorbed energy providing a maximum error of 0.078 J (1024 counts) for stored potential energy of the pendulum 80 J.
3. Results and discussion Cutting out small samples (templets) from RPV walls of some operating WWER-440 units has been carried out since 1990. This enables us to confirm both reliability of the trend curves specified by Russian Guide for estimation of radiation embrittlement of WWER-440 welds and the effectiveness of post-irradiation annealing. The templets were cut out from the internal surfaces of RPV without cladding. The data obtained in the present study were used for comparison of actual radiation embrittlement of RPV materials at different stages of operation (as-irradiated, after post-irradiation annealing and re-irradiation) with the predicted ones.
3.1. Justification of using subsize impact specimens for assessment of radiation embrittlement of RPV steel The admissible depth of templets taken from RPV surface generated a need for using subsize impact specimens for the determination of the DBTT of RPV steel. The maximum cross section of the specimens machined from these templets was 5 ×5 mm2. Therefore, it was necessary to develop a correlation between DBTT obtained using subsize impact specimens and standard Charpy specimens required by the Russian Guide (Calculation Standards for Strength of Equipment, 1989) for estimating the radiation embrittlement. To solve this problem the RRC ‘Kurchatov
Y.N. Korole6 et al. / Nuclear Engineering and Design 185 (1998) 309–317
Institute’ and CRISM ‘Prometey’ carried out experiments between 1980 and 1988 in order to determine the dependency and their relation on irradiation. Charpy specimens of 5× 5 and 10 × 10 mm2 made of steel 15Kh2MFA and its welds with different contents of impurities were tested. A significant part of these specimens was subjected to irradiation at 270°C with a fluence of 1×1020 n · cm − 2. A small part of the irradiated specimens was annealed after irradiation at 475°C for 100 h, which is the standard annealing for WWER-440 RPV. Some results of that research were published in (Kryukov et al., 1996; Leitz et al., 1996). The choice of a criterion to determine the DBTT using subsize specimens was based on the following approach. Due to the fact that in impact tests the DBTT is determined in a transition region by a fixed value of absorbed energy, the selection of this reference energy for impact specimens of different scale has been ensured by the condition of the constant ratio of this reference level to energy of fully ductile fracture, i.e. to the level of upper shelf energy (USE) region. Thus, it was supposed that for V-notched impact specimens of any size the following ratio is to be constant (Amaev et al., 1993a): KCVref/USE =const,
311
different scale (Fig. 1) shows that the following relation between the values of DBTT for 10× 10 and 5× 5 mm2 impact specimens in both as-received and irradiated conditions is valid: TT10 × 10 = 42× 0.92 TT5 × 5, °C
(2)
10 × 10
Standard deviation (s) for TT in Eq. (2) is 19°C. If the value of tangent is rounded to 1, then the standard deviation increases to 20°C. The corresponding equation is as follows: TT10 × 10 = 44× TT5 × 5, °C
(3)
It is worth noticing that the line of Eq. (3) lies within the 95% confidence band for the regression line (TT10 × 10 versus TT5 × 5) plotted using all the data shown in Fig. 1. The first and second regression coefficients of Eq. (2) were defined with a standard deviation of 3 and 0.05, respectively. Thus the free term of Eq. (2) can be varied with a confidence probability of 95% from 36 to 48. Standard deviation for the free term of Eq. (3) is 2.5, thus, the upper boundary of 95% confidence band for this parameter variation is 49°C. The following equation has been specified in 1991 to the Russian Standard for evaluating TT10 × 10 of RPV steel based on experimental data obtained using subsize impact bend specimens:
(1)
where KCVref is the reference level of absorbed energy. Hence, the ratio between reference levels of absorbed energy for impact specimens of different scales is defined by the ratio between the upper shelf energies for these specimens. Statistical treatment of the experimental data demonstrated that the reference absorbed energy of 47 J for standard Charpy specimens corresponds to the one of 6 J for impact bend specimens with section of 5×5 mm2. Alongside with a reference level of absorbed energy, some value of specimen lateral expansion is applied as a criterion for determination of DBTT. That is why, some amount of work was done for experimental substantiation of this criterion for subsize specimens. It was established that for specimens with section 5×5 mm2 this reference value is 0.35 mm. The analysis of the data from Kryukov et al. (1996) obtained by testing impact specimens of
Fig. 1. Comparison of testing standard Charpy and subsize impact bend specimens.
312
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Fig. 2. Orientation of standard Charpy and subsize specimens count out of trepan.
TT10 × 10 = 50 + TT5 × 5, °C,
(4a)
For the estimation of the 95% confidence band for the correlation between test results of standard Charpy and subsize impact specimens, the following equation was recommended: TT10 × 10 = 50 + TT5 × 5 92s, °C
(4b)
where s is 21°C.
3.2. Validation of the correlation between standard Charpy and subsize impact bend specimens For some reasons, post-irradiation annealing of NV NPP-2 was economically unexpedient and this unit was taken out of service in 1990. Decommission of NV NPP-2 enabled large samples of RPV (trepans) to be cut out of the wall throughout its thickness and to fabricate standard Charpy as well as subsize impact specimens (Fig. 2). A few pairs of series, i.e. standard Charpy and subsize specimens were fabricated from internal, external and intermediate parts of trepans in irradiated and annealed at regular conditions (at 475°C for 150 h). Two series of specimens were annealed at 560° for 2 h that simulated the unirradiated condition of the steel. All the post-irradia-
tion annealings were performed under laboratory conditions. The main difference between the data used for elaborating Eqs. (4a) and (4b) and the results of testing NV NPP-2 trepans is the long-term irradiation of the latter. Moreover, since it was a neutron flux gradient through the RPV wall different layers of the templets were exposed to irradiation to different fluences. Using the test results of NV NPP-2 trepans expanded the existing data set by the data with relatively high DBTT values (TT10 × 10 for the previous research programs was varied within the ranges −80} 140°C and −50} 236°C for the new NV NPP-2 test results). Thus, using the trepan test results was useful for validating the previously developed correlation dependence (Eqs. (2)–(4) and (4b)). The following correlation was established for impact specimens of different scale fabricated from the NV NPP-2 trepans (Fig. 3(a)): TT10 × 10 = 54+ 1.07 TT5 × 5, °C
(5)
The standard deviation of TT10 × 10 in Eq. (5) is 19.5°C. The slope of trend curve for materials with high DBTT increased in comparison with the one for materials with lower DBTT values (Fig. 1). However, the mean line specified by the Rus-
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313
Fig. 3. Comparison of test results of standard Charpy and subsize impact bend specimens.
sian Guide (Eqs. (4a) and (4b)) still lies in the 95% confidence region for regression line (the standard deviation for the coefficients of Eq. (5) were found to be 4.5 and 0.055, respectively). Fig. 3(b) presents a combination of the data obtained within the of TACIS-91 and Russian research programs developed in the 1980s (Kryukov et al., 1996). Total correlation dependence obtained from these data is closer to Eqs. (4a) and (4b) than Eq. (5): TT10 × 10 = 47 + 1.04 TT5 × 5, °C
the present investigation was to develop conservative estimation of the TT10 × 10 versus TT5 × 5 dependence. For this purpose the boundary of 95% confidence region of the regression line (Eq. (6)) was linearized and approximated by two lines (Fig. 4). The corresponding equations are as follows:
(6)
10 × 10
in Eq. (6) is 20°C. Standard deviation of TT The confidence region defined by Eqs. (4a) and (4b) and in Fig. 3 is nearly coincident. Regression analysis of the correlation TT10 × 10 versus TT5 × 5 for DBTT determined using the output criterion specified by Russian Guide (Calculation Standards for Strength of Equipment, 1989) produced the following regression line: TT10 × 10 = 45 + 0.96 TT5 × 5, °C
(7)
The corresponding dependence by the Russian Standard (Eq. (4a)) is even conservative in comparison with Eq. (7). The presented evaluation was found to be in good agreement with previously elaborated dependencies (Leitz et al., 1996). One of the aims of
Fig. 4. Conservative approach for correlation of Charpy and subsize specimens.
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TT10 × 10 = 52 + TT5 × 5 50), °C TT
10 × 10
5×5
= 52 + 1.09 TT
(TT
(8a) 5×5
]0), °C (8b)
The available experimental results (Fig. 3) prove that the regression dependence between DBTT evaluated from testing standard Charpy and subsize impact specimens and the TT10 × 10 versus TT5 × 5 dependence Eq. (4a) adopted in the Russian Standard nearly coincide. Thus, the Russian Standard (Eq. (4a)) is quite satisfactory in spite of the clearly defined tendency to acceleration of TT10 × 10 versus TT5 × 5 dependence with increasing DBTT. This effect can probably be contributed to modification of the material condition caused by long-term irradiation. To validate the authenticity of the relation specified by the Russian Standard (Eqs. (4a) and (4b)) the variation of the DBTT through the NV NPP-2 weld before and after post-irradiation annealing was measured using standard Charpy (Fig. 5(a)) and subsize impact bend specimens (Fig. 5(b)). It can be seen that the DBTT measured using standard Charpy and subsize impact bend specimens are quite comparable. The approach developed (Eqs. (8a) and (8b)) produced better assessment of DBTT changes in comparison with the Russian Standard (Eq. (4a)).
3.3. The results obtained for templets cut from NVNPP-3 and 4 RPV
Fig. 5. Variation of DBTT throughout NVNPP-2 RPV wall (weld metal) before and after post-irradiation annealing.
Small templets were cut out from the internal surfaces of NVNPP-2, 3 and 4 RPV and also from unit 2 at NPP ‘Kozoloduy’ to evaluate the actual condition of RPV materials between 1990 and 1995. The templets were cut out from all units, with the exception of NVNPP-2 before and after post-irradiation annealing and also after reirradiating. This paper includes the results of a study of these materials carried out at the ‘Kurchatov Institute’. Some results of that investigation have already been published (Leitz et al., 1996). The DBTT values for the materials of the above mentioned RPVs were determined for the templets before and after annealing as well as after re-irradiating. In addition, these values were
determined after special heat treatments, simulating a condition close to the initial (unirradiated) condition of welds. The study of templets was primarily aimed at a comparison between the actual and predicted values of radiation embrittlement, post-irradiation annealing effectiveness, and embrittlement due to re-irradiation. The experimental and predicted DBTT values of RPV materials under investigation in unirradiated, as-irradiated, annealed and re-irradiated conditions are compared in Fig. 6(a,b,c). Eqs. (8a) and (8b) was used for an estimation of the DBTT. The actual degree of radiation embrittlement for the NVNPP-4 was likewise found to be height than the inaccuracy in determination of DBTT in
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Fig. 6. Comparison of the experimental and predicted data on irradiation and annealing response of RPV welds.
315
316
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Fig. 6. (Continued)
unirradiated condition — TT0 (the calculated and experimental values of DTTF was found to be practically the same, see Fig. 6(a)). For NVNPP4, the predicted and experimental values of Charpy DBTT shifts caused by irradiation almost coincide. An estimation of DBTT (Eq. (4a)) specified by the Russian Standard and the new approach (Eqs. (8a) and (8b)) provide nearly the same results. This proves the presently adopted method of assessing radiation embrittlement of RPV steels to be valid. In the contrary, the method of evaluation of TT0 should be essentially modified. The residual embrittlement in NVNNP3 and NVNPP-4 welds after post-irradiation annealing does not exceed 20°C (Fig. 6). The result coincides with the predicted degree of recovery of weld metal properties due to annealing (Amaev et al., 1993b).
4. Conclusions (1) A relation between the values of the ductileto-brittle transition temperature obtained using standard Charpy and subsize impact bend speci-
mens was developed. The results of the study of trepans cut out from NVNNP-2 weld No. 4 and the results of previous Russian research programs were involved in elaborating these dependencies. (2) The presently valid Russian Standard at the DBTT measurement using subsize impact specimens was reliably justified. The correlation dependence proposed by Russian Standard was found to lie in the 95% confidence band of the regression line. A satisfactory correlation between DBTT shifts estimated using standard Charpy and subsize impact specimens was observed for trepans cut out from NVNNP-2 weld. (3) In order to increase the conservativity of Russian Standard, an attempt was made to develop a new approach for the TT5 × 5 versus TT10 × 10 dependence. For that purpose the upper boundary of the 95% confidence band of the regression line was linerarized and approximated by two lines. (4) Using subsize impact specimens fabricated from templets of NVNNP-3 and 4 welds, the actual condition of RPV steel was estimated in as-irradiated, annealed and re-irradiated conditions.
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