VVER-1000 RPV Research Articles

Influence of high fluence irradiation on reference temperature of VVER-440 RPV surveillance specimens

Master curve (MC) testing of VVER-440 RPV surveillance specimens treated for 27 years (∼200,000 h) in a surveillance channel of Metsamor-NPP was performed to investigate the influence of long term high fluence irradiation on RPV embrittlement. The surveillance chain consisted of both thermal aged specimens (above the core level) and irradiated specimens (inside the core). The reference temperature (To) values obtained from irradiated specimens are compared with the results from thermal aged specimens to characterize irradiation induced shifts in To values for both base and weld metal specimens. It was found that the high fluence irradiation up to a nominal fluence of 3.2 × 1025 n.m-2 at E > 0.5 MeV resulted large embrittlement with To shift values greater than 300 °C for both base and weld metal specimens. The obtained shifts in To values at these high fluence values were used to compare with the predictions from PNAE procedure in the Russian regulatory guide outside its validity range. It was found that the measured shift in reference temperature for the weld metal was well below the predicted value while, the shift in reference temperature for base metal was largely under-predicted at these high fluence values.

Prediction of radiation embrittlement of VVER-1000 RPV welded seam material at lifetime extension up to 60 years and more

The correctness of the model assumption about additivity of the contributions of hardening (formation of dislocation loops and radiation-induced precipitates) and non-hardening (formation and accumulation of grain boundary segregations) mechanisms to the final radiation embrittlement of VVER-1000 RPV weld materials has been verified. The paper presents the results of the experiment to obtain predicted values of the critical embrittlement temperature of weld metal with nickel content of 1.59–1.88% using accelerated irradiation in the IR-8 research reactor from states characterized by different starting level of grain boundary segregation. The procedure of accounting for the flux effect for the values of critical embrittlement temperature obtained using accelerated irradiation has been proposed. The conservativity of the weld metal radiation embrittlement dependence has been verified based on the modal assumption of additivity of the contributions of the strengthening and non-strengthening mechanisms up to the values of fast neutron fluence ~75–1022 neutrons/m2.

Effect of the Operation Factors on the Properties of the Heat Affected Zone in VVER-1000 RPV according to the Results of Surveillance Specimens

Effect of the Operation Factors on the Properties of the Heat Affected Zone in VVER-1000 RPV according to the Results of Surveillance Specimens

The regularities of creep deformation and failure of the VVER’s pressure vessel steel 15Kh2NMFA-A in air and argon at temperature range 500–900 °C”

• Creep tension tests of VVER vessel steel 15Kh2NMFA-A at 500–900 °C in air and Ar and duration up to 100 hrs. • Test specimens were made of the fragment of unirradiated VVER-1000 RPV. • Secondary creep rate and time to failure were determined. • Singularity of creep process is due to the phase transformations in range from 770 to 820 °C. • The creep mechanism in 15Kh2NMFA-A steel is the same in temperature range from 700 to 900 °C. The paper presents the results of the determination the creep strength properties and both the microstructure and regularities of high-temperature creep behavior of unirradiated ferritic-pearlitic steel 15Kh2NMFA-A within the temperature range from 500 to 900 °C and the creep process duration up to 100 h. It is extremely necessary to obtain the such experimental data characterizing the creep strength properties of this steel under the condition of high-temperature creep to predict the behavior of the VVER’s RPV structure in the severe accident when the pressure vessel structure undergoes both force and high-intensive thermal action from the melted core materials. This ferritic-pearlitic steel 15Kh2NMFA-A (2% Cr, 1% Ni, 0.5% Mn, 0.5% Mo) is one of basic steel types used in fabrication of reactor pressure vessels of the VVER type designed in Russia. This investigation included several series of creep tension tests of the specimens made of 15Kh2NMFA-A steel. In addition, an analysis of the microstructure of the steel under study was carried out for a number of samples after the creep tests. For the creep tests, several sets of specimens made of the fragment of unirradiated VVER-1000 RPV. The specimen’s working part was 8 mm in diameter and the gauge length was ~50 mm. Isothermal creep tests under uniaxial tension were carried out in air up to 650 °C and in shielding argon medium within the temperature range from 700 to 900 °C. Basing on mathematical processing of test results, the values of steady-state creep rate and the values of corresponding parameters ( B ) and ( n ) were determined. The analysis of obtained results showed that within the temperature range from 750 to 900 °C, the creep mechanism of 15Kh2NMFA-A steel is the same. A sharp variation of parameters ( B ) and ( n ) (describing the steady-state creep rate of this steel) is observed within the temperature range from 500 to 750 °C. At temperature of more than 750 °C, the value log( B ) is varied within the range from −9.65 to −8.48 and the values of parameter ( n ) are within the range from 4.72 to 4.82. The results of creep tests show that both creep failure time and secondary creep rate values of the 15Kh2NMFA-A steel at 800 °C coincide very closely with the analogous ones for the specimen tests under analogous conditions at 850 °C. It is obvious that such singularity is due to the simultaneous presence of several phases in the 15Kh2NMFA-A steel during the phase transformations in temperature range from 770 to 820 °C. The formation of oxide layer on the specimen surface under the creep condition in air at temperature of 800 °C and more than the given one changes considerably the model of deformation and fracture of the investigated steel under the creep condition. We defined the parameters for the constitutive equation of the Larson-Miller type (LMP) for this steel by statistical processing the results of creep tests within the temperature range from 500 to 900 °C. The analysis of the obtained LMP equation for this steel revealed an important fact that within the range of temperature values Ac1-Ac3, where microstructure transformations of steel take place, we observe essential differences between experimental data and corresponding values defined by using the obtained LMP dependence. When estimating rupture time of this steel under the condition of high-temperature creep, such a difference between the specified values results in a considerable error (by more than the order of magnitude).

Complex study of grain boundary segregation in long-term irradiated reactor pressure vessel steels

Grain boundary (GB) embrittlement of reactor pressure vessel steel occurs at the long-term operation through the radiation-enhanced segregation of impurities. All the known techniques of studying the GB segregation have their advantages and disadvantages, and complete information can be obtained only through a comprehensive analysis by different methods.In this paper the level of GB segregation in VVER-1000 RPV steels with high nickel content was assessed by combined methods of Auger electron spectroscopy (by the P monolayer coverage and atomic concentration of other elements); atom probe tomography (by the Gibbsian interfacial excess) and fractographic analysis (by the maximum fraction of brittle intergranular fracture).The results obtained by different methods are in good agreement and show that phosphorus GB segregation increases with the increase of fast neutron fluence, GB concentration of Ni and Mn, the bulk concentration of Ni and Mn, and correlates with the fraction of brittle intergranular fracture.

APT-studies of phase formation features in VVER-440 RPV weld and base metal in irradiation-annealing cycles

This paper presents a thorough APT study of VVER-440 RPVs WM and BM samples cut out from the inner RPV surface. The features of phase formation (number densities, sizes and composition) in VVER-440 RPV steels under primary irradiation, recovery annealing, re-irradiation, recovery re-annealing and subsequent accelerated re-irradiation are identified to justify its lifetime extension up to 60 years by recovery re-annealing.It is shown that primary and re-irradiation is accompanied by formation of radiation-induced Cu-P-based precipitates whereas recovery annealing leads to their partial dissolution and coarsening of the undissolved ones. At this, for both WM and BM under primary annealing, Cu dissolves in the matrix, while under re-annealing Cu almost doesn't return to the matrix. In WM P fully dissolves in the matrix under annealing while in BM there is gradual matrix P depletion under operation (including recovery annealing) due to formation of P grain boundary segregation. Under the third irradiation cycle, Cu almost doesn't contribute to precipitation: in WM radiation-induced precipitates are P-based, enriched with Si, Ni and Mn, while in BM there are Si-based precipitates, enriched with Ni and Mn. In BM number density of precipitates is lower than in WM during all operation stages that causes its lower radiation embrittlement.

Comparative studies of brittle fracture mechanism for standard and reconstructed CT specimens made of VVER-1000 RPV materials

Before 2002, SE(B) specimens were included in surveillance specimen sets for operating VVER-1000 reactors. The transition from SE(B) specimens to CT specimens in the fracture toughness tests is caused by the necessity to reduce the scatter and the conservativeness of results caused by the scale increase and the loading scheme. To make such a transition possible, SE(B) specimens were reconstructed into CT specimens. The comparative fractographic analysis of standard and reconstructed CT specimens after the fracture toughness tests shows that the crack initiation in both types starts from the “origin.” The types of “origins” and their ratios do not change, and the test results and the fractographic analysis data for standard and reconstructed CT specimens are described by one analytical dependence CTOD(CID) for each material and state.

Monte Carlo evaluation of neutron irradiation damage to the VVER-1000 RPV

Neutron irradiation damage is of the most critical damage mechanisms in nuclear power plant's pressure vessel. Neutrons transfer their kinetic energy to target atoms of RPV that start to jump creating vacancies and interstitials known as Frenkel Pair (FP). The FPs are responsible for formation of defected clusters and microstructural modifications (e.g. phase reactions, segregations). These effects deteriorate physical and mechanical properties of the RPV steels among which are increasing the hardness and decreasing the embrittlement which results in limiting the life-time of RPV. The most sensitive location in the RPV related neutron irradiation damages is in the beltline region and adjacent to the reactor core. Welds and their heat affected zones (HAZs) in this region are of particularly importance due to higher probability flaws compared with the base metal. In this paper, Monte Carlo simulation of the detailed reactor core have led to identifying the areas maximum neutron flux in the RPV. Then the SPECTER and SRIM Monte Carlo codes are used to evaluate the neutron spectral-averaged DPA values for the beltline region of RPV.

Modernization of Cross Section Library for VVER-1000 Type Reactors Internals and Pressure Vessel Dosimetry

The broad-group library BGL1000_B7 for neutron and gamma transport calculations in VVER-1000 internals, RPV and shielding was carried out on a base of fine-group library v7-200n47g from SCALE-6 system. The comparison of the library BGL1000_B7 with the library v7-200n47g and the library BGL1000 (the latter is using for VVER-1000 calculations) is demonstrated on several calculation and experimental tests.

Influence of operation factors on brittle fracture initiation and critical local normal stress in SE(B) type specimens of VVER reactor pressure vessel steels

A complex of mechanical tests and fractographic studies of VVER-1000 RPV SE(B) type surveillance specimens was carried out: the brittle fracture origins were revealed (non-metallic inclusions and structural boundaries) and the correlation between fracture toughness parameters (CTOD) and fracture surface parameters (CID) was established. A computational and experimental method of the critical local normal stress determination for different origin types was developed. The values of the critical local normal stress for the structural boundary origin type both for base and weld metal after thermal exposure and neutron irradiation are lower than that for initial state due to the lower cohesive strength of grain boundaries as a result of phosphorus segregation.

Evolution of microstructure and mechanical properties of VVER-1000 RPV steels under re-irradiation

This is a comprehensive study of microstructure and mechanical properties evolution at re-irradiation after recovery annealing of VVER-1000 RPV weld and base metals as well as the effect of annealing on the microstructure and properties of base metal in the zone of the temperature gradient that is implemented during annealing using special heating device. It is shown that the level of radiation-induced microstructural changes under accelerated re-irradiation of weld and base metal is not higher than for the primary irradiation. Thus, we can predict that re-embrittlement of VVER-1000 RPV materials considering the flux effect will not exceed the typical embrittlement rate for the primary irradiation.

Evolution of structure and properties of VVER-1000 RPV steels under accelerated irradiation up to beyond design fluences

In this paper comprehensive studies of structure and properties of VVER-1000 RPV steels after the accelerated irradiation to fluences corresponding to extended lifetime up to 60years or more as well as comparative studies of materials irradiated with different fluxes were carried out. The significant flux effect is confirmed for the weld metal (nickel concentration⩾1.35%) which is mainly due to development of reversible temper brittleness. The rate of radiation embrittlement of VVER-1000 RPV steels under operation up to 60years and more (based on the results of accelerated irradiation considering flux effect for weld metal) is expected not to differ significantly from the observed rate under irradiation within surveillance specimens.

Thermal ageing mechanisms of VVER-1000 reactor pressure vessel steels

In this paper a complex of microstructural studies (TEM and SEM) and a comparative analysis of the results of these studies with the data of mechanical tests of temperature sets of VVER-1000 RPV surveillance specimens with exposure times up to ∼200,000h were conducted. Special annealing of control and temperature sets of SS which provides the dissolution of grain boundary segregation was performed to clarify the mechanisms of thermal ageing. It was demonstrated that during long-term exposures up to 200,000h at the operating temperature of about 310–320°C thermal ageing effects reveal themselves only for the weld metal (Ni content⩾1.35%) and are the result of grain boundary segregation accumulation (development of reversible temper brittleness). The obtained results improve the accuracy of prediction of the thermal ageing rate of VVER-1000 materials in case of RPV service life extension up to 60years.

Investigation of the Microstructure Factors Affecting the Brittle Fracture Initiation of Pre-Cracked Charpy-Type Samples of VVER-1000 RPV Steel

The correlation between fracture toughness parameter KJc and cleavage initiation distance (CID) for the three point bending (3PB) pre-crecked Charpy type specimens of VVER-1000 reactor pressure vessel base and weld metals was observed. Two types of brittle fracture origin sites were found: nonmetallic inclusions and grain or subgrain boundaries. It was shown that KJc values are shifted to the higher temperature area for weld metal with respect to base metal data. In case when the initiation origin is grain or subgrain boundary, the KJc values are higher for base metal at the same CID values. This indicates the higher crack resistance of base metal.

Evolution of weld metals nanostructure and properties under irradiation and recovery annealing of VVER-type reactors

The results of VVER-440 steel Sv-10KhMFT and VVER-1000 steel SV-10KhGNMAA investigations by transmission electron microscopy, scanning electron microscopy, Auger-electron spectroscopy and mechanical tests are presented in this paper. The both types of weld metals with different content of impurities and alloying elements were studied after irradiations to fast neutron (E>0.5MeV) fluences in the wide range below and beyond the design values, after recovery annealing procedures and after re-irradiation following the annealing. The distinctive features of embrittlement kinetics of VVER-440 and VVER-1000 RPV weld metals conditioned by their chemical composition differences were investigated. It is shown that the main contribution into radiation strengthening within the design fluence can be attributed to radiation-induced precipitates, on reaching the design or beyond design values of fast neutron fluencies the main contribution into VVER-440 welds strengthening is made by radiation-induced dislocation loops, and in case of VVER-1000 welds – radiation-induced precipitates and grain-boundary phosphorous segregations. Recovery annealing of VVER-440 welds at 475°C during 100h causes irradiation-induced defects disappearance, transformation of copper enriched precipitates into bigger copper-rich precipitates with lower number density and leads to almost full recovery of mechanical properties followed by comparatively slow re-embrittlement rate. The recovery annealing temperature of VVER-1000 welds was higher – 565°C during 100h – to avoid temper brittleness. The annealing of VVER-1000 welds leads to almost full recovery of mechanical properties due to irradiation-induced defects disappearance and decrease in precipitates number density and grain-boundary segregation of phosphorus. The re-embrittlement rate of VVER-1000 weld during subsequent re-irradiation is at least not higher than the initial rate.

Comparison of Attenuation Coefficients for VVER-440 and VVER-1000 Pressure Vessels

Abstract The paper summarizes the attenuation coefficient of the neutron fluence with E > 0.5 MeV through a reactor pressure vessel for vodo-vodyanoi energetichesky reactor (VVER) reactor types measured and/or calculated for mock-up experiments, as well as for operated nuclear power plant (NPP) units. The attenuation coefficient is possible to evaluate directly only by using the retro-dosimetry, based on a combination of the measured activities from the weld sample and concurrent ex-vessel measurement. The available neutron fluence attenuation coefficients (E > 0.5 MeV), calculated and measured at a mock-up experiment simulating the VVER-440-unit conditions, vary from 3.5 to 6.15. A similar situation is used for the calculations and mock-up experiment measurements for the VVER-1000 RPV, where the attenuation coefficient of the neutron fluence varies from 5.99 to 8.85. Because of the difference in calculations for the real units and the mock-up experiments, the necessity to design and perform calculation benchmarks both for VVER-440 and VVER-1000 would be meaningful if the calculation model is designed adequately to a given unit.

Evolution of the nanostructure of VVER-1000 RPV materials under neutron irradiation and post irradiation annealing

A high nickel VVER-1000 (15Kh2NMFAA) base metal (1.34 wt% Ni, 0.47% Mn, 0.29% Si and 0.05% Cu), and a high nickel (12Kh2N2MAA) weld metal (1.77 wt% Ni, 0.74% Mn, 0.26% Si and 0.07% Cu) have been characterized by atom probe tomography to determine the changes in the microstructure during neutron irradiation to high fluences. The base metal was studied in the unirradiated condition and after neutron irradiation to fluences between 2.4 and 14.9 × 10 23 m −2 ( E > 0.5 MeV), and the weld metal was studied in the unirradiated condition and after neutron irradiation to fluences between 2.4 and 11.5 × 10 23 m −2 ( E > 0.5 MeV). High number densities of ∼2-nm-diameter Ni-, Si- and Mn-enriched nanoclusters were found in the neutron irradiated base and weld metals. No significant copper enrichment was associated with these nanoclusters and no copper-enriched precipitates were observed. The number densities of these nanoclusters correlate with the shifts in the ΔT 41 J ductile-to-brittle transition temperature. These nanoclusters were present after a post irradiation anneal of 2 h at 450 °C, but had dissolved into the matrix after 24 h at 450 °C. Phosphorus, nickel, silicon and to a lesser extent manganese were found to be segregated to the dislocations.

Application of Subsize Specimens for Re-Irradiation Embrittlement Monitoring of the First Generation of VVER-440 RPV Steels

Abstract The first generation of VVER-440 Units were not supplied with RPV steel surveillance programs. Therefore, cutting out of samples (templetes) from the inner side of RPV wall is the only possibility for re-irradiation embrittlement monitoring of the first generation of VVER-440 RPV steels. Calculations showed that it is possible to cut out a sample of 7×60×95 mm (with consequent repair) from a RPV core weld. Thus, it is impossible to machine the standard 10×10×55 mm Charpy V-notch specimens to determine RPV steel radiation embrittlement. Therefore it is necessary to use subsize specimens (5×5×27.5 mm and 3×4×27 mm) for ductile-to-brittle transition temperature evaluation of templet material. The small samples were cut out from the inner surface of the all old Russian VVER-440 RPVs to assess the actual condition of the RPV material before and after post-irradiation annealing and after re-irradiation. Special monitoring programs of periodic sampling of RPVs and re-irradiation of the templet material in surveillance channels of VVER-440/213 RPVs were developed. The state of the art of application of subsize specimens for re-irradiation embrittlement monitoring of the first generation of VVER-440 RPV steels is discussed.

Radiation Embrittlement of Cr–Ni–Mo and Cr–Mo RPV Steels

Abstract Radiation embrittlement kinetics of Cr–Mo and Cr–Ni–Mo steels, used for manufacturing VVER-440 and VVER-1000 RPV steels, respectively, are under consideration. Radiation embrittlement of VVER-440 and VVER-1000 RPV steels exposed to irradiation for 17–22 years is analyzed. Extremely high levels of thermal aging of VVER-1000 RPV steels at 320°C is pointed out. The neutron flux effect problem and the problem of accelerated radiation embrittlement of welds with high nickel contents are emphasized. The effect of phosphorus, copper, silicon, manganese, and nickel contents on radiation embrittlement of VVER-440 and VVER-1000 RPV steels is studied using the Russian surveillance database and representative results of research programs. The trend curves for VVER-440 and VVER-1000 RPV steels are proposed. The mechanisms of radiation embrittlement of Cr–Ni–Mo and Cr–Mo RPV steels are supposed.

Influence of the Multigroup Approximation on VVER-1000 RPV Neutron/Gamma Flux Calculation

Abstract One-dimensional calculations of neutron and gamma group fluxes and radiation damage in terms of DPA have been carried out for VVER-1000 mock-up realized at LR-0 critical assembly, Rez, Czech Republic in order to evaluate the influence of energy dependence multigroup approximation on VVER-1000 RPV Neutron/Gamma flux calculation. Two calculational methods have been applied: Monte Carlo method (MC) with continuous energy dependence description and discrete ordinates (DO) method with multigroup energy dependence approximation. Two different multigroup neutron/gamma cross section libraries have been used: BGL1000—created especially for VVER-1000 application and BUGLE96—created for PWR application. A good consistency between the results for both methods and different cross section libraries is obtained for neutrons with energy above 0.1 MeV. The worst consistency is obtained for the thermal neutrons. For gammas a better consistency between DO and MC results is obtained when BGL1000 library is applied.