The influence of noise diagnostic techniques on the safety and availability of nuclear power plants

Progress in Nuclear Energy. 1985, Vol. 15, pp. 513-524

(1079-65311/85$0.00 + .511 Copyright (~) 1985 Pergamon Press Ltd

Printed in Great Britain.

THE INFLUENCE OF NOISE DIAGNOSTIC TECHNIQUES ON THE SAFETY A N D AVAILABILITY OF N U C L E A R POWER PLANTS W.

BASTL, R.

S U N D E R AND D .

WACH

Gesellschaft fur Reaktorsicherheit (GRS)mbH, D-8046 Garching, Federal Republic of Germany

ABSTRACT Diagnostic techniques based on vibration and noise analysis have been developed in FRG with the aim of incipient failure and malfunction detection in nuclear power plants. On the basis of the Vibration Monitoring System in the nuclear power station GKN, the measuring system, the diagnosis concept and the interpretation results will be discussed in more detail. In order to achieve full acceptance for these methods a completely understanding of signal sources, long-term trends and alert levels is necessary. Some newer results of successful interpretation of signature deviations are described. Discussion of the benefits shows that noise diagnostic techniques can increase the plant availability and improve the operational safety. KEYWORDS PWR; vibration monitoring system; spectra interpretations; long-term trends; neutron noise; subcooled boiling; mechanical vibrations; core barrel integrity; fuel pin vibrations; shaft vibrations. INTRODUCTION When looking at the history lines can be observed: -

of

failure

detection methods

development,

two main

the integral detection approach (signature approach) the dedicated detection approach

While the integral approach was based upon the idea as to "visualize" an undue deviation from a reference signal pattern corresponding to the normal status of the plant, the second approach tried to identify specific signals, where the interesting physical phenomena manifestes themselves as a part of dynamic signal. As one of the typical examples we recall the mechanical vibration of control rods as a source for reactivity perturbations, which finally lead to a typical fluctuation of the neutron flux signal. At a first glance the integral approach has its merits, because it looks to be complete from the beginning. But it has an important deficiency, because mostly it is just able to indicate that something is going abnormal, but not what it is. The latter information, however, is of utmost importance for the operator. For this reason we concentrated in further developing of the dedicated approach based upon two groups of signals -

signal bursts (structure borne sound) signal noise (neutron flux, pressure, displacement)

With respect to the first signal type it is referred to our presentation "Source location and mass estimation in loose parts monitoring of LWR's" in the Poster Session /i/. In our paper we deal with the analysis of continuous noise signals and its practical applications in online vibration analysis systems. Furthermore some specific findings are dealt with, which are either due to other physical phenomena showing up in the available signals or which were elaborated when investigating certain mechanical components. 513

W. BASTL etal.

514

ACCEPTANC OF EARLY FAILURE DETECTION TECHNIQUES In spite of great successes of online vibration analysis in the course of specific campaigns, and in spite of considerable progress of the measurement and analysis techniques, the development towards an operational system turned out to be a very difficult task. This was mainly due to the fact, that the traditional instrumentation of power plants follow the one sensor - one signal concept and that more complicated signal treatment in order to achieve higher level information was i ntroduced comparably slowly. In addition the new online vibration measurement procedure called for not only specific signal analysis (spectral analysis, coherence analysis), but also for an interpretation of the spectra or correlation functions. The interpretation in turn was lacking - for a long time - completeness and certainty. But recalling singular results like the measurement of excessive shield vibrations, core support structure and similar, the licensing authorities and the utilities realized the potential of the online vibration measuring systems, -

to to to to

give information of a potential mechanical deficiency in its status nascendi permit an observation of the further development of this deficiency judge on the severity of the deficiency and consequently decide if and when the plant has to be reduced in power or even shut down.

From these few points, the strong linkage between safety- and availability criteria already become evident. In any case the new measurement and analysis techniques provided an information tool about the mechanical integrity of the plant which was not available up to now. Such a tool is of specific importance for systems, which are not accessable during operation, and this holds for various components of the primary and secondary system. Today online vibration measuring systems have to be installed in all new German nuclear power plants. The RSK-guideline for PWRs (RSK = German Reactor Safety Commission) says: "The vibrational behaviour of the reactor pressure vessel internals has to be examined by suitable measurements already during the commissioning phase of the plant. Repetition of such measurements should also be possible during operation of the plant." As per nuclear standard KTA 3204 (KTA = Nuclear Safety Standard Commission) also a vibration monitoring system is required. Repetitive measurements and analyses with VMS shall be performed three times per fuel cycle. The basic idea of these standards is -

-

to have signals available pertinent to reactor vibration to perform periodical measurements of these signals to analyze the signals for undue deviation.

This idea was not at least governed by the successful use of, e.g. neutron flux signals for the detection of excessive vibration of structural parts inside the reactor vessel, and by the experience that it is necessary to have available the signal history, if strange deviations during operation of the plant need to be interpreted. Unfortunately there is still some hesitation from the operator to make full use of the abilities of the systems. As already mentioned before one of the main reasons was the incompleteness of the signal interpretation. The vibration analysis is based upon the power density spectra and the correlations of various signals. In order to achieve full acceptance it is necessary to completely understand these spectra within the frequency range of interest. This means detailed interpretation of the signal patterns excellent knowledge of verified signal changes extension of analysis results to other power plants of different size long-term signal behaviour, especially under changed operation conditions, stretch-out operation etc. During the recent years we performed extensive investigations to achieve this goal. Some of the most important steps are treated in the next chapter. It is hoped that the results will contribute to a full acceptance of online vibration monitoring systems by the operator. The merits of vibration monitoring are evident.

Influence of techniques on the safety and availability of plants

SIGNATURE

515

I N T E R P R E T A T I O N AS A BASIS FOR C O R R E C T D I A G N O S I S

A t the o c c a s i o n of SMORN III the p r o t o t y p e system of a V i b r a t i o n M o n i t o r i n g System (VMS) as i n s t a l l e d in the N e c k a r w e s t h e i m p l a n t GKN has b e e n d e s c r i b e d /2/. The s y s t e m includes sensor types and sensors locations as i n d i c a t e d in figure i. The sensors u s e d are: -

-

4 i n d u c t i v e absolute d i s p l a c e m e n t sensors on closure head of R P V (A-signals) 6 r e l a t i v e d i s p l a c e m e n t sensors, 2 b e l o w each m a i n c o o l a n t pump, m e a s u r i n g in two h o r i z o n t a l d i r e c t i o n s (R-signals) 8 e x - c o r e n e u t r o n sensors o f the s a f e t y - i n s t r u m e n t a t i o n (X-signals) 4 p i e z o p r e s s u r e sensors l o c a t e d at i n l e t / o u t l e t pipes (P-signals).

This sensor c o n f i g u r a t i o n research programmes the demonstrated: -

was used from the b e g i n n i n g of our work. In c u r r e n t suitability of the following sensor-types could be

6 eddy c u r r e n t sensors, 2 at each m a i n c o o l a n t pump shaft, m e a s u r i n g in two h o r i z o n t a l d i r e c t i o n s (W-signals). 12 incore n e u t r o n sensors, a x i a l l y d i s t r i b u t e d at two d i f f e r e n t core p o s i t i o n s (coded No 22 and No 23).

VIBRATION

PLANT:

MONITORING SYSTEM

w=o

INSTRUMENTATION: 20 neutron nolse sensors 4 pressure noise sensors 18 vibration sensors

wzR Loop 2

MEASUREMENTS:

6 preoperatlonel measurements 3 start up/shut down measurements 31 power operation measurements during 8 core cycles

STORAGE:

40 signal PCM tape recording

ANALYSIS:

32 channel software packages

At

~,,, lle I -

Gemelnechafts Kernkraftwork Neckar (GKN) 3-Loop-PWR, 855MW

Mo

x*o

Fig.l: V i b r a t i o n and noise sensor p o s i t i o n s in PWR p r i m a r y c i r c u i t

Fig.2: A p p l i c a t i o n of noise diagnostic techniques in GKN

The incore d e t e c t o r s are a r r a n g e d in lances in diametrol p o s i t i o n s near the center of the core. E a c h lance houses 6 detectors l o c a t e d at d i f f e r e n t core heights. It s h o u l d b e m e n t i o n e d that the incore signals up to n o w are not p a r t of the s t a n d a r d VMS. Though their interpretation is r a t h e r d i f f i c u l t b e c a u s e of the strong influence of local effects, there are several benefits: there are no b a s i c o b j e c t i o n s for c o n n e c t i n g incore sensors to specific analysis tools, b e c a u s e they are as a rule n o t p a r t of the r e a c t o r safety system one radial core p o s i t i o n is e q u i p p e d w i t h several sensors in d i f f e r e n t heights, so that to some e x t e n t r e d u n d a n t i n f o r m a t i o n can be g a i n e d incores p r o v i d e an i m p o r t a n t source of signals from inside the r e a c t o r vessel as a w h o l e the incore signals p r o v i d e p o s s i b i l i t i e s for additional c o r r e l a t i o n analyses w h i c h is useful to i d e n t i f y signal sources. In the last years the VMS in GKN has b e e n c o n s i d e r e d as an integral o p e r a t i o n a l instrumentation. This is due to the following facts: -

p a r t of the

D u r i n g the startup phase of the p l a n t in 1975 and 1976 the internal and loop c o m p o n e n t v i b r a t i o n s were i n v e s t i g a t e d in detail (9 m e a s u r e m e n t s during cold and h o t functional tests). Since that 31 further "routine" m e a s u r e m e n t s have b e e n performed; a c c o r d i n g to the needs b e t w e e n three and five per fuel cycle.

516

W. BASTL et al.

Our efforts concentrated in a complete analysis and interpretation of the signal spectra, so as to achieve a firm basis for all undue deviation from normal readings. The full interpretation of the spectra was possible only by gradual adaption of our mechanical and hydrodynamical models to the measurements /3/,/4/ the performing of systematic periodical measuring campaigns and the excellent cooperation with the utility and the vendor. Owing to the indirect character of the used signals, the statistical functions of the signals measured with a vibration monitoring system usually contain the structural vibrations of primary circuit components only in a masked manner. Moreover, many component vibrations are measured in different ways. For interpretation of spectra it is necessary to gather all relevant information resulting from -

design features definition of modal parameters by shaker tests transient measurements including parameter variations operational measurements at steady state conditions structural model calculations and calculation of resonant frequencies backfitting of inspection results

-

The following APSD's of selected signals shall give a survey about the state of the art of interpretations. A high level in interpretation is a basic condition for general acceptence of these methods. Reference Spectra of t h e

Vibration Monitoring System

First of all, the pressure-noise spectra shall be discussed. Pressure fluctuat/ons are the essential exciting forces for structural vibrations. The pressure spectra from outlet-signals contain broad-banded background-fluctuations, superimposed by fluid resonances (Fig. 3). They can be explained by standing waves according to the primary piping system or for the volume control system. Due to different loop lengths, the frequencies differ from one signal to the other. The 25 Hz-Peak is a reaction from forced RPV-CB pendular vibration. The reason for this vibration are unbalanced forces of the pump shafts at rated speed acting via the shaft bearings, the pump housing and the pipings to the RPV. Analysing the relative displacement signals from inlet bend pipes of main coolant pumps, the above mentioned influences of standing waves and shaft vibrations can be recognized (Fig. 4). In addition to this, rigid body vibrations of main coolant pumps (MCP) and steam generators (SG) could be identified - in particular pendular vibrations in two directions, vertical and lateral vibration modes. 85C-MW-PWR

TSGpendulsr vibration (-M~)

~t~diflQ WaVe8 (M2, vo/tl'ae ¢on~ol lystem) ~ WaVi~B(AI4~ubet LOOp3) ~ 8 t 8 ~ 1 ~ wsvell (;~/4,ou¢~ Loeo 2)

! TMCP pend~" ,~braUon earn | I T M ~ Penddar v l b ~ t ~ (-re'v) | I ItSG pendukw vlbrdon ~-~,n

/ / l/

////

~

/

~

/

TForced vlb~tlon of MCP |

kter~ ,~braUon

tMCP verllcalvlwal~on

APSD

i

10 ~

i

:,v ,Jl J

10

-

10 -~

10-'

o

10

20

30 FREQU(~NCY40

Fig.3: Interpretation of pressure noise spectra

0

Fig.4:

10

20

Interpretation of vibration spectra

30

FEQUENCY 40

pump

R11

Influence o f techniques on the safety and availability of plants

517

;This leads to the interpretation of RPV vibration spectra as shown in figure 5. Dominating peaks in the spectra can be related to eigenfrequencies of RPV and internals: pressure vessel pendular- and vertical-vibrations, core barrel (CB) beam mode, pendular vibrations of core (C), upper core barrel (UCB) and secondary core support (SCS) structure. It should be pointed out that all pendular vibrations of RPV/internals show different pendular directions with different bearing stiffness and different frequencies. For this reason, the corresponding frequency peaks in two different spectra are split up according to the angular positions of the sensors. Standing waves, shaft-vibrations and rigid-body vibrations of all main coolant pumps and steam generators act as external excitations to the pressure vessel. The forced pendular vibration of RPV at 25 Hz shows a deterministic behaviour. [CIIBEAM bration ICP> vfbratlce fMCP vibratio~ /tSG vibration

MODE 850-MW-PWR RPV-C PENDULAR VIBRATION RPV-CB PENDULAR VIBRATION RPV VERTICAL VIBRATION RPV-UCB PENDULAR VIBRATION t Forced vibration of RPV ~tion j RPV-SCS PEND.

N~SDI~ II

TSW

8SO-MW-PWR

t temperature noise /ttFA VIBRATION (one,rid fr,,} Ill IIFA BEAM MODE (bo~ en~ damps) |11 | | u F A BEAM MODE t,,~,.o Jg II I l f C B BEAM MODE

~v~[[

hw

,F~ed ~ , , ~ ~ R ~

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'

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10~

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X2U X3U X4U

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~F"

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10 ~ 0

10

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30 FREQUENCy 40

0

10

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30 FREQUENCY 40

Fig° 6 : Interpretation of ex-core neutron noise spectra

Fig.5 : Interpretation of pressure vessel vibration spectra

Signal fluctuations of ex-core neutron detectors are generated when the spatial energy distribution is changed by dynamic processes. Besides fluctuations in the coolant density, e.g. standing waves or temperature fluctuations, such processes include the vibrations of internal structures (Fig. 6). Relevant vibrations causing variations of the thickness of the water layer between fuel assemblies and detectors or influencing the energy distribution of the neutron flux density in the annulus are 'core barrel beam mode vibrations' 'core barrel shell mode vibrations' and 'fuel assembly (FA) fundamental beam mode vibrations' and 'higher harmonics' 850-MW-PWR temperature nese ft FA VIBRATION (o~e end free) ///ttFA BEAM MODE (both endsC,Isn%oed) ///I/ ?TFA BEAM MODE (1.Harm) ||| | | i | TtCB-BEAM MODE /1/// /I// tfFA BEAM MOOE (2,Ham.) /1/ / l II// II RPV-UCB PENDULAR VIBRATION NAPSDJU ~ tSWU// ~w F~ vibration of RVP TSW "*

J

1 0 "6

,,, ~J

10-' ~ 10 "'

*2 x13 .23x15 -23x1@

i

vlbratk~n (-se) vixatlon (-RPV) Sd0harmonlc shaft vibration , Unbelanced shaft forces Shaft vibration

7

p

I ~ W1D nW1R

10 ~

I

10=

i

10'

'" I.

10 0 1010"

0

Fig.7:

1

Interpretation of incore neutron noise spectra

.z

0

Fig.8:

10

20

30 FREQUENCY 40

Interpretation of shaft vibration spectra

"*

W. BASTL etal.

518 Reference

Spectra of other Plant Signals

The incore neutron noise instrumentation is also a very sensitive indicator for fuel assembly vibrations. Figure 7 shows spectra of six axially distributed SPNdetectors, the fuel assembly beam modes are marked. Core barrel beam modes and vibrations of the upper core barrel are excitations for the fuel assemblies. Standing waves of the coolant and temperature fluctuations are additional driving forces in incore neutron noise. Spectra of the shaft instrumentation of main coolant pump 1 (MCPI) are shown in figure 8. Besides the wellknown 25 Hz peak (shaft unbalance at rated speed) the spectra show strong peaks at 12.5 Hz and 37.5 Hz. The 12.5 Hz peak is the subharmonic shaft vibration - a nonlinear vibration of the shaft in hydrodynamic bearings. Finally, pendular eigenfreguencies of the pump housing are visible. Alert Levels

in normal

and strech-out

Operation

If particular internal vibrations shall be monitored, it is necessary to give the operators an idea of the allowed frequency window and the alert level. Of course these values are dependent on the plant status, e.g. normal i00 ~ operation, reduced power operation or stretch-out. The complete interpretation of the spectra in addition with a basic knowledge of the long-term behaviour of the structure resonances are essential requirements to define alert levels for vibration monitoring. Three examples shall be given: core barrel clamping - loss of axial preload (Fig. 9, left) secondary core support coupling - loss of screw-prestressing (Fig. 9, middle) - fuel assembly integrity - reduced spacer clamping conditions (Fig. 9, right)

Fig.9:

Long-term

trend and alert levels

for vibration monitoring

The first example is concerned with the detection of significant loss of axial preload of the core barrel upper support flange. The core barrel beam mode vibration can be separated in APSD of absolute displacement spectra as well as in ex-core neutron noise spectra. The resonance peaks are split due to different pendular directions and the corresponding sensor sensitivities. Pursuing this peaks for several core-cycles one can state a very constant behaviour. During stretch-out conditions, a frequency-shift caused by decreasing coolant temperatures and increasing water density and water masses can be observed. Basic attention for vibration monitoring will be concentrated to the clamping conditions of core barrel-pressure vessel, realized by hold-down springs. Experiences from other plants show that a relaxation of hold-down springs will change the dynamic characteristic of core barrel beam mode unmistakably: Decreasing of the integrated spring forces to 70~ of admissible values will reduce the eigenfrequencies of core barrel vibration to 94~. This limitation is marked besides the long-term trend. Although the beam mode vibration shows a direction-dependent eigenfrequency, a running-out of the allowable area is easily detectable. The second example is concerned with the pendular vibration mode of the secondary core support structure. The SCS eigenfrequency can be identified in spectra of RPV vibration sensors very clearly. The resonance frequencies are different for the orthogonal directions. This has been considered when drawing the allowed frequency window (shaded area). Investigations of the long-term trend indicate a systematic

Influence of techniques on the safety and availability of plants

519

decrease of SCS pendular eigenfrequencies due to material properties of less then 0.i Hz per year, a distinct increase can be realized during stretch-out conditions. In another plant, the resonance frequency of the SCS was found to have shifted downwards by 0.4 Hz within four months. This frequency shift indicated that the coupling of the SCS structure versus RPV was becoming weaker. It was found out by torque-measurements that 6 of 32 screws had no pre-stressing. Referring to the GKN-plant, we transferred this factual situation by setting an alert level as shown in the figure. The third example is concerned with ex-core neutron noise signals in the frequency range of the fuel assembly beam modes, showing a distinct long-term trend. Comparing different measurements a systematic decrease of this frequency-peak during the lifetime of the core can be observed. The cause of this frequency shift can be explained by radiation effects reducing the coupling between fuel rods and the spacers. During stretch-out conditions of the plant, a second frequency-shift can be observed. Reasons are pellet-cladding interactions (PCI). Due to the limited "seeing area" of the detectors, the major contributions to the detector signal come from new fuel assemblies standing in outer core regions. After refueling and replacing of fuel assemblies, initial conditions are the same for every fuel cycle. Assuming that structure dynamics from second core fuel assemblies also do influence the ex-core-signals, a progressive reduction of the eigenfrequency must be considered. Therefore, an alert level like the inserted boundary can be postulated. A falling below this alert level would signify an undue decreasing of fuel assembly spacer clamping conditions. EXAMPLES OF USEFUL DIAGNOSIS

INFORMATION IN NOISE SIGNATURES

Successful interpretation of signature deviations and predictions of their structure-mechanical causes have been reported in various publications during the last years (e.g. /2/-/6/). In this paper some newer results shall be presented derived from spectra, coherence and phase functions. The first two examples are taken from signals of the standard VMS as installed in all new German PWR-plants, the other three are concerned with investigations being performed with the aim to apply noise diagnostic techniques also to other available plant signals for improvlng the availability and safety of the plants. Analysis of Co~e Barrel Clamping Conditions by means of Neutron Noise In order to get high level of confidence it is important in the most cases to analyse several different signal sources. At the occasion of SMORN-III spectra of absolute displacement gauges (A-signals) have been shown, which indicated a relaxation of hold-down springs for the upper core support and the core barrel. Of course the changed vibration behaviour of these internals influenced also other VMS signals like the ex-core neutron noise. Neutron noise analysis were rather difficult, because the changing boron concentration during the core cycle led to rising noise sources. For this reason we concentrated upon the comparison of different neutron flux signals at the same point in time. In fig.10 the APSDs of the neutron flux signals XlU to X4U are shown, which were taken in the third month of the core cycle. Characteristic increases of the signals X2U and X4U (as compared to XlU and X3U) can be noted in the range between 6 and 7,5 Hz. This is in accordance with the observed behaviour of the A-signals at 7.5 Hz. The reduced clamping fource evidently influenced the core barrel beam mode vibration, which in turn caused a sectorial amplification of the fuel element resonance frequencies at 6-7 Hz (first harmonic of the fuel assembly beam mode). From this it can be concluded that the relaxation of hold-down springs started in the X2- and X4-sectors, which is also to be seen from the final inspection result gained at the end of the core cycle time. The radial bars in fig.10 illustrate the measured hold-down forces of all 112 spring piles, the admissible range is marked. Reviewing Core Barrel

Integrity by Means of Neutron Noise Analysis

It is wellknown that core barrel vibration can be detected by means of neutron noise analysis. Systematic long-term investigations of neutron signals confirmed the possibility to use it as an important indicator for the overall core barrel integrity. The core barrel beam mode vibration serves as an integrity criteria for the area of the CB nozzles and the flange clamping conditions. The shell mode vibration is used as an indicator for the integrity of the CB shell and the core baffle plates.

520

W. BASTLet al.

850-MW-PWR

B: CBBEAMMODE S: CBSHELLMODE

V

ABS.PHASEIsd i

,3e

o

Fig.10:

C h a n g e d n e u t r o n spectra due to CB c l a m p i n g force r e l a x a t i o n

--~

f

,

I~

4O ~,EOU~NCV

~

Fig.ll: M o n i t o r i n g of core barrel integrity

While the b e a m mode can be d e t e c t e d also by means of the A-signals, the shell mode can be m e a s u r e d v i a the n e u t r o n flux only. The s e p a r a t i o n of the two v i b r a t i o n modes is p e r f o r m e d b y looking at the p h a s e - b e h a v i o u r of adjacent (90 ° ) and opposite (180 ° ) located sensor pairs: b e a m mode

shell mode

opposite signal pairs

180 ° phase

0 ° phase

adjacent signal pairs

0 ° and 180 ° phase changing

180 ° p h a s e

The left hand diagrams in figure II r e p r e s e n t 4 c o h e r e n c e / p h a s e functions of 8 n e u t r o n noise signals a r r a n g e d at two axial core levels, the right h a n d diagrams show 4 c o h e r e n c e / p h a s e functions of the 4 adjacent n e u t r o n noise signals. F o l l o w i n g the results as given b y the coherence functions and the a s s o c i a t e d phase behaviour, the 9.8 to 10.4 Hz range represents the CB beam mode vibration, the 23.5 to 24.5 Hz range the CB shell mode vibration. As d i s c u s s e d in figure 2 up to n o w 31 m e a s u r e m e n t s have b e e n p e r f o r m e d over 8 core cycles. The analysis results showed an u n c h a n g e d v i b r a t i o n b e h a v i o u r over all this time. Therefore b y means of these analyses it was p o s s i b l e to judge on the integrity of the core barrel system c o n s i s t i n g of cylindrical shield, lower core plate, spacers and baffle plates. This was of c o n s i d e r a b l y help, when m e c h a n i c a l p r o b l e m s in similar plants r e q u i r e d closer investigations. Subcooled Boiling Velocity Measurements

in PWRs b y Incore N e u t r o n N o i s e C o r r e l a t i o n

Figure 12 shows results of c o r r e l a t i o n analysis of incore n e u t r o n noise signals. Coherence and phase functions were c a l c u l a t e d b e t w e e n detectors of the same measuring string. From d e t a i l e d i n v e s t i g a t i o n s we know that incore n e u t r o n noise consists of a n u m b e r of d i f f e r e n t noise sources. The m o s t i m p o r t a n t are shown in figure 7: m a i n l y sources due to v i b r a t i o n s b u t also due to s t a n d i n g waves in the c o o l a n t and t e m p e r a t u r e fluctuations. In addition to these sources - and analog to the incore noise sources in BWRs - also the streaming of c o o l a n t i n h o m o g e n i t i e s t h r o u g h the "seeing area" of the detector are sources of n e u t r o n noise. Whereas v i b r a t i o n s and standing waves p r i m a r i l y cause either 0 ° or 180 ° phase in the crosss p e c t r u m of same-string-detectors, the s t r e a m i n g noise source is c h a r a c t e r i z e d b y a linear p h a s e behavior. The s u p e r p o s i t i o n of such sources was d i s c u s s e d a l r e a d y in c o n t e x t with the B W R - n e u t r o n noise 1974 /7/. C h a r a c t e r i s t i c are a l t e r n a t i n g dips ("sinkfrequencies") and peaks in the c r o s s - p o w e r spectral d e n s i t y and the coherence function b e i n g e q u i d i s t a n t in frequency and d e p e n d e n t on the t r a n s i t time ~ of the c o o l a n t (inhomogenities) b e t w e e n the detectors. The phase oscillates either around

Influence of techniques on the safety and availability of plants

~

~

~t!

521

~ i ........... -~ . . . . . . . .

o

t~~

Fig.12:

Subcooled boiling: Velocity measurements

Fig.13:

Sinkfrequency results three different PWRs

of

zero or around an - in average - frequency proportional linear line. The dips and peaks in the coherence in figure 12 are indicated by small arrows, also the first zero crossing of the phase. The derived delay times or velocities respectively are in good agreement with estimated velocities of subcooled boiling. Similar measurements at other plants with different geometrical sizes (see figure 13) are in agreement with these findings. The results which have been presented by us already during the 16th Informal Meeting on Reactor Noise 1983 in Budapest, are meanwhile confirmed by similar measurements in the Borssele-reactor /8/. The investigations show that also for PWRs a method for monitoring unusual boiling within the core is available. However, up to now the realization of such a monitoring system seems to be not needed in German plants due to the open design of the used fuel assemblies. Abnormal Fuel Pin Vibrations Incore Neutron Noise Sources

as

Figure 14 shows two spectra of incore neutron detectors with distinct deviations between ca. i0 Hz and 30 Hz resulting in a broad peak around 23 Hz. Theoretical estimations showed that the fuel pin eigenfrequencies between spacer grids are ca. 70 Hz, if they are normally clamped at the spacer grids, however, they are ca. 24 Hz, if the fuel pins are freely movable in one spacer grid position. Therefore a method for early detection of spacer grid force relaxation seems to be possible.

Fig.14:

Abnormal

fuel pin vibrations

D e t e c t i o n of Bearing Anormalies Measurements

in Main Coolant Pumps by Shaft Vibration

The last example reflects the diagnosis potential of pump shaft vibration measurements p e r f o r m e d by an eddy current gauge. The sensors had been mounted after contacts between pump shaft and a gasket ring had occurred. The signal analysis gave interesting results in one of the three pumps (s. Fig. 15): Here the sub-

522

Fig.15:

W. BASTL et al.

Shaft vibration spectra of main coolant pumps

Fig.16: Temperature dependency of subharmonic shaft vibration

harmonic of the rated frequency (12,5 Hz) and its higher harmonics (e.g. 37,5 Hz) appear very clearly. It is known from turbine investigations that such subharmonics at half rated speed frequency and the higher harmonics indicate a higher clearance of the shaft bearings. Regarding the design of a main coolant pump shown in the upper right corner of Fig. 15, two radial and one axial bearing of the pump shaft can be recognized. To localize the origins for the great bearing clearance, measurements of different coolant temperatures have been performed: The 12,5 Hz peak has a strong dependency of the primary coolant temperature. In figure 16 this temperature dependence of the spectra is shown. This behaviour leads to the diagnosis that only the lower radial bearing is responsible for the "subharmonic". An inspection of MCP-I lower radial bearing was not made up to now, but measurements of the clearances are planned for the next maintenance. In the meantime we had some studies of MCP shut-down characteristics. It was obvious that the greatest relative amplitudes between pump shaft and pump housing appeared during pump trip. To minimize the danger of contact damages it is necessary to limit the slowing time to a minimum. Due to the fact that the first-turned-off pump shows minimum slowing time - reason for this is the differential pressure of the still working other pumps - it was recommended to the operators: If plant shut-down is necessary - put MCP-I out of action first. Till now no further contacting problems occurred.

SUMMARY AND CONCLUSIONS Development work on early failure diagnosis based on noise analysis techniques has been performed world-wide in many countries. Methods and systems are available now and are applied in nuclear power plants with success. Also in the Fed. Rep. of Germany research and development efforts led to intensive use of these techniques. The primary circuits of all new German plants are monitored by early failure detection systems. Extensive operational experiences have been gathered in the last years. Precise diagnosis and failure predictions could be given owing to a high standard in the interpretation of the signal source compositions. The present actitvities are being directed to investigations, how to apply noise analysis techniques also to other plant signals and how to use them for improving operational procedures (like preparation of repairings, inspections or maintenance). The availability of comparably low-cost modern data reduction and analysis systems gives further impacts to general agreement and to more applications in the plants. The utilities have noticed and acknowledged meanwhile, that these new techniques provide many benefits which are very useful with respect to both: safety as well as availability. The most important benefits are

Influence of techniques on the safety and availability of plants

523

detection and diagnosis of structure deficiences and process anomalies on-line and early before critical situations are reached. Countermeasures can be found by the operators in sufficient time without stress (reduced probability for human errors). Damages will be limited, consequent damages can be avoided. Costly shut-down and repairing times are reduced;

-

detection of material fatigue in components, bearings and clampings, either due to design errors or when approximating the end of the expected operation life time (aging problems);

-

-

reduced shut-down times due to in-time providing of spare incipient failures are already known during plant operation;

-

potential

-

reduction of radiation loads to the personnel by means of dedicated inspections;

-

verification of the integrity of primary same-typed plants defects are identified;

-

ensuring plant operation licences, when - due to lack of spare components or repairing tools - the plant goes into operation with provisional repaired components (additional monitoring measures);

for a reduction

of inspection

components,

if

times and elongation of test intervals;

circuit

components,

on-line assessment and integrity verification of reactor circuit components after unusual loadings or stresses;

when

internals

in

other

and primary

diagnosis potential in unusual operation conditions when tools for the assessment of systems status and process behaviour are needed (e.g. in a post-accident operation). As it can be seen from this list of benefits the utilization of noise diagnostic techniques is manifold. Pure economic aspects as well as safety and licensing aspects are concerned. But since there is a strong interdependence between safety and availability in most cases we must not distinguish for that. As a whole we can state that noise diagnostic techniques exert a very positive influence to nuclear power plant operation and will play an important role in the future. It is worth to continue research and development work by including other plant signals and components and by making use of more automized systems for the detection and diagnosis procedures. Already existing monitoring systems should be intensively used and the operational experience should be enlarged. Investigations have to be performed, in how far computer-aided systems (like expert systems) can support the operator in normal and anormal situations. Finally it should be mentioned that meanwhile also other industries have noticed the developments and the status in the nuclear field and are going to apply similar techniques for their particular problems. This can be taken as an indicator that the basic work of early failure detection by means of stochastic signal analysis which has been evaluated within nuclear safety programmes, was a good work and was of programmatic nature. REFERENCES (1)

Olma, B.J., Source Location and Mass Estimation in Loose Parts Monitoring LWR's (SMORN IV poster session). To be published in Prog. Nucl. Energy

(2)

Bastl, W., Wach, D., Experiences with noise LWR's, Proq. Nucl. Energy 9 (1982) 505.

(3)

Bauernfeind, V., Evaluation of the Exciting Forces Causing Vibrations Primary Components, Proc. of Third Keswick International Conference, tion in Nuclear Plant, May ii-1%, 1982

(4)

Sunder, R., Wach, D., Reactor Diagnosis Using Vibration and Noise Analysis in PWRs, Proc. of International Symposium on Operational Safety of Nuclear Power Plants, Marseilles, May 2-6, 1983, p. 281-299

surveillance

systems

of

in German of PWR Vibra-

W. BASTLetal.

524

(5)

W a c h , D., S u n d e r , R., W e i n g a r t e n , J., " M o n i t o r i n g s y s t e m s a n d d i a g n o s i s t e c h niques for early failure detection in nuclear power plants", VGB Kraftwerkst e c h n i k , 64. J a h r g a n g , H e f t 2, F e b r u a r y 1 9 8 4

(6)

W a c h , D., T r e n d s i n E a r l y D i a g n o s i s o f F a i l u r e i n R e a c t o r I n t e r n a l s a n d P r i mary Circuit Components, IAEA-Seminar on Diagnosis of and Response to A b n o r m a l O c c u r e n c e s a t N u c l e a r P o w e r P l a n t s , D r e s d e n / D D R , 1 2 - 1 5 J u n e 1984, to b e p u b l i s h e d i n I A E A - S R - 1 0 5

(7)

W a c h , D., K o s ~ l y , G., I n v e s t i g a t i o n o f t h e j o i n t e f f e c t o f l o c a l a n d g l o b a l driving sources in incore neutron noise measurements, Atomkernenergie Kernt e c h n i k 24 ( 1 9 7 4 ) 2 4 4

(8)

T ~ r k c a n , E., O g u m a , R., I m p r o v e d N o i s e A n a l y s i s M e t h o d s f o r o n - l i n e T e s t i n g of incore Instrumentations and the Determination of Power Reactor Param e t e r s , N e t h e r l a n d s E n e r g y R e s e a r c h F o u n d a t i o n , E C N - 1 4 8 , J a n u a r y 1984.

-DISCUSSION-

PUYAL

Just a question about instrumentation. In most german plants~ vibration measurements are performed with absolute displacement sensors. Low frequency spectra don't seem to be very different from those obtained with accelerometers, the use of which is much more common anywhere else. Could you indicate the frequency range of these A.D. sensors and the reason fo their choise compared to accelerometers ?

WACH

First of all, we do not have only absolute displacement gauges. Where possible -for instance for the loop vibrations, we use relative displacement sensors (inductive differential principle). At the top of the reactor vessel four absolute displacement sensors are used : lowresonant spring-mass-systems developed by KWU as shown in the paper of Mr. WEHLING. The frequency range is from 0.5 Hz up to 150 Hz with a linear transfer function and above that response up to 1200 Hz. Reasons for applying these sensors are : ability for remote calibration~ linear transfer, and low frequency response. But I guess it is not a question of using these sensors or accelerometers (assumed the electronic channels are good enough). It is important to measure the RPV vibrations and it was interesting for me to hear form the last speaker Hr. FRY, that also the U.S.A. are going now to apply vibration measurements as done by the european contries since some time.

BERNARD

Have you any evidence and experience of possible contacts between fuel assemblies (and that could possibly change with time) in the core of the reactors that you are monitoring with incore neutron noise ?

WACH

In all German nuclear power plants, where incore neutron noise measurements were performed by us~ we are sure~ that no fuel assembly interactions/ contacts do exist. We know one plant in Germany, where fuel assembly contacts are continuously present with varying intensity, however incore measurements where not performed by us. So I can not answer your question, if there is a response in the incore neutron noise.