D. Blind

Strain-induced corrosion cracking of low-alloy steels in LWR systems — case histories and identification of conditions leading to susceptibility☆

Abstract The term “strain-induced corrosion cracking” (SICC) is introduced to describe crack formation involving dynamic straining, but in the absence of obvious, cyclic loading. Its origins in slow-strain-rate testing and in corrosion failures in boiler systems are described and the links with “classical” stress corrosion cracking and low-cycle corrosion fatigue are identified. Four areas, in which SICC of low-alloy steels in LWR systems has occurred, are described in detail and the typical features are used, together with literature data from laboratory testing, to identify conditions leading to susceptibility. Indications are given of remedial measures and of areas in which further work is necessary.

New observations on the crack growth rate of low alloy nuclear grade ferritic steels under constant active load in oxygenated high-temperature water

Abstract Within the scope of reactor safety research attempts have been made over several decades to determine corrosion-assisted crack growth rates. National and international investigations have been performed on both an experimental and an analytical basis. A compilation of internationally available experimental data for ferritic steels exhibits a scatter of crack growth rates of up to 5 decades. This was one of the reasons for commencing further experimental investigations focused on the evaluation of corrosion-assisted crack growth rates. These experimental studies were performed under constant, active, external load on 2T-CT specimens of the materials 20 MnMoNi 5 5 with 0.009 and 0.020% S (similar to A508 Cl.3), 22 NiMoCr 3 7 with 0.006% S (similar to A508 Cl.2) and 17 MnMoV 6 4 with 0.017% S. The tests were carried out in deionized oxygenated high-temperature water (240°C; 0.4 and 8.0 ppm O 2 ). For K I values up to 60 MPa m 1 2 , the experimental results showed no significant dependence between corrosion-assisted crack growth rates and the stress intensity factor, the oxygen content of the medium or the sulphur content of the steel. Here it is important to note, that in this K I region the high crack growth rates after the onset of cracking due to loading are decreasing and finally come to a standstill after a short period of time as compared with operational times of plants. Consequently, the determination of crack growth velocities as corrosion-assisted crack advance divided by the test duration, so far practised worldwide, results in wrong crack growth rate values in the above-mentioned range of loading up to 60 MPa m 1 2 . Based on a test duration of 1000 h, the average crack growth rates are below 10 −8 mm s −1 for K I ≤ 60 MPa m 1 2 . When applied to a single start-up and service period of one year, this would formally lead to an average crack growth rate of 2·10 −9 mm s −1 (equivalent to 0.06 mm per year). At K I values between 60 and 75 MPa m 1 2 the average corrosion-assisted crack growth rates increase significantly. It can be observed experimentally that the crack propagates during the whole period of the test. Consequently the calculation of crack growth velocities as corrosion-assisted crack advance divided by the test duration as mentioned earlier can be applied as a first estimate. Finally, for K I values ≥ 75 MPa m 1 2 high crack growth rates up to 10 −4 mm s −1 can be observed. In this region the average crack growth rates are also in quite good agreement with a theoretically based crack growth model.

Formation and growth of cracking in feed water pipes and RPV nozzles

Abstract The observation of numerous small and large cracks in ferritic feed water pipes of boiling (BWR) and pressurized water reactors (PWR) in the last few years has led to basic research into the causes of cracking and the crack growth mechanisms. In horizontal feed water pipe sections connected to nozzles of reactor pressure vessels (RPV) of BWRs as well as of steam generators (SG) of PWRs, circumferential macro and micro cracks were detected. These cracking phenomena could be observed in base material of pipes as well as in weld seam regions. The examination of the stress state displayed that the cracked pipe regions have been exposed to a number of cyclic thermal transients (thermal shock, flow stratification) during start-up (hot stand-by) and shut-down periods of the plants. During thermal transient periods, local and global cyclic stresses in the referred pipe cross sections have been induced which in interaction with the influence from environment (in operation as well as in shut-down periods) and local geometrical imperfections led to the initiation and formation of macro and micro cracks. In the reactor water clean-up system of BWR through which reactor water is fed from the RPV to the main feed water line, two longitudinally welded elbows have been detected to be severely cracked. Both elbows have been subjected to an internal pressure corresponding to RPV and additionally to a relevant “in-plane” bending moment. These longitudinal cracks were found to be started from the inner elbow surface. In one case the longitudinal crack was situated in the base material and was enlarged to leakage. In the second elbow the longitudinal crack was located in the heat affected zone (HAZ) of a longitudinal weld. In both cases the macro cracks started either from corrosion pits located in defective areas of the magnetic protection layer or from geometrical notches (weld root). The semi-elliptic small cracks got linked to more extended shallow cracks. Formation and growth mechanism of these cracks have been studied at the MPA Stuttgart in laboratory under simulated operation conditions which were held as realistic as possible compared with those in nuclear power plants. The results of experimental studies in laboratory as well as conclusions based on the above mentioned cracking phenomena in piping have been used as basic information for a realistic design of large scale (RPV) thermal shock experiments under operation conditions. The formation and growth mechanism of these cracks and their detection by means of NDE during thermal transients at the inner surface of RPV nozzle and at the adjacent cylindrical areas of RPV shell will be described.

Solidification and segregation in heavy forging ingots

Abstract The solidification process of heavy ingots was reproduced by the establishment of isocarbon lines. From these data conclusions can be drawn in order to improve ingot quality. The order of individual elements in which they tend to segregate macroscopically was evaluated as well as the influence of ingot making on macro-segregation. A-segregations, and especially MnS inclusions aligned to them, appear as most negative to the properties of forgings, e.g. toughness and weldability. Localized segregation is tolerable if the surrounding matrix is sufficiently tough.

Fracture mechanics evaluation of cracked components with consideration of multiaxiality of stress state

Abstract The fracture-mechanics evaluation of cracked components is an essential part of safety analyses. This evaluation is usually based on one-parametric evaluation procedures without taking into account the multiaxiality of the stress state. Considering the multiaxiality of the stress state across the flawed component cross-section, it is possible to recognize and extend the limits of application of fracture mechanics, which among others, are given by the limited transferability of fracture-mechanics material laws. Within the scope of the research project “Phenomenological Vessel Burst Tests-Phase IV”, T-branches and elbows with dimensions like the primary coolant lines of PWR plants were investigated. In addition to the experimental investigations, extensive numerical calculations were performed by means of the finite element method (FEM). To determine the stress and the gradient of multiaxiality across the ligament of the component, 3-D finite-element analyses were carried out concerning elastic-plastic material behaviour. The evaluation with regard to crack initiation has been proven by experimental results as well as the qualitative assessment of the fracture behaviour on the basis of the multiaxiality analyses.

Investigation methods for the detection and study of stress-relief cracking

Abstract The occurrence of grain boundary damage and stress-relief cracking within heat-affected zones of pressure vessel weldments and claddings has led to numerous investigations. The metallographic and fractographic as well as microanalytical methods used are reported in respect of the results obtained and the given limits. Within the range of microanalysis the application of the electron beam microprobe is described for the detection of micro segregations as well as companion elements and trace elements, Auger spectroscopy for the investigation of thin grain boundary films, transmission electron microscopy especially for the identification of precipitations, and the photoemission electron microscope for the analysis of transformation and precipitation behaviour, as well as the so-called phase analysis for better knowledge in the field of kinetics of phases. Using the results of these methods, which were applied on real welded joints as well as on weld simulation tests, it was possible to prove the validity of the chosen parameters for the welding simulation. Further efforts will be made to find more clear correlations between the contents of alloying elements in the base material and the precipitation or crack behaviour in the coarse grain zone of the HAZ.

Planning and use of condition-dependent NDT for the interaction between fracture mechanics and quantitative NDT

The proof of safety for reactor components is generally based on non-destructive testing results, which much be evaluated in reference to their safety relevance. The use of fracture mechanics concepts in flaw evaluations requires the quantification of the dependencies between flaw geometry, material characteristics, and load condition. The solution consists of the combination of optimal test preparation using computer-aided design software modules and modeling of the inspection, a quantitative flaw testing using qualified reconstruction methods and qualification procedures of the material parameters like fracture toughness. Besides the discussion of the condition-dependent strategies of lifetime management many results from large-scale round-robin testing concerning crack initiation parameters versus fracture toughness or J-integral or ray tracing in connection with defect sizing in a CAD-environment are shown.

Experience in the replacement of safety related piping in German boiling water reactors

Abstract In the German boiling water reactors (BWR) of the 69 series and their forerunner plant, high-strength low-alloy ferritic materials were used for a large number of pipings both inside and outside the pressure boundary (PB). The choice of this type of material led to comparatively thin-walled piping which, at that time, had been designed and manufactured in accordance with the codes and standards applying in the Federal Republic of Germany. Due to material properties resulting from production in a conventional manner, design features which did not sufficiently meet the requirements for nondestructive testability, and defects caused during processing, mainly in the area of circumferential welds, the ferritic pipings inside the PB were replaced in the course of a plant upgrading by new piping designed and manufactured according to the basis safety concept. The improvements and experience gained during backfitting of five German BWR plants are part of the German safety strategy and can be summarized as follows: 1. (1) Exclusion of large fractures on the basis of an optimized quality level for the piping. 2. (2) Elimination of need for the pipe whip restraints which existed in the former piping. 3. (3) Limited reduction of the former scope of inservice inspections, mainly as a consequence of improved weld quality and optimized weld performance. 4. (4) Reduction of personnel radiation exposure, e.g. by reduced number of welds and by manufacture of welds using automatic equipment, as well as by improved nondestructive testing. 5. (5) Availability values for backfitted BWR comparable to German PWR values. The pipings made of stabilized austenitic materials, which are arranged inside and outside the containment of the BWR plants, were not replaced since their quality level has been proved to be sufficient even on the basis of the present standards.

Influence of repair welding on cyclic thermal shock behaviour of a RPV nozzle corner

Abstract In piping as well as in RPV-nozzles and in surrounding parts of the cylindrical vessel shells, repeated cold water injections have been observed leading to crack initiation and subcritical crack growth. In the framework of the German Reactor Safety Research Programme, cyclic thermal shock tests have been carried out at the RPV of the decommissioned HDR (Heissdampfreaktor) near Frankfurt applying extremely conservative temperature transients. The crack propagation under access of oxygenated pressurized water was evaluated by fracture mechanics methods. The validation by non-destructive examination, as well as by destructive testing at a trepan, proved the fracture mechanics results to be conservative. The depression at the nozzle corner caused by the trepan removal has been repaired by ‘temper bead’ welding without stress relief heat treatment, similar to the ‘half bead’ technique of the ASME Boiler and Pressure Vessel Code. The subsequent thermal shock stressing (1200 cycles) under similar parameters demonstrated, with respect to the weld repair: 1. (1) no material separation extending into the ferritic RPV-wall; 2. (2) the quality of repair welding is comparable to the quality of the as-delivered condition manufactured in the 1960s in a more conventional manner. The positive experience derived from this work may be helpful in the future in dealing with any cases of cracking in thick-walled large diameter vessels which could require repair welding when a post-weld stress relief heat treatment is not possible.

Failure behavior of a pipe system with a circumferentially orientated flaw - analytical and experimental investigations

Abstract At the german HDR-test-facility a pipe failure experiment was performed at a fullsize feedwater piping system under operating conditions of T = 240° C , p i = 10.6 MPa and with an elevated oxygen content in the pressure medium. The loading was internal pressure and a cyclic varying bending moment with an R- ratio of 0.5. The in form of a circumferentially orientated notch initially weakened piping system failed after a total number of 4773 loaded cycles with different frequencies in form of a small leak. The analyses of the fracture surface indicated the strongly growing influence of corrosion effects on the crack propagation rate with decreasing loading frequency. The cyclic crack growth and the leak-before-break behavior of the piping system could be explained on the basis of results of finite element calculations using ADINA-code.