Progress in Investigation of Wwer-440 Reactor Pressure Vessel Steel by Gamma and Mossbauer Spectroscopy

HR9800123

PROGRESS IN INVESTIGATION OF WWER-440 REACTOR PRESSURE VESSEL STEEL BY GAMMA AND MOSSBAUER SPECTROSCOPY

J. Hascik1. V. Slugen', J. Lipka1, RKupca2, R Hinca', I. Toth', R Grone', P. Uvacik1

'Department of Nuclear Physics and Technology, Slovak University of Technology, Ilkovicova3, 81219 Bratislava, Slovakia 2NPP Research Institute, Trnava, Okruznd 5, Slovakia

Abstract Gramma spectroscopic analyse and first experimental results of original irradiated reactor pressure vessel surveillance specimens are discussed in . In 1994, the new "Extended Surveillance Specimen Program for Nuclear Reactor Material Study" was started in collaboration with the nuclear power plants (NPP) V-2 Bohunice (Slovakia). The first batch of MS samples (after 1 year, which is equivalent to 5 years of loading RPV-steel) was measured and interpreted using the new four components approach with the aim to observe microstructural changes due to thermal and neutron treatment resulting from operating conditions in NPP. The systematic changes in the relative areas of Mossbauer spectra components were observed.

1 INTRODUCTION

The reactor pressure vessel (RPV) is probably the most important component of a nuclear power plant (NPP) and its condition significantly affects the NPP's lifetime and operational characteristics. One of the basic requirements in nuclear reactor technique is ensuring the sufficient safety margin and reliability of used materials during their operational mechanical, thermal or radiation treatment [1]. In framework of Extended Surveillance Specimen Program 24 specimens, designed especially for MS measurement, were selected and measured in "as received" state, before their placement into the core of the operated nuclear reactor. Mossbauer spectra, which correspond to the basic material samples, show typical behaviour of dilute iron alloys and can be described with three [2,3] or four sextets. Comparison of different Eastern and Western types of RPV-steels were presented in details in [4]. Their relative areas are close to the theoretical values calculated from a random distribution model of impurities in a b.c.c. structure (5% of 12 elements in total). Results confirmed MS sensitivity to detect also small differences in chemical composition or preparing technology of RPV

157 steel samples. In comparison with western types of RPV steels such as A533 Cl.l and A508 C1.3, the doublet fraction ascribed as Mn and/or Cr- substituted cementite is completely absent in 15Kh2MFA. Here are formed probably mainly O23C6, O7C3 and VC carbides. [1,4]. These spectra will be compared with MS spectra of in real operating conditions irradiated specimen (1-year into the nuclear reactor NPP Bohunice unit-3 and unit-4). The first batch of them was measured. The irradiation level of these specimens reached the maximal value presented in Table 1 is comparable with the real RPV- steel irradiation level after 5 years reactor operation (WER-440).

Table 1 - The maximal irradiation levels of studied specimens reached at the 4h unit at NPP V-2

Energy of neutrons Neutron flux [mV] Neutron fluency [m~2] >0.1MeV 5.1 xlO16 1.2 xlO24 > 0.5 MeV 2.8 xlO16 6.7 x 1023 >1.0MeV 1.5 xlO16 3.6 xlO23

2 RESULTS AND DISCUSSION

Room temperature Mossbauer spectroscopy measurements were carried out in transmission geometry on a standard constant accelerator spectrometer with a 57Co source in Rh matrix. The absorbers consisted of 25-40 u.m thick foils. Due to higher neutron embrittlement and ageing sensitivity of WER 440 (V-230) nuclear reactors, our study has been focused on the Russian 15Kh2MFA steel. The chemical composition of studied RPV-steel is shown in Table 2.

Table 2 - Chemical composition of used RPV steel 15Kh2MFA :.specimens Element C Si Mn Cr Ni Mo V S P Co Cu Base material 0'o) 0.140 0.31 0.37 2.64 0.20 0.58 0.27 0.017 0.014 0.019 0.091 Weld (%) 0.048 0.37 1.11 1.00 0.12 0.39 0.13 0.013 0.043 0.020 0.103

As the most suitable fitting model we used the four components fit with fixed sextet No2, which corresponds to the pure oc-iron with hyperfine field B = 33,0 T. MS parameters as areas under

sextets (Ax) and hyperfine fields (Bx) of RPV-steel specimens are selected in Tables 3 and 4 and the typical spectra in non-irradiated and irradiated state are depicted in Fig. 1 and Fig.2.

158 Table 3 - Comparison of RPV-steel specimens in non-irradiated and irradiated state at 3rd unit NPP Bohunice (abbreviations BM-N stand for: base material - not irradiated, BM-I base material - irradiated, WM-Nweld material - not irradiated, WM-I weld material - irradiated).

Specimen A4 B4 J%L k716 BM-N 24.3 35.0 33.3 7.4 33.8 33.0 30.6 28.5 k716 BM-I 31.6 25.1 37.3 6.0 33.7 33.0 30.7 28.6 k719BM-N 25.7 31.2 36.1 6.9 33.9 33.0 30.6 28.4 k719BM-I 31.8 23.8 38.1 6.3 33.8 33.0 30.7 28.2 k723 WM-N 17.2 41.9 34.1 6.8 33.8 33.0 30.6 28.5 k723 WM-I 23.8 34.0 35.3 6.9 33.7 33.0 30.6 28.4 Accuracy ±0.8 ±0.8 ±0.8 ±0.8 ±0.1 ±0.0 ±0.1 ±0.2

Table 4 - Comparison of RPV-steel specimens in irradiated and non-irradiated state at 4th unit NPP Bohunice.

A B B Specimen Al A2 A3 4 Bl 2 3 B4 1%) I%1 l%] J%) 01 m EL m k729 BM-N 27.9 30.5 35.5 6.1 33.8 33.0 30.6 28.4 k729 BM-I 31.6 25.0 37.3 6.0 33.7 33.0 30.6 28.0 k731BM-N 24.6 33.3 35.4 6.7 33.8 33.0 30.6 28.3 k731 BM-I 28.4 28.0 36.0 7.6 33.7 33.0 30.6 28.2 k735 WM-N 18.1 44.0 31.1 6.8 33.7 33.0 30.6 28.7 k735 WM-I 23.0 38.2 32.7 6.1 33.7 33.0 30.6 28.4 Accuracy ±0.8 ±0.8 ±0.8 ±0.8 ±0.1 ±0.0 ±0.1 ±0.2

159 1 1 1 ' 1 ' 1 ' 1 • 1 ' 1 • 1.00 0.98 r- Till - 0.96 — Iff III- 0.94 - 1 11- 0.92 - f1 ' -BM-I ] k716 1,1,1,1, -8-6-4-2 0246 8 -8-6-4-I i I i 2I • 0 2 4 6 8 velocity (mm/s) velocity (mm/s) Fig.l - Comparison of Mossbauer spectra of RPV- base material in non-irradiated and irradiated state

I i I , I • I i I , I i I • I , I I ' 1 i 1 > 1 i 1 i 1 • 1 1.00 1.00 0.98 0.98 - 0.96 - If I I I i iff ~ ^0.94 II ~ 0.96 1Tim 1- S 0.92 - 1! fi 11F - f 1 1 - 0.94 - 1 I - 0.90 - 1 f * * 1 0.88 0.92 - !k723 -BM-I •_ ! k723-BM- 11 ~ i i 1,1,1,1,1, | I , I.I.I -8-6-4-2 02468 -8 -6 -4-2024 6 8 velocity (mm/s) velocity (mm/s)

Fig. 2 - Comparison of Mossbauer spectra ofRPV-weld material in non-irradiated and irradiated slate The most significant change is observableJtLareas under first two components. The deterioral mechanism of RPV-steel specimens owing to fast neutron bombardment is shown in decrease of the ratio of pure oc-iron component presence. The significant percentual decrease of about 9,9%, 7,4%, 7,9%, 4,5%, 5,3% and 5,8% is observed in all of 6 specimens. As it was expected, the value of Mossbauer effect is lower of about 4% in the case of irradiated specimens. The total specific activity of the first batch of specimens (sample k716 BM-I with the weight of 25.6 mg) was 3.2 xlO7Bq/g). It is caused mostly due to presence of ^Co and 54Mn as presented in Table 5 and Fig.3.

160 Table 5 - Activities of the most detected miclides in RPV-steel specimen k716 BM-I (25.6 mg)

Nuclide Sbl24 Co 58 Co 60 Cr51 Fe59 J131 Mn54 Na24 Activity [Bq] 6438 1673 160750 22622 47952 37660 544790 143 Error ±71 ±45 ±206 ±293 ±175 ±2443 ±312 ±12

7 ^/-Mn-54 k716 BM-I 6- r

2- Co€0 Sb-124 1- Cr-51 Fe-59 \^<7 o J~~^~~^l . , , 1 j , 1 1 , 1 n-—r—T i 1—i—-1 1 1 1 1—1 1—T 1 1 1 P T 1 1 1 250 500 750 1000 1250 1500 Energy (kev)

Fig. 3 - Gamma spectroscopy spectrum of irradiated RPV-steel specimen k716-MB-I.

In order to study some microstructural changes in irradiated nuclear RPV-steel samples using positron-annihilation technique, a new three-detector set-up, suitable for a positron 1-Dimensional Angular Correlation of Annihilation Radiation (1D-ACAR) study of ^Co - containing materials, was developed [5]. This equipment was arranged especially for the irradiated RPV-steel specimens measured in framework of "Extended surveillance specimen program".

3 CONCLUSION

The obtained results from the present measurements indicate that MS is effective technique for the evaluation of microstructural changes in RPV-steels and, in combination with other spectroscopic methods (Positron annihilation techniques, Transmission electron microscopy, ...) can contribute to an increase of NPPs operational safety. Interpretations of the results obtained from angular correlation

161 PAS technique are in agreement with the findings from the Mossbauer effect experiments. It will be very interesting to study the second batch of the specimens, which was taken out of reactor after 2 operating years in July this year.

ACKNOWLEDGEMENT We would like to thank to IAEA (Research contract 9001/RBF) and partly to Grant 1/4286/97 support.

References [1] G. Brauer, W. Matz, Cs. Fetzer, Hyperfine Interaction 56 (1990) 1563-1568 [2] J. Lipka, J. Hascik, V. Slugen, L. Kupca, M. Miglierini, RGrone, I. Toth, K. Vitazek, J. Sitek, Proc. Int. Conf. Vol. 50, ICAMF95,1. Ortalli(ed), SIF, Bologna, 1996, p. 161 - 164. [3] V. Slugen, E. De Grave, P.M.A. De Bakker, Journal of Electrical Engineering, 48 (1997), p.95-98 [4] V. Slugen, IAEA Research report 9001/RBF, FEI SUT Bratislava, 1996, 66 p. [5] R. Grone, J. Hascik, V. Slugen, R. Lipka, P. Pietryzk, K. Vitazek, Nucl. Instr. & Meth. in Phys. Res. B 129, (1997), p.284-288

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