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VIBRATION MONITORING BY NEUTRON NOISE ANALYSIS AND ACCELERATION

MEASUREMENTS INSIDE OPERATING NUCLEAR POWER REACTORS

J. Runkel, D. Stegemann, J. Fiedler, P. Heidemann,Institute of Nuclear Engineering and Nondestructive Testing, University of Hannover (IKPH)

Elbestr. 38 A, D-30419 Hannover, Germanye-mail : [email protected]

R. Blaser, F. SchmidVibro-Meter SA

P.O. Box 1071, CH-1701 Fribourg, Switzerlande-mail : [email protected]

M. Trobitz, L. Hirsch and K.ThomaNuclear Power Plant Gundremmingen (KRB)

P.O. Box 300, D-89355 Gundremmingen, Germany

ABSTRACT

A miniature biaxial in-core accelerometer has been inserted temporarily inside the Travelling In- coreProbe (TIP) systems of operating 1300 MWel Boiling Water Reactors (BWR) during full poweroperation. In-core acceleration measurements can be performed in any position of the TIP-system. Thisprovides new features of control technologies to preserve the integrity of reactor internals.The radial and axial position where fretting or impacting of instrumentation string tubes or otherstructures might occur can be localised inside the reactor pressure vessel. The efficiency and long-termperformance of subsequent improvements of the mechanical or operating conditions can be controlledwith high local resolution and sensitivity. Low frequency vibrations of the instrumentation tubes weremeasured inside the core. Neutron-mechanical scale factors were determined from neutron noise,measured by the standard in core neutron instrumentation and from displacements of the TIP-tubes,calculated by integration of the measured in core acceleration signals. The scale factors contribute toqualitative and quantitative monitoring of BWR internal’s vibrations by the only use of neutron signals.

1. INTRODUCTION

A miniature biaxial accelerometer was developed for acceleration measurements in radioactive and hightemperature environments by Vibro-Meter. In co-operation with IKPH a special version of the sensor,which can be inserted inside the Travelling In-core Probe (TIP) tubes of Nuclear Power Reactors insteadof the TIP, was developed [1]. The geometrical dimensions of the accelerometer and the TIP are similar,therefore the accelerometer can be moved in the same way as a TIP. The coupling between the transducerand the inner wall of the tube is performed by means of a specially designed adapter spring (fig.1).

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A sufficient high coupling force foracceleration transmission and a sufficientlow frictional resistance for moving thesensor by means of pushing the cable hasbeen verified in a full scale model of a TIPtube and instrument string. The newsensor has been inserted in all TIP tubesof both units of the KRB during fullpower operation. The low frequencyvibrations of the reactor and core internalswere investigated [2,3]. In higherfrequencies flow induced impacts of somein-core instrument tube structures werelocalised. The German Company SiemensKWU developed a technology tostrengthen the impacting tubes withspecial sleeve tube. The sleeves were mounted during regular outages of the plants. After restarting itcould be verified by repetitive reactor external and internal acceleration measurements, that the impactintensity had been reduced drastically due to the improvement of the construction.

2. SENSOR DESIGN

The design of the biaxial accelerometer is effectively imposed by thegeometrical envelope permitted, i.e. that it should be attached with thelonger axis parallel to the tube that needs vibration measuring. This meansthat the transducer is sensitive in x and y directions and without sensitivityin the z axis where the length is relatively unimportant. The biaxialaccelerometer works in the so called ”bender” mode patented in 1992(European Patent No. 316498 and US Patent No. 5.117.696).

The whole assembly consisting of transducer head and spring (fig.1) doesnot have sharp edges and is designed to be easily moved forward andbackward instead of the travelling neutron probe inside TIP-tubes. Thesensing elements (fig.2) are using the piezoelectric effect to measurevibrations. The charge output of a piezoelectric ring is proportional to thedynamic compression applied to it. The piezoelectric stack is held togetherby means of a pre stressed rod. On the top of it there is a seismic massfixed to the piezoelectric stack.

In order to obtain a sufficient sensitivity the number of crystals together with the total seismic massoperating on them has to be optimised. The total mass working in the crystal stacks is composed of ahigh density material (tungsten) in order to get the maximum mass within the minimum volume. Thewhole sensing element is bound together under a high pre stress so that no parts may move with respectto each other especially under the maximum shock that is encountered. It is essential that the pre stressdoes not sensibly change with temperature, which necessitate the incorporation of a thermalcompensation material to match the rates of expansion between the diverse parts within the sensingelement and the pre stressed rod. Under the influence of inertial forces which act perpendicular to theaxis of the pre stressed rod, seismic mass bending load occurs which result in an increase of thecompression at one side and in an decrease of the compression at the other side of the piezoelectric rings.

Fig 1: Biaxial Accelerometer with Adapter Spring andinserted inside a TIP tube

Fig 2: Inner Construction

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Each piezoelectric ring has a four segment electrode mounted on its top face and a four segmentelectrode mounted on its bottom face. The opposite pairs of segments are connected together so as toprovide a biaxial accelerometer. The integrally attached hard-line cable compromises a MgO insulationand four conductors which are connected to the electrically separated sensing elements. The choice of thepiezoelectric material is driven by the operational temperature, the radiation and the design itself whichrequires very homogenous and stable materials. The natural hydrogen piezoelectric crystal defined as”Vibro-Meter VC2” was chosen so far. Although this material has been found not linear in terms ofsensitivity under high radiation at low frequencies (see next section), the previous applications show thatthe biaxial accelerometer can be successfully used for acceleration and vibration measurements insideoperating power reactors. Further developments will be in finding the best piezoelectric material for theapplication and its evaluation under high radiation.

3. RESULTS OF LOW FREQUENCY VIBRATION MEASUREMENTS INOPERATING 1300 MWel BWRS

The vibrations of BWR internals were analysed in low frequency range (below 10 Hz) by use of thesimultaneously measured signals of in-core neutron detectors and the in-core biaxial accelerometer. Thelater one was temporarily positioned at the neutron detector height inside of several instrumentationtubes during full power operation. The biaxial sensor could be moved inside the TIP tube to any positionup to the upper core grid (fig.3). It was inserted in the TIP tube at the position of the cable reel andpushed with the cable itself. Pre-operational tests in a full scale model of the TIP system have beenperformed for the successful optimisation of the adapter spring, which must be weak enough so that thedetector can be moved and strong enough so that the coupling force is sufficient to measure the tubevibrations in the frequency rage of interest (up to 1 kHz in case of impact detection, see next section).

The normalised auto power spectral densities (NAPSD) of in-core neutron noise signals, measured in oneof the 44 instrument strings of the BWR (fig.4, top of right side) show a significant peak at 2,5 Hz. It isknown that this peak is caused by a vibration of the instrumentation tubes between the upper and thelower core grid, without impacts or contacts of the guide tube to the surrounding fuel channel boxes[6,7]. The corresponding auto power spectral densities (APSD) of the accelerometer signals (fig.4, leftside) have a similar shape at the measuring positions inside the core, while they deviate as expected atmeasuring positions below the lower core grid and outside of the reactor pressure vessel. The amplitudesof the movements can be inferred from the acceleration signals by double integration in time domain aswell as in frequency domain and from the neutron noise spectra through neutron mechanical scale factorsrelating the random displacements to the relative change of the neutron signal. From the APSD ofdisplacement the Root Mean Square (RMS) value in x and y direction and the resulting vector can becalculated [5]. The procedure of calculation of the RMS is based on integrating the shape of a peak givenby a linear vibration equation, but the effect of non-linear sensitivity of the accelerometer in the lowfrequency range and under the influence of radiation has to be considered (see next paragraph). Theresulting RMSx,y has the dimension of mm. The RMSn value of neutron signals is calculated from theNAPSD in the same frequency range correspondingly. For correlated acceleration and neutron noisesignals, neutron-mechanical scale factors for different vibration modes of BWR instrument tubes (at 1,6Hz and at 2.5 Hz) and for a combined vibration of instrument tubes and fuel channel boxes atapproximately 4 Hz were determined [2,3]. This enables to estimate the displacements of thecorresponding vibrating components inside the core of the given BWR through the scale factors from theneutron noise spectra of signals measured by the standard in-core instrumentation. For a PWRapplication there have been published some corresponding results [4,5].

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Fig 3: In-core Instrumentation of a 1300 MWel BWR

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Fig 4: Acceleration and Neutron Noise Spectra

Under the influence of the high radiation inside the core a remarkable increase of the measuredamplitudes of the acceleration signals was observed in the low frequency range (fig.5). This reversible,frequency and radiation dependent sensitivity change of the biaxial in core accelerometer is not fullyclarified until now. But the effect must be taken into account and corrected for the determination of thescale factors.

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Further investigations will be done to find new piezoelectric materials for the specific application in highradiation and high temperature environment in order to determine and minimise the radiation influence tothe low frequency signal of the in core accelerometer.

Fig 5: Reversible and Frequency dependent Sensitivity Change of Accelerometer due to high Radiation

4. Detection of Impacting In-core Instrumentation Tubes Inside Operating 1300 MWel BWRs

The vibrations of all in-core instrumentation tubes of both units of the KRB are frequently analysedtwice in each fuel cycle by use of neutron noise analysis. Particular vibrations of single instrumentstrings were found and monitored subsequently [6]. The instrument tube vibrations and the effect ofpossible impacts to surrounding structures on the vibrations were analysed and analytically modelled[8,9]. The theoretical results were used to interpret the particular instrument tube vibrations. Thisinterpretation gave some hints that impacts of at least one of the instrument tubes against the surroundingstructures occurred. An detailed inspection of the relevant tube during a subsequent regular shut downwas made, and it was found, that the instrument guide tube was damaged at the position of the mountingbell (fig.3) and that flow induced impacts of the three telescoped tubes of that particular instrument stringhad caused a degradation of the structure in the area of the mounting bell.

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Fig 6: Acceleration signals measured in all TIP tubes of a 1300 MWel BWR

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Fig 7: Acceleration signals measured in one TIP tube at several axial positions inside thereactor pressure vessel and the core

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In order to detect even weak impacts, which might not influence the vibrations of the instrument tubesinside the core region and which therefore cannot be detected by neutron noise analysis, the new biaxialaccelerometer has been inserted in all TIP tubes during full power operation.

In the frequency range up to approximately 1 kHz (due to the sensor construction and the adaptation by aspring (fig.1) this is the maximum frequency of the measuring system) flow induced impacts of someother in-core instrument tube structures were localised, too.

The acceleration signals (x- and y-direction) measured inside of the TIP tubes at the mounting bellposition of all instrument strings are shown in figure 6 in time domain. In a few core position bursts,caused by impacting structures, are clearly visible. At those core positions, the biaxial in-coreaccelerometer has been moved to several axial positions up to the upper end of the TIP tube, in order tolocalise were the impacts were transmitted to the TIP tube. In figure 7 some examples of thesemeasurements in one of the instrument strings are given.

Comparing the signals in fig7, it is not clearly seen, inwhat axial position thestrongest bursts aremeasured. In order to indicatethe position were the impactsare created, statistical valuesof the acceleration signalswere calculated and some ofthe results are in fig.8. Thestandard deviation (fig.8,upper part) characterises theenergy content respectivelythe averaged amplitudes ofthe signals. The Kurtosisfactor (fig.8, middle part) isused to quantify the burstcontent of the signals. If thesignal fluctuations arenormally distributed and arenot influenced by bursts, thenthe value of the Kurtosisfactor is three, while burstscause an increase of it. Forweighing the energy andburst content of the measuredacceleration signals wedefined the product of thestandard deviation and of thekurtosis factor as an intensityvalue of possible impacts.

The comparison of the statistical values calculated from measured acceleration signals at severalpositions inside a single TIP-tube before and after the guide tube has been sleeved (see below) shows

Fig 8: Statistical analysis of acceleration signals measured in oneTIP tube at several axial positions

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relatively high standard deviations and consequently relatively high intensity values (product of standarddeviation and Kurtosis factor) in the core region (fig.8). This is caused by the obvious increase of thesensitivity of the biaxial accelerometer when it was exposed to the high flux inside the core. But this hasno influence to the Kurtosis factors, which show a relative maximum in the axial vicinity of the in-coreinstrumentation guide tube mounting bell where the impacts happened. Obviously the bursts weretransmitted from the guide tube to the inner TIP tube through the water inside the gaps between thetelescoped tubes. This enables the localisation not only of the radial but also of the axial position wherefretting or impacting of instrumentation tube components or other structures might occur inside thereactor pressure vessel and core.

The German company SiemensKWU developed a technology tostrengthen the impacting tubes byspecial sleeve tubes inside theinstrument guide tube at theposition of the mounting bell. Thesleeves were mounted duringregular outages of the plants. Afterrestarting it could be verified byrepetitive reactor external andinternal accelerationmeasurements, that the impactshave been reduced drastically dueto the improvement of theconstruction (fig.9). After thisimprovement the Kurtosis factorscalculated from the accelerationsignals measured at the samepositions as before (fig.8) still show a maximum at the mounting bell position, because weak impactswere registered after strengthening the guide tube, but the intensity values demonstrate the remarkablereduction of the impact energies.

In figure 10 the impact intensity values of all in-core instrument tubes measured at the axial position ofthe guide tube mounting bell are compared before and after strengthening two of the guide tubes (thosewhich had the highest values before). It is clearly seen, that the sleeves reduced the impact intensity atthese positions and that the intensity did not increase at other positions, where low intensity was detectedpreviously and consequently no sleeves has been mounted. Repetitive measurements of the in-coreacceleration inside all of the TIP-tubes allows the early detection of degradation due to fretting andimpacting. The efficiency and long-term performance of subsequent improvements (sleeves) can becontrolled with high local resolution and sensitivity.

Fig 9: Acceleration signals measured in a TIP tube beforeand after the installation of a sleeve

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Fig 10: Reduction of impacts due to the improvement of the construction of guide tubes,comparison of statistical values

CONCLUSIONS

A miniature biaxial in-core accelerometer was successfully used to detect even weak impacts and tolocalise the radial and axial position of impacting components inside the reactor pressure vessel duringfull power operation. The measuring system can be used without any interference to the regular reactoroperation and without constructive changes of the existing in-core instrumentation. This provides a newand high-sensitive diagnosis tool to control and preserve the integrity of reactor internals.

In low frequency range and under the influence of high radiation inside the reactor core a nonlinearsensitivity of the sensor was observed, which has to be considered in determination of the neutronmechanical scale factors.

REFERENCES

[1] Runkel, Laggiard, Stegemann, Heidemann, Blaser, Schmid and Reinmann, ”IncoreMeasurements of Reactor Internals Vibrations by Use of Accelerometers and NeutronDetectors”, Proceedings of Specialist’s Meeting on In-core Instrumentation and Reactor CoreAssessment, Mito-shi, Japan, 1996, OECD publication 1997

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270°0°

Without sleeve Improved with sleeve

Product of standard deviation and kurtosis of the measurements at axial position of mounting bell

180° 180°270° 270°

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[2] Stolle, ”Schwingungsuntersuchungen an Instrumentierungslanzen von Siedewasserreaktoren”,Diplom-work IKPH, 1997

[3] Stegemann, Runkel, Fiedler and Heidemann, “Ergebnisbericht zu Schwingungsmessungeninnerhalb der Fahrkammerrohre von LVD-Lanzen des Blockes C der KernkraftwerkeGundremmingen, 11.BE-Zyklus”, Internal IKPH / KGB Report No.: KGB/BC/BEZ.11/05.97,1997

[4] Laggiard, Fiedler, Runkel, Starke, Stegemann, Lukas, and Sommer, “Vibration Measurements inPWR Obrigheim by Use of Incore Accelerometers”, Progress In Nuclear. Energy , vol. 29, 1995

[5] Laggiard, Runkel, Stegemann, Fiedler, “Determination of Vibration Amplitudes and Neutron-Mechanical Scale Factors Using Incore Accelerometers in NPP Obrigheim”, Proceedings ofSMORN VII, Avignon 1995, OECD Publication 1996

[6] Runkel, Laggiard, Stegemann, Fiedler, Heidemann, Mies, Oed, Weiß and Altstadt “Applicationof Noise Analysis in Two BWR Units of Nuclear Power Plants Gundremmingen”, Proceedingsof SMORN VII, Avignon 1995, OECD Publication 1996

[7] Altstadt and Weiß “Numerische Untersuchungen zum mechanischen Schwingungsverhalten einernassen LVD-Lanze”, report produced for IKPH, 1993

[8] Laggiard, Runkel and Stegemann “One-dimensional bimodal model of vibration and impactingof instrument tubes in a BWR”, Nuclear Science and Engineering, 115, 62, 1993

[9] Laggiard, Runkel and Stegemann “Three-dimensional model of vibration and impacting ofinstrument tubes in a BWR and transfer from mechanical to neutron noise”, Nuclear Science andEngineering, 120, 124, 1995


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