A complex research of RPV 15Kh2NMFA class 1 steel in different states was performed: in the initial state and after irradiatiion at different temperatures (50-300∘C). Low temperature (50-140∘C) irradiation was shown to cause the formation of dislocation loops of ultrahigh density. At the same time radiation-induced precipitates, being the main radiation-induced hardening phase at irradiation temperature of VVER reactors (∼300∘C), were absent. This leads to a higher radiation embrittlement rate after low-temperature irradiation as compared with irradiation at operating temperatures of VVER-1000. The radiation embrittlement coefficient values are 8.7 and 1.45 for (120-140)∘C and (290-315)∘C irradiation temperature, respectively

Bukina, Z V

Fedotova, S V

Frolov, A S

Gurovich, B A

Krikun, E V

Kuleshova, E A

Maltsev, D A

Zhurko, D A

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The Effect of Reducing Irradiation Temperature on the Structure and Radiation Embrittlement Mechanism of RPV Steels

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 KnE Materials ScienceMIE-201715th International School-Conference “New materials – Materials of innovative energy:development, characterization methods and application’’Volume 2018Conference PaperThe Effect of Reducing IrradiationTemperature on the Structure and RadiationEmbrittlement Mechanism of RPV SteelsE. A. Kuleshova1,2, B. A. Gurovich1, A. S. Frolov1, S. V. Fedotova1, D. A. Maltsev1,E. V. Krikun1, D. A. Zhurko1, and Z. V. Bukina11National Research Center «Kurchatov institute» Akademika Kurchatova sqr. 1, Moscow,123182,Russia2National Research Nuclear University «MEPhI» (Moscow Engineering Physics Institute)Kashirskoe highway 31, Moscow,115409, RussiaAbstractA complex research of RPV 15Kh2NMFA class 1 steel in different states was performed:in the initial state and after irradiatiion at different temperatures (50-300∘C). Lowtemperature (50-140∘C) irradiation was shown to cause the formation of dislocationloops of ultrahigh density. At the same time radiation-induced precipitates, being themain radiation-induced hardening phase at irradiation temperature of VVER reactors(∼300∘C), were absent. This leads to a higher radiation embrittlement rate afterlow-temperature irradiation as compared with irradiation at operating temperaturesof VVER-1000. The radiation embrittlement coefficient values are 8.7 and 1.45 for(120-140)∘C and (290-315)∘C irradiation temperature, respectively.1. IntroductionNumerous studies demonstrated occurrence of two embrittlementmechanisms, whichrelative contribution can change during irradiation: hardening mechanism - (formationof radiation defects and radiation-induced precipitates) and non-hardeningmechanism- (formation of grain boundary impurities segregation).Irradiation temperature has a significant influence on the processes in steel thatcontrol each of the embrittlement mechanisms. On the one hand, the temperaturedecrease has to slowdown the point defect of crystal lattice and decrease the prob-ability of vacancy – interstitial atom annihilation that will increase the radiation dam-age effectiveness, i.e. increase the rate of radiation defects accumulation. On theother hand, the slowdown of the point defect and atoms migration, i.e. the diffu-sion processes slowdown, can results in decreasing both the intensity of radiation-induced phase transformation and the rate of grain boundary segregation accumu-lation and, consequently, has to affect the radiation embrittlement rate of the steelHow to cite this article: E. A. Kuleshova, B. A. Gurovich, A. S. Frolov, S. V. Fedotova, D. A. Maltsev, E. V. Krikun, D. A. Zhurko, and Z. V. Bukina, (2017),“The effect of reducing irradiation temperature on the structure and radiation embrittlement mechanism of RPV steels” in 15th International School-Conference “New materials – Materials of innovative energy: development, characterization methods and application’’, KnE Materials Science, pages406–413. DOI 10.18502/kms.v4i1.2191Page 406Corresponding Author:E. V. Krikunevkrikun@mail.ruReceived: 21 December 2017Accepted: 15 April 2018Published: 6 May 2018Publishing services provided byKnowledge EE. A. Kuleshova et al. Thisarticle is distributed under theterms of the Creative CommonsAttribution License, whichpermits unrestricted use andredistribution provided that theoriginal author and source arecredited.Selection and Peer-reviewunder the responsibility of theMIE-2017 ConferenceCommittee. KnE Materials Science MIE-2017at low-temperature irradiation as compared to similar steels that were irradiated athigher temperatures.The most important characteristics for RPV-steels of nuclear VVER-type reactors aretheir resistance to thermal and radiation embrittlement. Radiation component of theductile to brittle transition temperature (DBTT) shift can be represented as:∆𝑇 𝐹𝐾 = 𝐴𝐹 ⋅ (𝐹𝐹0)𝑚(1)where∆𝑇 𝐹𝐾 – radiation component of DBTT shift,𝐹 – fluence,𝐹0 – normalization fluence,𝐴𝐹 – radiation embrittlement coefficient, m – experimentally determined parameter.The influence of irradiation on the steel structure degradation and the coefficient ofradiation embrittlement A𝐹within the temperature range (50-315)∘C was investigated.2. Materials and methodsThe paper focuses on VVER-1000 RPV 15Kh2NMFA class 1 base and weld metal spec-imens in the initial state and after accelerated irradiation in a research reactor upto fast neutron fluences (E>0.5 MeV): at irradiation temperatures (50-140)∘C upto fluences(4,5-8) 1022m−2; at irradiation temperatures (290-310)∘C up to fluences∼4,51023m−2. Table 1 shows the chemical composition of 15Kh2NMFA class 1 steel.Table 1: Chemical composition of 15Kh2NMFAA class 1 steel base metal (BM) and weld metal (WM).Material Composition, wt. %C Mn Si Ni Cr Mo V Cu P SBM 0.16 0.53 0.31 1.23 2.32 0.62 0.11 0.03 <0.005 <0.003WM 0.08 0.98 0.24 1.18 1.92 0.59 0.017 0.04 <0.008 0.006The mechanical tests (static tension and impact tests) were carried out to deter-mine the yield strength change and DBTT temperature shift (ΔT𝐾). Furthermore, todetermine radiation-induced structural components - radiation-induced precipitatesand radiation defects - analytical research methods were used: transmission electronmicroscopy (TEM) («Titan 80-300», FEI, USA); scanning electron microscopy (SEM)(«Supra 40-VP», «Merlin», Zeiss, FRG) and atom-probe tomography (APT) («LEAP4000 HR» Cameca, France). Grain boundary segregationwas studied by Auger electronspectroscopy (AES) (PHI 700, Physical Electronics, Japan-USA).DOI 10.18502/kms.v4i1.2191 Page 407 KnE Materials Science MIE-20173. Phase composition study of 15Kh2NMFA class 1 steeland its weld in the initial state and afterirradiation in (50-315) ∘C temperature rangeTo reveal regularities of radiation hardening the structural studies were performed formaterials in the initial state and after irradiation to a variety of fast neutron fluencesin different temperature ranges.3.1. The study of initial hardening carbide and carbonitride phasesPhase analysis of steel specimens in the initial state showed (according to microdiffraction patterns) that the base metal along with tempered bainite contains hard-ening phases: Me7C3, (Mo,Cr)2C type carbides, V(C,N) carbonitrides and non-metallicinclusions (manganese sulfides MnS and silicon oxides SiO2). The weld metal containsno carbonitrides. The number density of carbide phases also differs for the weld andbase metal (see Table 2) due to the higher carbon content in the base metal.Table 2: The number densities N and average sizes d of initial hardening phases in the initial state andafter irradiation for base and weld metal specimens in (50-315)∘C temperature range.Material State Number density (ρ) and average size (d) of second phasesMe7C3, V(C,N) Me2Cd, nm N, 1019m−3 d, nm N, 1022m−3 d, nm N, 1020m−3BM Initial 40-50 4.7±0.6 6-10 1.3±0.2 15-25 1.0±0.2BM F1 = 5.12 ⋅ 1022m−2 45-55 5.2±0.7 4-8 1.4±0.3 15-25 0.8±0.2BM F2 = 8.5 ⋅ 1022m−2 45-55 5.5±0.7 4-8 1.1±0.3 20-30 1.2±0.2WM Initial 45-55 1.9±0.2 - - 31±4 4.0±1.1WM F = 4.0 ⋅ 1021m−2 50-60 2.1±0.3 - - 25-35 3.0±1.1Thus, the irradiation did not cause a significant change of the initial hardening phasesin the studied base and weld metal specimens: the number density and size of secondphases match the corresponding values in the initial state (within the experimentaldata scatter). This means that the carbide and carbonitride phases do not additionallycontribute to radiation hardening (the yield strength change) after irradiation in thespecified temperature range.DOI 10.18502/kms.v4i1.2191 Page 408 KnE Materials Science MIE-20173.2. The study of radiation-induced hardening phasesand grain boundary segregation after irradiation in(50-140) ∘C temperature rangeIn 50-140∘C temperature range radiation-induced structural changes aremostly causedby formation of different types of dislocation loops with no precipitates formation.Figure 1 and Table 3 show the absence of radiation-induced hardening phases afterlow temperature irradiation in contrast to the irradiation in (290-315)∘C temperaturerange.dark field TEM-images 3d distribution maps of Ni (APT) Irradiation temperature 290-315°C    Dislocation loops Precipitates Irradiation temperature 50-140°C    Dislocation loops Precipitates  Figure 1: Radiation-induced structural elements at different irradiation temperature.Table 3 and fig. 1 show that low-temperature irradiation (50-140) ∘C leads to signif-icantly higher number density of dislocation loops as compared to the one for VVER-type temperature irradiation (∼ 300 ∘C). It can be seen that low-temperature irradiationup to ∼ 5-8 times lower fast neutron fluence leads to formation of ∼250-500 timeshigher density of dislocation loops with somewhat smaller sizes.This leads to a greater rate of radiation hardening than for VVER-1000 steel irradiatedat ∼300 ∘C: a close yield stress shift is achieved after irradiation at ∼ 300 ∘C at a fluenceDOI 10.18502/kms.v4i1.2191 Page 409 KnE Materials Science MIE-2017Table 3: Number densities N and average sizes <d> of radiation-induced structural elements in15Kh2NMFA class 1 steel after irradiation at different temperatures.Irradiationtemperature, ∘CFluence,1022m−2Dislocation loops PrecipitatesNumberdensity,1021m−3<d>, nm Numberdensity,1023m−3<d>, nm(50-70) ∘C 6 1100±200 3±1 - -(120-140) ∘C 8.5 600±100 4±1 - -(290-315)∘C 45.3 2.5±0.8 5±2 1.4±0.2 2.5±0.5of an order ofmagnitude greater (see fig. 2). Thus, lowering the irradiation temperatureled to a significant increase of radiation hardening.0 20 40 60 80 100 120050100150200250300  ∆σFigure 2: The dose dependence of yield strength for various irradiation temperatures.The average loops size increases with the irradiation temperature increase. Thus,radiation hardening is solely due to dislocation loops accumulation. At this, its densityis the higher, the lower is irradiation temperature.TEM-studies of steels after irradiation at different temperatures revealed mainly the<100> Burgers vector of dislocation loops for all the studied steels but dislocation with<111> Burgers vector were also observed.Dislocation loops in all the studied steels are mainly interstitial (its density is 1.5times higher than for vacancy-type dislocation loops).It was shown in [1] that the hardening due to <100> and 1/2<111> types interstitialsdislocation leads to a strong hardening in BCC metals.DOI 10.18502/kms.v4i1.2191 Page 410 KnE Materials Science MIE-2017Vacancy loops are also known [2] to cause higher radiation hardening as comparedto clusters. The higher is irradiation temperature, the lower is point defects contributionto radiation hardening.That is why hardening is controlled by dislocation loops at low temperatures andtheir contribution is higher than at higher temperatures.Fractographic studies demonstrated no brittle intergranular fracture in specimensirradiated at low temperature, indicating absence of grain boundary segregation. AESstudy neither revealed any grain boundary segregations as far as long-term liquidnitrogen cooling of the specimens has not resulted in getting a brittle intergranularfracture, and therefore gave no possibility to measure the grain boundary segregation.Thus, the results demonstrate that low-temperature irradiation causes no detectablegrain boundary segregation due to low diffusion mobility of atoms (primarily phos-phorus).In sum, irradiation temperature effect on nanostructure evolution is the following.3.3. Low-temperature irradiation (50-140∘C)In this temperature range radiation-induced structural changes aremostly formation ofdifferent types of dislocation loops with no precipitates formation. The average loopssize increases with the irradiation temperature increase. Thus, radiation hardening issolely due to dislocation loops accumulation. At this, its density is the higher, the loweris irradiation temperature.There is no segregation due to low diffusion mobility of atoms (primarily phospho-rus). In this regard only hardening mechanism is responsible for the ductile-to-brittletransition temperature shift.3.4. VVER-type RPV irradiation temperature (290 -315∘C)In this temperature range radiation-induced structural changes are both formationof dislocation loops and precipitates. Besides, there is formation of grain boundarysegregation. Radiation embrittlement is due to both hardening and non-hardeningmechanisms.Figure 3 shows the impact test results for determining DBTT shift for 15Kh2NMFAclass 1 steel specimens irradiated at different temperatures.Thus, the observed structural changes under the influence of low-temperature irra-diation lead to a significantly greater extent of radiation embrittlement compared withDOI 10.18502/kms.v4i1.2191 Page 411 KnE Materials Science MIE-20170 10 20 30 40 50 60050100150200250∆TK = 1.26F0.8    (290-310)°C  ∆Τ∆Τ∆ TK, °C∆T K = 8.7F0.8     (120-140)°C Figure 3: The dose dependence of the DBTT shift ΔТ for 15Kh2NMFA class 1 steel at different irradiationtemperature.irradiation at ∼300 ∘C, the operating temperature of VVER-1000 type RPV. Indeed, radi-ation embrittlement coefficient A𝐹 is 8.7 and 1.45 for irradiation at temperatures (120-140)∘C and (290-315)∘C, correspondingly, which significantly exceeds the normalizedA𝐹 values for these steels at the irradiation temperature (290-315)∘C [24].4. Conclusions1. 15Kh2NMFAA steel specimens were studied in initial state and after acceleratedfast neutron (E> 0.5 MeV) irradiation in IR-8 up to fluences (6 ⋅ 1022m−2÷ 4,5 ⋅1023m−2) in the temperature ranges (50-140)∘C and (290-315)∘C.2. Radiation hardening of 15Kh2NMFAA RPV steel at (290-315)∘C temperature irra-diation and fluences up to ∼ 4 ⋅ 1023 m−2 is mainly due to radiation-inducedprecipitates formation of high density and some contribution of radiation defects,while radiation hardening at low-temperature irradiation (50-140)∘C is causedonly by radiation defects formation – dislocation loops of ultrahigh density.3. This leads to a higher radiation embrittlement rate at low-temperature irradiation:A𝐹 values are 8.7 and 1.45 for irradiation temperature (120-140)∘C and (290-315)∘C, respectively.DOI 10.18502/kms.v4i1.2191 Page 412 KnE Materials Science MIE-2017References[1] D. Terentyev, D.J. Bacon, Yu.N. Osetsky 2010 Philosophical Magazine 90 (7–8) 1019–1033[2] Stoller R.E. 1993 Proc. Int. Conf. “Effects of Radiation on materials”.–Philadelphia: ASTMSTP 1175. 301-307.[3] Regulations to control the mechanical properties of the metal of operating VVER-1000-type RPVs based on surveillance specimens test results 1.3.2.01.0061 – 2009.DOI 10.18502/kms.v4i1.2191 Page 413

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The Effect of Reducing Irradiation Temperature on the Structure and Radiation Embrittlement Mechanism of RPV Steels

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