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Fluence rate effects on irradiation embrittlement of model alloys
International Journal of Pressure Vessels and Piping 82 (2005) 373–378 www.elsevier.com/locate/ijpvp
Fluence rate effects on irradiation embrittlement of model alloys L. Debarberisa, F. Sevinia,*, B. Acostaa, A. Kryukovb,1, D. Erakb a
European Commission, Joint Research Centre, Institute for Energy, P.O. Box 2, 1755 ZG Petten, The Netherlands b Russian Research Centre, Kurchatov Institute, Moscow, Russian Federation Received 10 January 2003; revised 31 July 2004; accepted 22 October 2004
Abstract The Reactor Pressure Vessel (RPV) material of Nuclear Power Plants (NPP) is exposed to neutron irradiation during its operation. Such exposure generally induces degradation of the mechanical and physical properties of the materials: e.g. an increase of the ductile to brittle transition temperature (DBTT) and a decrease of the upper shelf impact energy. At a given irradiation temperature, dose and neutron spectrum, the sensitivity of materials to neutron irradiation depends on their chemical composition. In particular, elements like phosphorus, P, copper, Cu, and nickel, Ni play a key role in RPV steels. The effect of fluence rate on irradiation embrittlement of RPV materials is also a key issue for the correct interpretation of accelerated data and surveillance data in view of reactor pressure vessel life assessment of nuclear reactors. Much effort was done in the last decades to tackle such issues and quite contradictory results have been obtained. Model alloys can successfully be used to study embrittlement mechanisms and the effect of fluence rate. A parametric study of the response to neutron irradiation of 32 different model alloys with systematic variation of elements (Ni from 0.004 to w2 wt%, P from 0.001 to 0.039 wt%, Cu from 0.005 to w1 wt%) was completed by some members of the European Network AMES. The irradiation of the 32 model alloys took place in the LYRA rig at the High Flux Reactor (HFR) of the Joint Research Centre, Petten, The Netherlands. Some model alloys were also irradiated in commercial reactors, namely in Rovno Nuclear Power Plant (NPP), Ukraine, and Kola NPP in Russia. Data available on these model alloys are presented and analysed in this paper, proving to be very important for the study of fluence rate effect. q 2004 Elsevier Ltd. All rights reserved. Keywords: Irradiation; Embrittlement; Fluence rate; Transition temperature
1. Neutron embrittlement The degradation of RPV steel properties caused by neutrons and gamma rays is due to the formation of vacancies, interstitials and transmutation reactions. Matrix damage as well as precipitation and grain boundary segregation of impurities are considered to play a major role in mechanical properties degradation of RPV steels. Thermal ageing may also combine with neutron embrittlement. The embrittlement of steels exposed to radiation fields (neutron and g-radiation mainly) is directly and indirectly caused by displacements of atoms from their original positions due to collisions by energetic particles. A neutron * Corresponding author. Tel.: C31 224 565139; fax: C31 224 565636. E-mail address: [email protected] (F. Sevini). 1 Visiting Scientist at the Joint Research Centre, Petten. 0308-0161/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijpvp.2004.10.002
collision with an atom of the host metal, for example, creates an event where several atoms interact in a billiard ball fashion and move from their original positions. After the collision, a so-called displacement cascade is formed containing several vacancies in the middle, surrounded by a cloud of interstitial atoms. Most of the defects recombine and a relatively small amount of point defects survive to become freely migrating point defects, which, together with collapsed cascades, take part in the radiation induced damage processes. The radiation damage of RPV materials is proportional to the number of neutrons hitting the material with high enough energy to induce atom displacement(s). The neutron fluence values in embrittlement studies are expressed on the basis of displacement per atom (dpa) or as the number of fast neutrons, with energies exceeding a certain threshold energy (e.g. EO1 MeV). However, it has been calculated that smaller displacement cascades provide
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less opportunity for recombination and for this reason moderated neutrons can be relatively more damaging. Due to these spectral effects on radiation damage, it is difficult to accurately compare embrittlement results gained in different reactor environments.The reactor pressure vessels of commercial power plants are subject to embrittlement mainly due to exposure to high-energy neutrons from the core. Thermal neutrons and g-rays are also additional sources of damage. For a given exposure, copper, phosphorus and nickel are recognised as the major deleterious element of concern for irradiation degradation of steel mechanical properties. For the majority of pressure vessels, the sensitivity of the steels can be described by trend relationships which are formulated on the basis of the concentration of the main deleterious elements in the steels; the so called ‘chemistry factor’. The understanding of irradiation embrittlement of the pressure vessels of nuclear reactors is a key issue for plant lifetime assessment and much effort has been done in the last decades to tackle this complex issue. For the understanding of mechanisms of embrittlement, and the role of elements like Ni, Cu and P, the use of model alloys has proven to be a key methodology in connection with the utilization of material testing reactors or suitable channels of commercial ones. The statistical analysis of data coming from the surveillance programmes is critical for direct understanding of vessel commercial steel behaviour. The above-mentioned methods rely on accelerated data obtained at fluence rates greater or much greater that those experienced by the vessel. The verification of possible effects related to fluence rate is crucial for the proper interpretation of accelerated data.
2. The model alloys project at JRC-IE The model alloys study carried out in the context of AMES network activities [1] is focussing on the objective of understanding the role and the influence of the content of impurities such as phosphorus, copper and nickel on the mechanical properties of steels [2]. The model alloys have been offered by the Russian Research Center ‘Kurchatov Institute’. The irradiation programme of the Model Alloys project was carried out in the LYRA irradiation facility, in the High Flux Reactor at Petten. The LYRA rig is a dedicated re-loadable irradiation facility, designed and tailored specifically to the needs of the AMES research programme in the High Flux Reactor Petten. The rig is located in the reactor pool, provided with a g-heating shield designed in order to minimize the thermal gradients and the specimen target temperature is maintained by means of a complex system of heating plates. The LYRA rig can accommodate a very large number of specimens, and control the irradiation temperature during the irradiation by means of eight independent electrical heater plates. The rig
allows for irradiation of a large number of specimens at very low thermal gradients, including the possibility for in-pile annealing, as described in several publications [3]. The testing of the 32 reference model alloys sets was successfully completed at the AMES Laboratory. The materials were fully characterised by means of Charpy Impact test, hardness measurements and STEAM, i.e. Seebeck–Thomson Effect on Aged Materials, a nondestructive technique based on the thermoelectric power, developed at the AMES Laboratory [4]. The testing of the 32 sets of irradiated samples was then carried out at VTT Finnish Research Laboratories. The results obtained were very consistent with the expectations: ductile-to-brittle transition temperature (DBTT) shifts increase with the content of the key elements [5,6]. For each level of nickel, the combined effects of copper and phosphorus to increase the DBTT shift can be clearly and consistently observed. Copper influence is always clearly observed especially for the high to very high content sets. Phosphorus plays a clear role in further increase of the transition temperature shifts. The effect of nickel is always very clear and some synergism with phosphorus and copper contents can be observed.
3. Effect of fluence rate on model alloys Seven model alloys, at low and high nickel content, irradiated in LYRA were irradiated also in Kola NPP (Russian Federation) with higher fluence but comparable fluence rate, while five of them were also irradiated in Rovno NPP (Ukraine), at different fluence rate but up to the same fluence (Fig. 1). The specimens were packed into special containers that were assembled in chains and loaded in free surveillance channels of Rovno NPP Unit 1 (Rovno-1) and Kola NPP Unit 3 (Kola-3). During irradiation in Kola-3 the specimens were in contact with the coolant, while in Rovno-1 they were irradiated in hermetic containers. The irradiation temperature was evaluated to be around 270–275 8C.
Fig. 1. Model alloys common to the three irradiation programmes.
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Table 1 Content of Ni, Cu P in model alloys and mechanical properties Model alloy
Ni (wt%)
Cu (wt%)
P (wt%)
Rp02 (MPa)
Rm (MPa)
Am (%)
A5 (%)
637 640 638 175 181 178 176
0.01 0.01 0.01 1.1 2.0 1.2 1.1
0.11 0.42 0.11 0.12 0.12 0.42 0.12
0.013 0.012 0.039 0.012 0.010 0.012 0.039
234 293 260 317 338 388 351
352 384 363 423 428 474 447
19.2 18.0 21.6 21.6 19.3 16.1 20.0
39.9 35.1 40.4 37.6 35.9 29.6 34.2
Rovno-1 has a core with dummy assemblies, while Kola3 has a full core, the neutron fluence rates in the surveillance channels of these units are then different:
significantly different fluence rate is studied in order to observe directly the effect of fluence rate on such model alloys.
† Rovno-1: w3!1011 n cm K2 s K1 (‘low EO0.5 MeV † Kola-3: w2!1012 n cm K2 s K1 (‘high EO0.5 MeV.
5. Post-irradiation results
flux’), flux’),
5.1. Comparison of LYRA–Kola data The irradiation period was approximately 1 year for both experiments. The neutron fluences accumulated by the specimens were then, respectively.
The composition (Cu, P and Ni content) and mechanical properties of the seven model alloys are given in Table 1.
The comparison of the model alloys irradiated in LYRA and KOLA is very important in order to verify the consistency of the data sets at an equal fluence rate (fLYRAZw3!1012 cmK2 sK1, EO0.5 MeV). The accumulated fluence is very different in the two cases: w8 and 80!1018 cmK2 (EO0.5 MeV) for LYRA and Kola, respectively. As can be seen in Fig. 2, the data sets are very consistent; a 1/3 power dependence of the obtained shifts is observed and the role of nickel to increase the temperature shifts is well defined. Phosphorus and copper effects can also be consistently observed, as in Fig. 3.
4.1. Charpy impact testing results
5.2. Comparison of LYRA–Rovno data
Standard Charpy V-notched specimens of the model alloys were impact tested before and after irradiation. Ductile-to-brittle transition temperature shifts and upper shelf energies (USE) were calculated from hyperbolic tangent curve fit of Charpy impact data. The 47 J impact energy criterion was chosen to evaluate the DBTT of full-size Charpy specimens (10! 10!55 mm3), and the 1.9 J impact energy criterion for mini-Charpy specimens (3!4!27 mm3). Table 2 summarises the results obtained for reference temperatures, Tk0, and transition temperature shifts, DTk. As can be seen in Table 1, different contents of phosphorus and copper are present, allowing some interesting conclusions to be drawn on element influence in connection with the fluence rate effect. The comparison of the results obtained in LYRA and in Kola NPP, at equal fluence rate was studied in order to verify the consistency of the data sets. The comparison of the results obtained in LYRA and in Rovno NPP, at equal accumulated fluence but at
The comparison of the model alloys irradiated in LYRA and Rovno is a direct visualization of the fluence rate effect since the data are obtained for equal accumulated fluence and very different fluence rates; LYRA fluence rate is w10 times greater than that in Rovno (fRovnoZw3! 1011 cmK2 sK1, EO0.5 MeV). The obtained transition temperature shifts in LYRA at higher fluence rate and in
† F1Z1!1019 n cmK2 for Rovno-1, EO0.5 MeV † F2Z8!1019 n cmK2 for Kola-3, EO0.5 MeV
4. Model alloy characterization
Table 2 Charpy impact test results: initial transition temperature and shifts Model alloy
637 640 638 175 181 178 176
Tk0 (8C)
K60 K72 K22 K47 K66 K60 K52
DTk (8C) at F1Z1!10 19 n cmK2 ROVNO
HFR
– 120 – 164 162 236 217
30 115 120 120 175 180 150
DTk (8C) at F2Z 8!10 19 n cmK2, KOLA 46 119 141 223 268 305 347
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Fig. 4. Comparison of LYRA–ROVNO data sets.
Fig. 2. Comparison LYRA–KOLA data sets; Ni effect.
Rovno at lower fluence rate are plotted in Fig. 4 as a function of nickel content. As can be seen in Fig. 4, for some alloys, independently of nickel content, there is practically no difference between the transition temperature shift obtained at lower and at higher fluence rate. Alloys containing phosphorus present instead a significant difference in shift. This is more evident in Fig. 5, where DBTT shift is shown as a function of phosphorus content.
6. Results analysis Various countries adopt different formulae to forecast the change of transition temperature due to neutron embrittlement. See for example the US Reg. Guide [7] or the Russian Guide [8]. The latter is the reference design formula for irradiation embrittlement of WWER Reactor Pressure Vessels. This guide foresees for WWER-440 RPV (low Nickel contents): DTk Z 800ðP C 0:07CuÞF1=3
(1)
The quantity 800(PC0.07Cu) is called C.F. or Chemistry factor, and depends only on copper and phosphorus content but not on Nickel.
Fig. 3. Comparison of LYRA–KOLA data sets; P effect.
We can observe that the Russian Guide foresees an exponent of 1/3 for the fluence dependence of the transition temperature shift. In the case of Russian WWER-1000 RPV material with Ni content O1% wt, the Russian Guide adopts a different formula, where the effect of Nickel is anyway not explicitly taken into account: DTk Z ½230ðCu C 10PÞ C 20F1=3
(2)
The above formulae contain a power 1/3 dependence of the fluence, which reflects the results obtained. It does not take into account the effect of fluence rate. To take into account this as well as other effects, a semimechanistic model was recently developed.
7. Semi-mechanistic modelling of neutron embrittlement The simplified semi-mechanistic radiation embrittlement model defines the irradiation shift of transition temperature evaluated by Charpy impact as the result of the contribution of three additive major mechanisms [9]. One of the basic assumptions is the nickel–phosphorus synergetic effect.
Fig. 5. Comparison of LYRA–ROVNO data sets.
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377
Hence DBTTshift (ductile-to-brittle transition temperature shift) is modelled as follows: DBTTshift Z MD C CuPr C OPrSg
(3)
where MD matrix damage CuPr copper dominated precipitations OPrSg other co-precipitations and segregations Writing explicitly the various partial effects, DBTTshift is the following: DBTTshift Z aF0:5 C b½1 K expðKF=Fsat Þ F K Fstart C c=2 1 C Tangh d
(4)
where
Fig. 7. Predictions of fluence rate effects according to the semi-mechanistic model.
a the matrix damage parameter F the neutron fluence (in 1018 n sK1 cmK2, EO1 MeV) b represents the maximum saturation value of the DBTT shift due to precipitations and segregations Fsat represents the fluence at which saturation effects begin (fluence at which 66% of b is reached) c represents the maximum saturation value of the shift due to segregation Fstart represents the fluence at which segregation starts d represents the velocity of increase of the effect to saturation It has been observed that the coefficients b and c can be assumed to be simply dependent on Cu and P initial concentrations (threshold values for copper are also considered; Cuthresholdw0.035 wt%). A linear dependence can be assumed as a first approximation: b Z b1 Cu;
c Z c1 P
where Cu, P are the Cu and P concentrations in wt% b1 and c1 are constants This model has been tested successfully on model alloys and demonstrated rather well also for VVER-440 welds
and VVER-1000 surveillance data analysis, see for example [10–13].
8. Fluence rate effect evaluation by means of the semi-mechanistic model The proposed model confirms the observations that fluence rate dependence mainly occurs at intermediate fluences (far from saturation) and for high Cu and P contents, (see Fig. 6). From the physical point of view, fluences obtained at higher fluence rates are accumulated at significantly shorter time, thus possibly reducing the effect of time-dependent processes like diffusion. It is generally assumed that fluence rate does not influence matrix damage [14]. Therefore, in the proposed model, it can only influence the time-dependent parameters: Fstart, Fsat. In particular, the time-dependent parameters are supposed to increase at increasing fluence rate. No effects are in fact observed for model alloys with low nickel content, or for alloys with high nickel content in combination with low phosphorus content. Marked effects are observed for model alloys with high nickel contents in combination with phosphorus, confirming the synergetic hypothesis of the semi-mechanistic model. Examples of model predicted fluence rate effects are given in Fig. 7. Further experimental verification is planned.
9. Summary and conclusions
Fig. 6. Observed field for fluence rate effects on model alloys.
The model alloy studies carried out within the framework of AMES focused on understanding the role and
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the influence of the content levels of elements such as phosphorus, copper, and nickel on embrittlement. The adopted matrix of model alloy compositions covers several conditions typical of different reactor systems. Such model alloys, even if the results are not immediately transferable to commercial RPV steels, are very suitable for study and the results obtained can be the basis to understand mechanisms and better focus irradiation experiments on commercial steels. The irradiation programme of the model alloys project was carried out in the LYRA pool-side facility in the High Flux Reactor of Petten. The reference samples were tested at the JRC Laboratory, and the irradiated samples in VTT. The obtained DBTT shifts demonstrate clear and consistent trends of influence of the various elements, and allow reliable prediction formulae to be developed. A unique opportunity for verifying the existence of fluence rate effects on model alloys embrittlement was offered by the comparison of the results obtained on six specific alloys, which were also irradiated in Rovno and Kola NPP. The main conclusions and achievements are summarised as follows: , The compositional matrix of the model alloys covers a wide range of P, Cu and Ni contents typical of different eastern and western reactor systems. , Comparison of the LYRA data and Kola data demonstrate full compatibility of data obtained at similar fluence rate in all cases. , Comparison of the LYRA data and Rovno data demonstrate that fluence rate is responsible for a marked effect for certain model alloys, in particular: † No effects are observed for model alloys with low nickel content, or for alloys with high nickel content in combination with low phosphorus content. † On the contrary, marked fluence rate effects are observed for model alloys with high nickel contents in combination with phosphorus. † The presented data support the prediction of the Semi-mechanistic model model of neutron embrittlement, based on the nickel-phosphorus synergism.
References [1] Sevini F, Debarberis L, To¨rro¨nen K, Gerard R, Davies LM. Development of the AMES network throughout the 4th and 5th EURATOM framework programmes. Int J Pressure Vessels Piping 2004;81:683–94. [2] Debarberis L, Sevini F, Acosta B, Kryukov A, Nikolaev Y, Amaev AD, et al. Irradiation embrittlement of model alloys and commercial steels: analysis of similitude behaviors. Int J Pressure Vessels Piping 2002;79(8–10):637–42. [3] Debarberis L, von Estorff U, Crutzen S, Beers M, Stamm H, de Vries MI, et al. LYRA and other projects on RPV steel embrittlement: study and mitigation of the AMES Network. Nuclear Eng Des 2000; 195(2):217–26. [4] Acosta B, Sevini F. Evaluation of irradiation damage effect by applying electric properties based techniques. Nuclear Eng Des 2004; 229(2–3):165–73. [5] Debarberis L, To¨rro¨nen K, Sevini F, Acosta B, Kryukov A, Nikolaev Y, etal. Experimental studies of Copper, Phosphorus and Nickel effect on RPV model alloys at two different fluences. Sixth international conference on materials issues in design, manufacturing and operation of NPP equipment, St Petersburg, Russia, VTT; 19–23 2000. [6] Debarberis L. The effect of Nickel,Phosphorusand Copperin irradiation embrittlement of RPV steel model alloys. NATO international workshop on RP vessel embrittlement, Varna, Bulgaria; September 2000. [7] Eason ED, Wright JE, Odette GR. Improved embrittlement correlations for reactor pressure vessel steels. NUREG/CR-6551 MCS 970501. [8] Stress analysis standards for equipment and piping at nuclear power plants. Russian Code PNAE G-7-002-86. Moscow. [9] Debarberis L, Kryukov A, Gillemot F, Valo M, Morozov A, Brumovsky M, et al. Use of the semi-mechanistic analytical model to analyse radiation embrittlement of model alloys: Cu and P effects. Int J Problems Strengths 2004 [ISSN 0556-171X, UDC 539.4]. [10] Debarberis L, Kryukov A, Gillemot F, Valo M, Morozov A, Brumovsky M, et al. Use of the semi-mechanistic analytical model to analyse embrittlement of model alloys; Cu and P effects. Mater World 2003;4(1) [www.materialsworld.uni-miskolc.hu]. [11] Debarberis L, Kryukov A, Gillemot F, Valo M, Nikolaev A, Brumovsky M, et al. Ni inclusion in semi-mechanistic analytical model for radiation embrittlement of model alloys and combined effects with Cu and P. International conference on irradiation embrittlement, Gus, Russia 2004. [12] Debarberis L, Kryukov A, Erak D, Kevorkyan Yu, Zhurko D. Advanced methods for VVER RPV embrittlement assessment. Int J Pressure Vessels Piping 2004;81:695–701. [13] Debarberis L, Acosta B, Sevini F, Kryukov A, Zhurko D. Mechanistic interpretation and forecasting of radiation damage of high Ni RPV materials data. IAEA international conference on irradiation embrittlement, Gus, Russia 2004. [14] Conclusions from the EPRI-CRIEPI workshop on dose rate effects in RPV materials. Olympic Valley, CA: Squaw Creek Resort; November 12–14, 2001.
Fluence rate effects on irradiation embrittlement of model alloys L. Debarberisa, F. Sevinia,*, B. Acostaa, A. Kryukovb,1, D. Erakb a
European Commission, Joint Research Centre, Institute for Energy, P.O. Box 2, 1755 ZG Petten, The Netherlands b Russian Research Centre, Kurchatov Institute, Moscow, Russian Federation Received 10 January 2003; revised 31 July 2004; accepted 22 October 2004
Abstract The Reactor Pressure Vessel (RPV) material of Nuclear Power Plants (NPP) is exposed to neutron irradiation during its operation. Such exposure generally induces degradation of the mechanical and physical properties of the materials: e.g. an increase of the ductile to brittle transition temperature (DBTT) and a decrease of the upper shelf impact energy. At a given irradiation temperature, dose and neutron spectrum, the sensitivity of materials to neutron irradiation depends on their chemical composition. In particular, elements like phosphorus, P, copper, Cu, and nickel, Ni play a key role in RPV steels. The effect of fluence rate on irradiation embrittlement of RPV materials is also a key issue for the correct interpretation of accelerated data and surveillance data in view of reactor pressure vessel life assessment of nuclear reactors. Much effort was done in the last decades to tackle such issues and quite contradictory results have been obtained. Model alloys can successfully be used to study embrittlement mechanisms and the effect of fluence rate. A parametric study of the response to neutron irradiation of 32 different model alloys with systematic variation of elements (Ni from 0.004 to w2 wt%, P from 0.001 to 0.039 wt%, Cu from 0.005 to w1 wt%) was completed by some members of the European Network AMES. The irradiation of the 32 model alloys took place in the LYRA rig at the High Flux Reactor (HFR) of the Joint Research Centre, Petten, The Netherlands. Some model alloys were also irradiated in commercial reactors, namely in Rovno Nuclear Power Plant (NPP), Ukraine, and Kola NPP in Russia. Data available on these model alloys are presented and analysed in this paper, proving to be very important for the study of fluence rate effect. q 2004 Elsevier Ltd. All rights reserved. Keywords: Irradiation; Embrittlement; Fluence rate; Transition temperature
1. Neutron embrittlement The degradation of RPV steel properties caused by neutrons and gamma rays is due to the formation of vacancies, interstitials and transmutation reactions. Matrix damage as well as precipitation and grain boundary segregation of impurities are considered to play a major role in mechanical properties degradation of RPV steels. Thermal ageing may also combine with neutron embrittlement. The embrittlement of steels exposed to radiation fields (neutron and g-radiation mainly) is directly and indirectly caused by displacements of atoms from their original positions due to collisions by energetic particles. A neutron * Corresponding author. Tel.: C31 224 565139; fax: C31 224 565636. E-mail address: [email protected] (F. Sevini). 1 Visiting Scientist at the Joint Research Centre, Petten. 0308-0161/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijpvp.2004.10.002
collision with an atom of the host metal, for example, creates an event where several atoms interact in a billiard ball fashion and move from their original positions. After the collision, a so-called displacement cascade is formed containing several vacancies in the middle, surrounded by a cloud of interstitial atoms. Most of the defects recombine and a relatively small amount of point defects survive to become freely migrating point defects, which, together with collapsed cascades, take part in the radiation induced damage processes. The radiation damage of RPV materials is proportional to the number of neutrons hitting the material with high enough energy to induce atom displacement(s). The neutron fluence values in embrittlement studies are expressed on the basis of displacement per atom (dpa) or as the number of fast neutrons, with energies exceeding a certain threshold energy (e.g. EO1 MeV). However, it has been calculated that smaller displacement cascades provide
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less opportunity for recombination and for this reason moderated neutrons can be relatively more damaging. Due to these spectral effects on radiation damage, it is difficult to accurately compare embrittlement results gained in different reactor environments.The reactor pressure vessels of commercial power plants are subject to embrittlement mainly due to exposure to high-energy neutrons from the core. Thermal neutrons and g-rays are also additional sources of damage. For a given exposure, copper, phosphorus and nickel are recognised as the major deleterious element of concern for irradiation degradation of steel mechanical properties. For the majority of pressure vessels, the sensitivity of the steels can be described by trend relationships which are formulated on the basis of the concentration of the main deleterious elements in the steels; the so called ‘chemistry factor’. The understanding of irradiation embrittlement of the pressure vessels of nuclear reactors is a key issue for plant lifetime assessment and much effort has been done in the last decades to tackle this complex issue. For the understanding of mechanisms of embrittlement, and the role of elements like Ni, Cu and P, the use of model alloys has proven to be a key methodology in connection with the utilization of material testing reactors or suitable channels of commercial ones. The statistical analysis of data coming from the surveillance programmes is critical for direct understanding of vessel commercial steel behaviour. The above-mentioned methods rely on accelerated data obtained at fluence rates greater or much greater that those experienced by the vessel. The verification of possible effects related to fluence rate is crucial for the proper interpretation of accelerated data.
2. The model alloys project at JRC-IE The model alloys study carried out in the context of AMES network activities [1] is focussing on the objective of understanding the role and the influence of the content of impurities such as phosphorus, copper and nickel on the mechanical properties of steels [2]. The model alloys have been offered by the Russian Research Center ‘Kurchatov Institute’. The irradiation programme of the Model Alloys project was carried out in the LYRA irradiation facility, in the High Flux Reactor at Petten. The LYRA rig is a dedicated re-loadable irradiation facility, designed and tailored specifically to the needs of the AMES research programme in the High Flux Reactor Petten. The rig is located in the reactor pool, provided with a g-heating shield designed in order to minimize the thermal gradients and the specimen target temperature is maintained by means of a complex system of heating plates. The LYRA rig can accommodate a very large number of specimens, and control the irradiation temperature during the irradiation by means of eight independent electrical heater plates. The rig
allows for irradiation of a large number of specimens at very low thermal gradients, including the possibility for in-pile annealing, as described in several publications [3]. The testing of the 32 reference model alloys sets was successfully completed at the AMES Laboratory. The materials were fully characterised by means of Charpy Impact test, hardness measurements and STEAM, i.e. Seebeck–Thomson Effect on Aged Materials, a nondestructive technique based on the thermoelectric power, developed at the AMES Laboratory [4]. The testing of the 32 sets of irradiated samples was then carried out at VTT Finnish Research Laboratories. The results obtained were very consistent with the expectations: ductile-to-brittle transition temperature (DBTT) shifts increase with the content of the key elements [5,6]. For each level of nickel, the combined effects of copper and phosphorus to increase the DBTT shift can be clearly and consistently observed. Copper influence is always clearly observed especially for the high to very high content sets. Phosphorus plays a clear role in further increase of the transition temperature shifts. The effect of nickel is always very clear and some synergism with phosphorus and copper contents can be observed.
3. Effect of fluence rate on model alloys Seven model alloys, at low and high nickel content, irradiated in LYRA were irradiated also in Kola NPP (Russian Federation) with higher fluence but comparable fluence rate, while five of them were also irradiated in Rovno NPP (Ukraine), at different fluence rate but up to the same fluence (Fig. 1). The specimens were packed into special containers that were assembled in chains and loaded in free surveillance channels of Rovno NPP Unit 1 (Rovno-1) and Kola NPP Unit 3 (Kola-3). During irradiation in Kola-3 the specimens were in contact with the coolant, while in Rovno-1 they were irradiated in hermetic containers. The irradiation temperature was evaluated to be around 270–275 8C.
Fig. 1. Model alloys common to the three irradiation programmes.
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Table 1 Content of Ni, Cu P in model alloys and mechanical properties Model alloy
Ni (wt%)
Cu (wt%)
P (wt%)
Rp02 (MPa)
Rm (MPa)
Am (%)
A5 (%)
637 640 638 175 181 178 176
0.01 0.01 0.01 1.1 2.0 1.2 1.1
0.11 0.42 0.11 0.12 0.12 0.42 0.12
0.013 0.012 0.039 0.012 0.010 0.012 0.039
234 293 260 317 338 388 351
352 384 363 423 428 474 447
19.2 18.0 21.6 21.6 19.3 16.1 20.0
39.9 35.1 40.4 37.6 35.9 29.6 34.2
Rovno-1 has a core with dummy assemblies, while Kola3 has a full core, the neutron fluence rates in the surveillance channels of these units are then different:
significantly different fluence rate is studied in order to observe directly the effect of fluence rate on such model alloys.
† Rovno-1: w3!1011 n cm K2 s K1 (‘low EO0.5 MeV † Kola-3: w2!1012 n cm K2 s K1 (‘high EO0.5 MeV.
5. Post-irradiation results
flux’), flux’),
5.1. Comparison of LYRA–Kola data The irradiation period was approximately 1 year for both experiments. The neutron fluences accumulated by the specimens were then, respectively.
The composition (Cu, P and Ni content) and mechanical properties of the seven model alloys are given in Table 1.
The comparison of the model alloys irradiated in LYRA and KOLA is very important in order to verify the consistency of the data sets at an equal fluence rate (fLYRAZw3!1012 cmK2 sK1, EO0.5 MeV). The accumulated fluence is very different in the two cases: w8 and 80!1018 cmK2 (EO0.5 MeV) for LYRA and Kola, respectively. As can be seen in Fig. 2, the data sets are very consistent; a 1/3 power dependence of the obtained shifts is observed and the role of nickel to increase the temperature shifts is well defined. Phosphorus and copper effects can also be consistently observed, as in Fig. 3.
4.1. Charpy impact testing results
5.2. Comparison of LYRA–Rovno data
Standard Charpy V-notched specimens of the model alloys were impact tested before and after irradiation. Ductile-to-brittle transition temperature shifts and upper shelf energies (USE) were calculated from hyperbolic tangent curve fit of Charpy impact data. The 47 J impact energy criterion was chosen to evaluate the DBTT of full-size Charpy specimens (10! 10!55 mm3), and the 1.9 J impact energy criterion for mini-Charpy specimens (3!4!27 mm3). Table 2 summarises the results obtained for reference temperatures, Tk0, and transition temperature shifts, DTk. As can be seen in Table 1, different contents of phosphorus and copper are present, allowing some interesting conclusions to be drawn on element influence in connection with the fluence rate effect. The comparison of the results obtained in LYRA and in Kola NPP, at equal fluence rate was studied in order to verify the consistency of the data sets. The comparison of the results obtained in LYRA and in Rovno NPP, at equal accumulated fluence but at
The comparison of the model alloys irradiated in LYRA and Rovno is a direct visualization of the fluence rate effect since the data are obtained for equal accumulated fluence and very different fluence rates; LYRA fluence rate is w10 times greater than that in Rovno (fRovnoZw3! 1011 cmK2 sK1, EO0.5 MeV). The obtained transition temperature shifts in LYRA at higher fluence rate and in
† F1Z1!1019 n cmK2 for Rovno-1, EO0.5 MeV † F2Z8!1019 n cmK2 for Kola-3, EO0.5 MeV
4. Model alloy characterization
Table 2 Charpy impact test results: initial transition temperature and shifts Model alloy
637 640 638 175 181 178 176
Tk0 (8C)
K60 K72 K22 K47 K66 K60 K52
DTk (8C) at F1Z1!10 19 n cmK2 ROVNO
HFR
– 120 – 164 162 236 217
30 115 120 120 175 180 150
DTk (8C) at F2Z 8!10 19 n cmK2, KOLA 46 119 141 223 268 305 347
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Fig. 4. Comparison of LYRA–ROVNO data sets.
Fig. 2. Comparison LYRA–KOLA data sets; Ni effect.
Rovno at lower fluence rate are plotted in Fig. 4 as a function of nickel content. As can be seen in Fig. 4, for some alloys, independently of nickel content, there is practically no difference between the transition temperature shift obtained at lower and at higher fluence rate. Alloys containing phosphorus present instead a significant difference in shift. This is more evident in Fig. 5, where DBTT shift is shown as a function of phosphorus content.
6. Results analysis Various countries adopt different formulae to forecast the change of transition temperature due to neutron embrittlement. See for example the US Reg. Guide [7] or the Russian Guide [8]. The latter is the reference design formula for irradiation embrittlement of WWER Reactor Pressure Vessels. This guide foresees for WWER-440 RPV (low Nickel contents): DTk Z 800ðP C 0:07CuÞF1=3
(1)
The quantity 800(PC0.07Cu) is called C.F. or Chemistry factor, and depends only on copper and phosphorus content but not on Nickel.
Fig. 3. Comparison of LYRA–KOLA data sets; P effect.
We can observe that the Russian Guide foresees an exponent of 1/3 for the fluence dependence of the transition temperature shift. In the case of Russian WWER-1000 RPV material with Ni content O1% wt, the Russian Guide adopts a different formula, where the effect of Nickel is anyway not explicitly taken into account: DTk Z ½230ðCu C 10PÞ C 20F1=3
(2)
The above formulae contain a power 1/3 dependence of the fluence, which reflects the results obtained. It does not take into account the effect of fluence rate. To take into account this as well as other effects, a semimechanistic model was recently developed.
7. Semi-mechanistic modelling of neutron embrittlement The simplified semi-mechanistic radiation embrittlement model defines the irradiation shift of transition temperature evaluated by Charpy impact as the result of the contribution of three additive major mechanisms [9]. One of the basic assumptions is the nickel–phosphorus synergetic effect.
Fig. 5. Comparison of LYRA–ROVNO data sets.
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Hence DBTTshift (ductile-to-brittle transition temperature shift) is modelled as follows: DBTTshift Z MD C CuPr C OPrSg
(3)
where MD matrix damage CuPr copper dominated precipitations OPrSg other co-precipitations and segregations Writing explicitly the various partial effects, DBTTshift is the following: DBTTshift Z aF0:5 C b½1 K expðKF=Fsat Þ F K Fstart C c=2 1 C Tangh d
(4)
where
Fig. 7. Predictions of fluence rate effects according to the semi-mechanistic model.
a the matrix damage parameter F the neutron fluence (in 1018 n sK1 cmK2, EO1 MeV) b represents the maximum saturation value of the DBTT shift due to precipitations and segregations Fsat represents the fluence at which saturation effects begin (fluence at which 66% of b is reached) c represents the maximum saturation value of the shift due to segregation Fstart represents the fluence at which segregation starts d represents the velocity of increase of the effect to saturation It has been observed that the coefficients b and c can be assumed to be simply dependent on Cu and P initial concentrations (threshold values for copper are also considered; Cuthresholdw0.035 wt%). A linear dependence can be assumed as a first approximation: b Z b1 Cu;
c Z c1 P
where Cu, P are the Cu and P concentrations in wt% b1 and c1 are constants This model has been tested successfully on model alloys and demonstrated rather well also for VVER-440 welds
and VVER-1000 surveillance data analysis, see for example [10–13].
8. Fluence rate effect evaluation by means of the semi-mechanistic model The proposed model confirms the observations that fluence rate dependence mainly occurs at intermediate fluences (far from saturation) and for high Cu and P contents, (see Fig. 6). From the physical point of view, fluences obtained at higher fluence rates are accumulated at significantly shorter time, thus possibly reducing the effect of time-dependent processes like diffusion. It is generally assumed that fluence rate does not influence matrix damage [14]. Therefore, in the proposed model, it can only influence the time-dependent parameters: Fstart, Fsat. In particular, the time-dependent parameters are supposed to increase at increasing fluence rate. No effects are in fact observed for model alloys with low nickel content, or for alloys with high nickel content in combination with low phosphorus content. Marked effects are observed for model alloys with high nickel contents in combination with phosphorus, confirming the synergetic hypothesis of the semi-mechanistic model. Examples of model predicted fluence rate effects are given in Fig. 7. Further experimental verification is planned.
9. Summary and conclusions
Fig. 6. Observed field for fluence rate effects on model alloys.
The model alloy studies carried out within the framework of AMES focused on understanding the role and
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the influence of the content levels of elements such as phosphorus, copper, and nickel on embrittlement. The adopted matrix of model alloy compositions covers several conditions typical of different reactor systems. Such model alloys, even if the results are not immediately transferable to commercial RPV steels, are very suitable for study and the results obtained can be the basis to understand mechanisms and better focus irradiation experiments on commercial steels. The irradiation programme of the model alloys project was carried out in the LYRA pool-side facility in the High Flux Reactor of Petten. The reference samples were tested at the JRC Laboratory, and the irradiated samples in VTT. The obtained DBTT shifts demonstrate clear and consistent trends of influence of the various elements, and allow reliable prediction formulae to be developed. A unique opportunity for verifying the existence of fluence rate effects on model alloys embrittlement was offered by the comparison of the results obtained on six specific alloys, which were also irradiated in Rovno and Kola NPP. The main conclusions and achievements are summarised as follows: , The compositional matrix of the model alloys covers a wide range of P, Cu and Ni contents typical of different eastern and western reactor systems. , Comparison of the LYRA data and Kola data demonstrate full compatibility of data obtained at similar fluence rate in all cases. , Comparison of the LYRA data and Rovno data demonstrate that fluence rate is responsible for a marked effect for certain model alloys, in particular: † No effects are observed for model alloys with low nickel content, or for alloys with high nickel content in combination with low phosphorus content. † On the contrary, marked fluence rate effects are observed for model alloys with high nickel contents in combination with phosphorus. † The presented data support the prediction of the Semi-mechanistic model model of neutron embrittlement, based on the nickel-phosphorus synergism.
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