In the last years many hydrotests, cyclic fatigue tests and thermal shock tests have been carried out on reactor pressure vessels, which exhibited crack-like defects, with the aim to develop the conditions of detectability of growing and non-growing cracks by acoustic emission (AE). The AE generated during these experiments has been recorded and analyzed. Techniques for evaluation of AE-data have been developed which allowed the separation of AE-events from crack growth and crack surface friction by risetime. Crack growth can be detected with high sensitivity at thermal shock and fatigue tests, whereas the detectability of crack surface friction increases, as the amount of compressive stresses between the crack surface grows. At hydrotest only a low level AE is produced by non-growing cracks due to the lack of compressive stresses.

Usually burst-like acoustic emission (AE) is localized by triangulation. For continuous AE, e.g. from leakages, this method is not feasible. Therefore a new method for localization of continuous AE has been developed. It is based on a phase-controlled probe array which consists of many single sensor elements. The AE signals received by the different sensor elements are delayed according to their time-of-flight differences from the source to the single elements of the receiver array. By choosing special combinations of time differences between the array elements the directivity pattern of the sensitivity of the array can be changed, e.g. rotated in the plane of a large plate. Thus, the source direction can be determined by one array. Some preliminary experiments with an artificial noise source, positioned on a large steel plate, have been performed and have demonstrated the feasibility of this approach.

In the framework of the German HDR reactor safety programme a selected area of the reactor pressure vessel was loaded with 9500 thermal cycles. In 3 saw cut regions several crack fields were generated and monitored with on-line and off-line NDE-procedures. With potential drop and ultrasonic testing, crack front profiles could be described off-line with sufficient resolution. During the trials the crack growth was measured with stationary potential drop and ultrasound probes. With on-line acoustic emission crack growth of about 5 mm could be detected. On the basis of an interpretation model a classification of the acoustic emission signals in crack growth and crack surface friction phenomena was possible. The crack growth during the cooling phase is caused by fatigue; the reason for crack growth during the heating up phase is corrosion influence. All cracks are growing at different time periods and have different stress conditions.

To demonstrate and to extend the performance of acoustic emission testing as a method of detecting and classifying flaws, six institutes conducted acoustic emission measurements in the course of various loading tests on a medium-sized, thick-walled vessel (model of a reactor pressure vessel) containing natural flaws. This paper will present a description of the vessel and the preparation of the flaw patch with 14 natural flaws, the performance of the loading tests to simulate pressure test and operating conditions existing in the primary systems of pressurized water reactors. and especially the conduction of acoustic emission measurements by using various monitoring systems.In the pressure tests conducted with slowly rising pressures, only one flaw was detected unequivocally by the acoustic emission monitoring, although several flaws had grown in the test phases between the pressure tests. Cyclic loading over prolonged periods of time produced clear signals of larger flaws, which calculations and subsequent destructive investigations showed to have grown. The small flaws which, most probably, had not changed, could not be detected.

During thermal and mechanical loading NDT-techniques were applied on pipe nozzles and in the cylindric pressure vessel wall. With the acoustic emission macroscopic crack growth at the tube could be detected. After an analysis of the energy and amplitude of the signals criteria for a prewarning of the failure of the tube line could be defined. For the thermal shock test on nozzles and in the cylindric wall small initiating cracks could be indicated with the acoustic emission analysis in the cladding. It was possible to differentiate between crack growth and crack surface friction noise.For online quantification of crack growth at some cracks stationary potential drop probes were applied. A resolution of crack growth of about 0,5 mm could be reached. In off-line-mode small defects in crack fields could be detected. With the ALOK procedure defects of more than 3 mm depth and 5 mm length also in the crack fields could be detected and resolved. With electro-dynamic exited SH-waves all defects larger than 10 mm depth could be detected. With the potential drop technique an exact characterization of the crack profile is possible, if the cladding thickness is known.

Within the German HDR-programme simulations of different accident conditions are performed in a real reactor plant out of operation. Pressurized thermal shock tests on a saturated steam outlet nozzle have been conducted under simulated realistic operation conditions (t = 300° C, p = 11 MPa) for the past three Years. On the one hand the tests are to improve our understanding of crack formations and crack growth under thermal shock conditions and on the other hand to evaluate and to improve the suitability of non-destructive test methods for detection and quantitative description of natural cracks and crack fields both in on-lineapplication and off-line.

On a cladded test specimen with 2 mm cylindrical holes in different depths, measurements with SH waves, longitudinal waves (creeping waves) and Rayleigh waves from the clad as well as from the ferritic side were performed. Investigations with the multifrequency eddy current prototype were also carried out.

On a cladded test specimen with 2 mm cylindric holes in different depths measurements with SH-waves, longitudinal waves (creeping waves), Rayleigh-waves as well from the clad as from the ferritic side were performed. Also investigations with the multifrequency eddy current prototype were carried out.

Within the framework of the HDR reactor safety research program, thermal shock tests on a saturated steam outlet nozzle edge have been conducted under simulated realistic operation conditions (T = 300°C, p = 11 MPa) for the past three years. On the one hand, the tests are to improve our understanding of crack formation and crack growth under thermal shock conditions and, on the other hand, to evaluate and improve the suitability of non-destructive test methods for the on-line surveillance, the detection and quantitative description of natural cracks and crack fields.

Within PISC-I, which was finished at the end of the seventies, three uncladded 200 mm thick plates were available. In this program ten European countries were participating. The test specimens came out of the American HSST-program. A series of testing according to the ASME-procedure were performed and in addition several alternative techniques. The manufacturing defects dominated but were unrealistic large. There was a lack of small cracklike defects.

The analysis of acoustic emission measurements during cyclic thermal-shock tests on a pressure vessel nozzle (HDR) with cracks indicated a time-dependent emission of signals that partially contradicts the usual fracture mechanics approach (according to Paris).

Cyclic thermal shock tests have been performed on a RPV nozzle (HDR) with cracks in order to apply and to assess technics available for the description of real cracks (non-destructive testing, fracture mechanics calculation). In this paper the application of dye penetrant testing, potential drop testing and acoustic emission measurements on the nozzle is considered. Some experimental results are described in detail; assessments have been possible on the basis of destructive testing (fractography). The determination of crack parameters for correlating these NDT results with fracture mechanics data are discussed such as crack depth, a, cracklength, crack area and crack growth. Crack growth is considered over a period of cycles, a(N), per cycle, da/dN, and during a cycle, da/dt. A selection of numerical NDT findings is presented and compared with the results of fracture mechanics calculations. The investigation leads to the following conclusions: 1.1. The crack dimensions found by NDT are not quite conservative (fractography).2.2. As the performed fracture mechanics calculations overestimated the crack growth advanced NDT data like da/dN indicate starting points for improvements of the calculations (influence of mean stress).3.3. Although acoustic emission measurements give no quantitative fracture mechanics parameters qualitative correlations yield decisive understanding of dominating crack growth mechanisms (corrosion).4.4. Reversely different comparisons between acoustic emission and fracture mechanics results seem to confirm the classification of acoustic emission signals (crack surface friction, crack growth).

One of the focal points in the discussion about the safety of nuclear power plants is the integrity of the reactor pressure vessel.

In the reactor safety projects “integrity of components”, PISC, HDR and “full size vessel” many components with defects and lower bound structure conditions are available. Relevant loading states are simulated. These projects guarantee the further development of all NDT tools, the possibility to prove and to improve the capability of the techniques for the definition of defects, and for the determination of the state of the material (toughness) and the loading conditions. The relevant safety significance of NDT for the description, especially of the area of end of life of a component is demonstrated in such a way.

The capability of the acoustic emission monitoring system of Prine (Gard Corporation, Illinois) should be demonstrated for a multi-layer submerged arc weld of a 250 mm test plate. The question had to be answered, if the measuring system developed by Prine and the evaluation models are suitable for production control of multi-layer submerged-arc welding, respectively if defect detection with interpretation of defect type, classification of size and triangulation is possible. The Prine system works with regard to electronic equipment as well as the interpretation quite satisfactory. The agreement of the defect indication with the defects visible during welding is very well. Hot and cold cracks as well as slag inclusions were indicated and normally evaluated correctly. Pores were not detected during welding. Partly a lot of low signals were realized, especially if residual stresses (closure seam) were expected. These small indications at the sensitivity level of the system could not be precisely explained. A clear evaluation of the results will be possible after completing nondestructive and destructive testing of the weld.