Elsevier

Nuclear Engineering and Design

Volume 265, December 2013, Pages 201-209
Nuclear Engineering and Design

Comparison of four NDT methods for indication of reactor steel degradation by high fluences of neutron irradiation

https://doi.org/10.1016/j.nucengdes.2013.06.020Get rights and content

Highlights

  • Results of 4 NDT methods on highly irradiated steel are normalized and compared.

  • Two of the methods (MAT and HV) correlate well with DBTT.

  • Magnetic Adaptive Testing gives the most sensitive and the best correlated results.

  • Measurements and sample shapes for an NDT surveillance program are suggested.

Abstract

Results of three magnetic nondestructive methods, Magnetic Barkhausen Emission (MBE), magnetic minor loops Power Scaling Laws (PSL) and Magnetic Adaptive Testing (MAT), and of one reference mechanical measurement, Vickers Hardness (HV), applied on the same series of neutron heavily irradiated nuclear reactor pressure vessel steel materials, were normalized and presented here for the purpose of their straightforward quantitative mutual comparison. It is uncommon to carry out different round-robin testing on irradiated materials, and if not answering all open questions, the comparison alone justifies this paper. The assessment methods were all based on ferromagnetism, although each of them used a different aspect of it. The presented comparison yielded a justified recommendation of the most reliable nondestructive method for indication of the reactor steel irradiation hardening and embrittlement effects. The A533 type B Class 1 steel (JRQ), and the base (15Kh2MFA) and welding (10KhMFT) steels for the WWER 440-type Russian reactors were used for the investigations. The samples were irradiated by high-energy neutrons (>1 MeV) with up to 11.9 × 1019 n/cm2 fluences. From all the applied measurements, the results of MAT produced the most satisfactory correlation with independently measured ductile-brittle-transition temperature (DBTT) values of the steel. The other two magnetic methods showed a weaker correlation with DBTT, but some other aspects and information could be assessed by them. As MAT and MBE were sensitive to uncontrolled fluctuation of surface quality of the steel, contact-less ways of testing and more conveniently shaped irradiated nuclear pressure vessel steel samples were suggested for future measurements.

Introduction

Pressure vessel of each nuclear reactor is its indispensable part. It is the container, which keeps inside and covers the radioactive fuel and most of the thermal exchange medium and which, therefore, may never be allowed to degrade to an unsafe level.

The principal material of the container, i.e. the nuclear reactor pressure vessel steel (RPVS), must be therefore of a very high quality at the start of the reactor industrial service, and actual level of its quality must be monitored and known during its whole service life. One of the basic ways of inspection of the RPVS quality is provided by the surveillance program. The surveillance program uses a number of surveillance samples, which are positioned inside the reactor from the very beginning of its industrial service and which thus – at any moment of time – experience the same (or even a little larger, because they are closer to the fuel core than walls of the pressure vessel) neutron fluence as the RPVS walls.

Presently, the surveillance samples are typically V-notched blocks 55 × 10 × 10 mm3 of the RPVS. At prescribed times several of them are taken out from the RPV and tested by the well-known destructive Charpy impact method. Usually minimum about a dozen of surveillance samples must be used for a single Charpy test and the main resulting information of each test is the actual value of the ductile-brittle-transition-temperature (DBTT) of the RPVS at the moment of the test. The Charpy tests are mechanical, they directly measure toughness of the RPVS and as they are destructive, the used surveillance samples can never be returned back into the RPV for any next Charpy investigation. All the surveillance samples must be put into the RPV at the reactor start and no others can be added for the same purpose later.

If, during lifetime of the reactor, it turns out that the RPV keeps its structural integrity longer than conservatively estimated at the time of the RPV construction (which is nowadays frequently the case), and that its safe, profitable industrial service could be easily prolonged far over the originally planned period, there are no available surveillance samples for the prolonged period any more. And this is only one of the points where it is clear, that employment of nondestructive tests instead, or at least parallel with the destructive ones would be very helpful.

In contrast to destructive investigations, any nondestructive test does not measure directly the material property (usually a mechanical one) which is to be determined. For instance, no nondestructive test can directly assess quantitative values of DBTT. Instead, however, it can measure quantitative values of another physical property (e.g. magnetic coercivity), which is influenced by the same elementary origin (e.g. by presence and quality of structural material defects). Then, the only way how to get the desired quantitative information about the destructive-test-based required property (DBTT) from nondestructive measurement of an indirect one (coercivity), is to settle a trustworthy one-to-one correlation between the two properties in a series of comparative experimental investigations. Each nondestructive test, before it can be reliably applied, must be painstakingly and rigorously correlated to the relevant destructive one in a long, checked and re-checked series of detailed exploration.

This is probably the main reason why not a single nondestructive test was so far not only unable to replace the destructive Charpy ones, but not even to be applied in parallel as an auxiliary one – which would lay a very sound foundation for verification of the needed correlation. Nevertheless, a number of applicable nondestructive tests were suggested, are currently tested, and the mosaic of the necessary correlations is presently slowly built up.

Magnetic nondestructive tests are an important section of all the possible ones, especially because of their simplicity and also as it is known and understood that mechanical and magnetic properties of ferromagnetic construction materials are really very closely correlated: regularity of microstructure of ferromagnetic construction materials (e.g. of RPVS) and density and quality of its defects, has a strong, kind of “parallel” influence on both the mechanical and magnetic parameters of the same material.

Some magnetic nondestructive tests of various RPVS were lately described in a number of publications (see, e.g. Takahashi et al., 2006a, Dobmann et al., 2008, Vandenbossche, 2009, Gillemot Pirfo, 2010, Pirfo Barroso and Horváth, 2010, Kobayashi et al., 2012a, Minov, 2012). However, few of them were performed at mechanically intact RPVS, which would form a continuous series from not-irradiated samples up to samples of the same RPVS and the same shape, which would be subjected to high fluence of neutrons in a nuclear reactor.

We present here three series of such degraded samples of three different RPVS, all samples of the same prismatic shape, mechanically intact. These samples were irradiated in the 10 MW Budapest Research Reactor at 290 °C up to the fluence of 11.9 × 1019 n/cm2 by high-energy neutrons (>1 MeV), which corresponded approximately to about 20 years of service of a pressure vessel in a WWER440 reactor, and by kindness of Dr. F. Gillemot (KFKI Atomic Energy Research Institute in Budapest) they were made available to any interested researcher. This made it possible to collect in the present paper results of examination of the same samples by one reference mechanical property and three different nondestructive methods. They were investigated by (a) Vickers Hardness (HV) (Pirfo Barroso and Horváth, 2010), (b) Magnetic Barkhausen Emission (MBE) (Pirfo Barroso and Horváth, 2010), (c) magnetic minor loops Power Scaling Laws (PSL) (Kobayashi et al., 2012a, Kobayashi et al., 2012b), and (d) Magnetic Adaptive Testing (MAT) (Tomáš et al., 2013).

The purpose of the present paper is

  • to collect here all the obtained results of those four methods,

  • to normalize them so that they can be quantitatively compared,

  • to compare them with each other,

  • to discuss their sensitivity and reliability, and finally

  • to conclude about applicability of the most suitable of them for nondestructive evaluation of the RPVS degradation in a future RPV.

Section snippets

Samples

All the samples had rectangular prismatic shape with dimensions 30 × 10 × 10 mm3 with all surfaces originally machined (milled) to the quality usual for the normal Charpy samples, and with short alpha-numeric sample-labels gently hammered into the 10 × 10 mm2 faces of each sample. They were prepared from three types of nuclear RPVS: the A533 type B Class 1 steel known as JRQ (a special type of heat-treated steel with increased impurity, optimized for investigation of correlation between radiation

Results

All the results we intend to compare were published in the above-cited papers (Pirfo Barroso and Horváth, 2010, Kobayashi et al., 2012a, Kobayashi et al., 2012b, Tomáš et al., 2013). However, as they were measured by different methods, and different physical variables were used in the available plots, it is impossible to compare them directly in the shape they were originally published. The purpose of all the four measurements was to show to what extent the original RPVS was degraded by the

Discussion

The normalized curves presented in Section 3.2 are mutually comparable. From the first look at them it is evident that Fig. 2a (HV) and d (MAT) is more similar to one another than to any of the two other figures (Fig. 2b (MBE) and c (PSL)). It is also seen that Fig. 2b (MBE) and c (PSL) is not very similar to one another either (Fig. 3).

To be able to compare the individual degradation curves obtained for the same series of samples (the same material and orientation), and measured by each of the

Conclusions

Results of four nondestructive methods (HV, MBE, PSL and MAT) applied on each of the five series of various nuclear RPVS materials, heavily irradiated by high-energy neutrons, were collected, normalized to comparable shapes and introduced in this paper for the purpose of their straightforward mutual quantitative comparison. To our knowledge no such a broad comparison of different nondestructive magnetic methods was published yet.

The presented comparison and evaluation made it possible to choose

Acknowledgements

The authors are very grateful to F. Gillemot, Á. Horváth, M. Horváth and R. Székely (Atomic Energy Research Institute, Budapest, Hungary) for providing the irradiated RPVS samples and for making possible the measurements in the hot laboratory of the Atomic Energy Research Institute. This work was supported by Hungarian Scientific Research Fund (project A08 CK 80173) and by project No. TA02011179 of the Technical Agency of the Czech Republic. SPB is funded via a grant from the RCUK Energy

References (21)

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