Ferenc Gillemot

The role of chemical composition in WWER RPV irradiation embrittlement

The main chemical elements influencing WWER RPV steel radiation embrittlement are copper, phosphorus, nickel and manganese. Copper and phosphorus are the main influencing elements for WWER-440 RPVs. The influence of copper is implemented in the forming of nanocluster centres rounded by other elements which are the effective barriers to dislocation movements. Phosphorus segregates inside the grains, interacting with matrix defects and attracted to the Cu-type precipitates. Phosphorus also migrates to grain boundaries through diffusion processes. Due to the higher nickel content and very low copper and phosphorus present in WWER-1000 RPV steel, the main influencing element for this RPV type is nickel. The manganese content also has to be taken into account due to synergism of Ni and Mn. The semi-mechanistic model is based on key embrittlement mechanisms of chemical elements influence: matrix damage, irradiation-induced precipitation and element segregation proposed.

Overview of reactor pressure vessel cladding

Licence extension of NPPs requires a new safety analysis of the plant covering the extended operating life. One of the most important safety considerations is the integrity of the RPV (Reactor Pressure Vessels). The RPV material suffers several ageing-related effects during operating service: neutron (and gamma) irradiation embrittlement, thermal ageing, low-cycle fatigue, thermal fatigue and corrosion. Generally, the most severe effects are irradiation and the thermal embrittlement of the RPV belt near the fuel core zone, which is often referred to as the beltline. Nearly all operating RPVs are covered inside with a stainless steel layer called RPV cladding. The RPV cladding generally made by welding results in very rough grain size and its mechanical and thermal properties are different from the RPV base material, causing high residual stresses. The thickness of RPV cladding is 2?10 mm. Since the RPV cladding was previously considered only as an anticorrosive layer the mechanical properties and the role in the RPV integrity were not properly studied. In this review, the existing relevant informations on RPV cladding properties are summarised.

Small Specimen Testing Applied at Surveillance Extension

The WWER-440 V-213 type RPVs are equipped with the surveillance specimens used to predict the embrittlement of the RPV material. These specimens are located near the core, resulting in accelerated irradiation. After four years of operation only one overexposed set remains in each unit. To obtain a more extensive database for life time calculation and life time management, a new, more extended surveillance pron has been elaborated and implemented. Due to the limited availability of archive materials reconstructed Charpy V specimens and small size smooth and notched tensile bars were used in the program. To develop this surveillance extension, an extensive laboratory study on use of reconstructed Charpy and subsize tensile specimens was performed. Methods for K 1 a measurements on small size specimens were also studied.

IAEA Coordinated Research Project on Master Curve Approach to Monitor Fracture Toughness of RPV Steels: Final Results of the Experimental Exercise to Support Constraint Effects

The precracked Charpy single-edge notched bend, SE(B), specimen (PCC) is the most likely specimen type to be used for determination of the reference temperature, T0 , with reactor pressure vessel (RPV) surveillance specimens. Unfortunately, for many RPV steels, significant differences have been observed between the T0 temperature for the PCC specimen and that obtained from the 25-mm thick compact specimen [1TC(T)], generally considered the standard reference specimen for T0 . This difference in T0 has often been designated a specimen bias effect, and the primary focus for explaining this effect is loss of constraint in the PCC specimen. The International Atomic Energy Agency (IAEA) has developed a coordinated research project (CRP) to evaluate various issues associated with the fracture toughness Master Curve for application to light-water RPVs. Topic Area 1 of the CRP is focused on the issue of test specimen geometry effects, with emphasis on determination of T0 with the PCC specimen and the bias effect. Topic Area 1 has an experimental part and an analytical part. Participating organizations for the experimental part of the CRP performed fracture toughness testing of various steels, including the reference steel JRQ (A533-B-1) often used for IAEA studies, with various types of specimens under various conditions. Additionally, many of the participants took part in a round robin exercise on finite element modeling of the PCVN specimen, discussed in a separate paper. Results from fracture toughness tests are compared with regard to effects of specimen size and type on the reference temperature T0 . It is apparent from the results presented that the bias observed between the PCC specimen and larger specimens for Plate JRQ is not nearly as large as that obtained for Plate 13B (−11°C vs −37°C) and for some of the results in the literature (bias values as much as −45°C). This observation is consistent with observations in the literature that show significant variations in the bias that are dependent on the specific materials being tested. There are various methods for constraint adjustments and two methods were used that reduced the bias for Plate 13B from −37°C to −13°C in one case and to − 11°C in the second case. Unfortunately, there is not a consensus methodology available that accounts for the differences observed with different materials. Increasing the Mlim value in the ASTM E-1921 to ensure no loss of constraint for the PCC specimen is not a practicable solution because the PCC specimen is derived from CVN specimens in RPV surveillance capsules and larger specimens are normally not available. Resolution of these differences are needed for application of the master curve procedure to operating RPVs, but the research needed for such resolution is beyond the scope of this CRP.Copyright

Investigation of effects of long-term thermal aging on magnetization process in low-alloy pressure vessel steels using first-order-reversal-curves

We have investigated effects of long-term thermal aging at 550°C up to 10000 h on major-loop coercivity, hysteresis scaling of minor loops, and first-order reversal curves (FORCs) for low-alloy pressure vessel steels with low and high Ni contents. While major-loop coercivity and minor-loop coefficient of the scaling exhibit a gradual decrease with aging for high-Ni steel, those for low-Ni one are very weakly dependent on aging time. On the other hand, we found that FORC distribution becomes steep along both axes of interaction and switching fields and the peak shifts toward a lower switching field for both steels. Considering that there is no significant development of nanoscale precipitates during the aging as revealed with small-angle neutron scattering experiments, a relaxation of lattice strain in a matrix, possibly associated with diffusion of Ni atoms, may dominate magnetic properties at 550°C.

Insight on the inconsistencies of Barkhausen signal measurements for radiation damage on nuclear reactor steel

This paper focuses on the use of magnetic measurements, using Barkhausen signals to determine the irradiation effects, attempting to predict fracture toughness changes on nuclear reactor structural materials and correlating these measurements to mechanical testing and microstructure. For this study, two types of nuclear reactor materials were investigated: one sensitive to irradiation effects, the JRQ IAEAs reference material (A533B- -type); and one resistant material, 15KH2MFA WWERs reactor pressure vessel steel. The samples were carefully identified within the original heat block, i.e. forged or rolled plate. These calibrated samples were irradiated at different neutron fluences up to 1023 n/m2. We show how microstructural anisotropy can mask the irradiation effects in the magnetic measurements. A correlation between irradiation effects and the magnetic measurements is explained based on this study.