Belgian experience in noise monitoring of nuclear power plants

Progress in Nuclear Energy, 1988, Vol. 21, pp. 13-18.

0079--6530/88 $0.00 + .50 1988 Pergamon Press pie

Printed in Great Britain.

BELGIAN EXPERIENCE IN NOISE MONITORING OF NUCLEAR POWER PLANTS R. BAEYENS, G . BOON a n d G. D ' A N s LABORELEC, Rodcstraat, 125 - B1630 Linkebeck, Belgium

ABSTRACT Seven nuclear power plants are operating now in Belgium. PWR's either from Westinghouse or Framatome type.

They are all

Two are two loop reactors, the others are three loop reactors. Neutron noise monitoring was performed since the commissioning of the first unit with ex-core neutron detectors on a regular basis. In-core neutron noise monitoring was started later on. Loose parts monitoring systems were installed later. The experience gained with these two types of monitoring will be described in this paper, as well as a new turbogenerator vibration monitoring system which is now installed on two of our nuclear power plants.

1.

INTRODUCTION

The Belgian nuclear power stations are installed on two sites - Doel and Tihange. At Doel we have DoeI 1 - 2 started up in 1974-75 - two loop reactors of 400 MWe each. Doel 3 - started up in 1982 - three loop 900 MWe. Doel 4 - started up in 1985 - three loop 980 MWe and at Tihange Tlhange 1 - started up in 1975 - three loop 870 MWe. Tihange 2 - started up in 1983 - three loop 900 MWe. Tlhange 3 - started up in 1985 - three loop 980 ~4We. The load factor of these units are very good and the overall capacity factor for the nuclear units was 81.8 • in 1985. Several monitoring programs were set-up since the start-up of the first unit (Doel I). Among these the noise monitoring techniques were employed on a regular basis for all the units since their first start-up. The experience and status of these techniques will be summarized in this paper.

2.

NEUTRON NOISE

In the field of neutron noise analysis some preliminary tests were performed on the BR3 reactor in the frame of a Euratom contract.

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It was then applied on a periodical basis since the first start-up of the first nuclear unit. Ex-core neutron nolse analysis is performed on a monthly basis on all the units and it is a regulatory requirement that a report should be issued on the results at the end of each fuel cycle, in order to be able to take eventual corrective actions during the refuelling period. With ex-core neutron noise analysis, the main core vibration modes are being monitored. Thls means : - the pendular movement of the core barrel and the integrity of the hold down spring. -

Higher order modes such as thermal shield modes and shell modes of the core (enveloppe).

Some thermohydraullc phenomena were also studied by this technique. For such an analysls, the eight channels are tape recorded (four upper and lower parts of the power lonisation chambers). The recorded data are then analysed by a fourier analyser which provides the normalised auto and cross spectra as well as the coherence functions. This operation is completely controlled by a computer and the spectra are stored on disks for archival. The characteristics of the peaks are also computed. As said before, this is being made on a monthly basis, but the operator has also an online monitoring system which gives the normallsed rms value of the different noise components on a banalised recorder. This simple monitoring system is able to isolate the in and out of phase components of the noise for opposite chambers. If fast evolution is detected, Laborelec is called in to make a spectral analysis of the signals. With this monitoring system it is possible to have an early warning if some structural changes occur. In-core neutron noise analysis with movable in-core neutron detectors is being performed at least twice per fuel cycle since problems with fuel assemblies were suspected. Wlth the in-core neutron noise analysis, we are mainly interested in the vibrations of the fuel assemblies and thermohydraulics. The main problems to which neutron noise has contributed to the diagnostics and monitoring in Belgium were : -

Baffle Jetting. Thimble vibration. Nucleate boiling detection. Fuel pin vibration. Integrity of the hold down spring. Primary flow fluctuations. Sensor testing.

The main effort now is to automate the neutron noise analysis and incorporate artificial intelligence techniques. The introduction of expert systems is envisaged in order to make the operator as autonomous as possible. The potentialities of the noise techniques are not fully investigated up to now and an effort should be made on sensor testing for instance.

S,

LOOSE PARTS MONITORING

First experience with accelerometric instrumentation on the primary loops were obtained at the BR3 and Doe1 i and 2 plants. A t T i h a n g e 1, t h e i n s t a l l a t i o n was modified to obtain an elementary loose parts detection system. The rectified output signal of a general purpose ampllfier was used for level detection and alarm. An audio output was also installed.

Belgian experience in noise monitorin 8

At the construction of Doel 3 - 4 and Tlhange 2 - 3 a Rockwell vibration and loose parts monitoring system was ordered. The accelerometers were installed at the following locations : 2 at the bottom of the reactor vessel 2 on the reactor vesselhead 2 on the hot plenum of each steam generator These installations perform a dual signal conditioning for vibrations and loose parts. The vibration monitoring is ensured by a low pass filter combined with a rectifler-threshold detector for alarms. For loose parts monitoring the sensor responses are bandpass filtered at the sensor resonance and again a rectifier-threshold detector activates the alarms. An audio output and tape recorder were installed as well as a locator and spectrum analyaer on two of the four units. It turned out that lots of false alarms were issued in operation, due to noisy wiring and poor sensor locations and/or monitoring. While recently commissioning a Westinghouse DMIMS (digital metal impact monitoring system) at Doel I - 2 plants, these problems were properly solved. This system features an advanced digital signal treatment to reduce false alarms, while maintaining a good sensitivity to loose parts impacts. In practice the alarm levels were set at ten times the background noise. An alarm starts an analog tape recorder during a preprogrammed time. These raw data can be later analysed off-line and compared with base line data which were obtained by extensive calibration. This analysis requires noise experts. Minor problems were encountered with this installation but operating experience was very encouraging. During the 15 year experience with loose parts detection only one loose part was reported with certainty. It was detected by the DMIMS system on the secondary side of a steam generator and its estimated mass was about 100 grams. Probably a lot of small loose parts were not detected by the other systems due to a lack of sensitivity. This leads us to formulate certain remarks about loose parts monitoring systems. The installation of sensors is very crltlcal. The most probably impact locations for real loose parts should be well studied and the shockwave path should be as short as possible. Mounting studs are to be avoided. Using mechanical punches with the proper amount of kinetic energy according to reg. guide 1.133 is not a good choice to calibrate loose parts monitoring systems. One reason may be that they do not produce a pure elastic impact, as it is the case with real loose parts. Pendulum techniques seem more appropriate. Furthermore, advanced pattern recognition techniques should reduce the rate of false alarms. Already available digital logics represent a good step in this direction and may be further improved.

4.

VIBRATION MONITORING OF LARGE ROTATING MACHINERY

A few y e a r s t r o u b l e s h o o t i n g v i b r a t i o n a l p r o b l e m s w i t h l a r g e t u r b o s e t s l e a d s t o t h e c o n c l u s i o n t h a t most e x i s t i n g m o n i t o r i n g s y s t e m s f a i l t o p r o p e r l y i n t e g r a t e t h e v i b r a t i o n v e c t o r s a t t h e r o t a t i n g s p e e d and t w i c e t h e r e o f i n t o t h e i r a l a r m l o g i c . Also l a c k i n g i s the p e a k - p e a k v s . f u n d a m e n t a l v i b r a t i o n r a t i o i n d i c a t i n g t h e p r e s e n c e of o t h e r f r e q u e n c i e s t h a n t h e f u n d a m e n t a l , which may be c a u s e d by such s e r i o u s p r o b l e m s a s s h a f t c r a c k s or a h o s t of u n s t a b l e s e l f - e x c i t e d v i b r a t i o n s ( e . g . o i l or s t e a m w h i r l s ) . In c a s e t h e r a t i o f a r e x c e e d s one, i t i s i m p o r t a n t t o o b t a i n a f u l l d e s c r i p t i o n of t h e f r e q u e n c y s p e c t r u m t o f u r t h e r diagnose the vibration origin. For s h a f t s u p p o r t e d i n s l e e v e b e a r i n g s , a n o t h e r i n t e r e s t i n g f e a t u r e c o n s i s t s of v i s u a l i s t n g t h e j o u r n a l p o s i t i o n w i t h i n t h e b e a r i n g s u c h a s d e s c r i b e d by its clearances.

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A real-time on-llne system has been developed to add the above capabilities to those already marketed by numerous manufacturers, such as Bode, Nyquist and Campbell-llke analyses at variable speeds for successive coastdown and start-up signatures. It has been implemented in 2 fossil-fueled and 2 nuclear plants to monitor the main turblne-generators over 2 years. As a result, about 20 more systems have been ordered since. The c u r r e n t i m p l e m e n t a t i o n o f t h e v e c t o r m o n i t o r i n g i s among t h e c i a t e d by p r a c t i t i o n e r s a n d , a s s u c h , t h e s o l e one p r e s e n t e d h e r e .

facilities

most appre-

U n b a l a n c e s a r e i n d e e d t h e m o s t common c a u s e f o r v i b r a t i o n s . They m o s t l y i n f l u e n c e t h e vibration vectors at the running speed (fundamental). In large turbosets, the latter usually undergo significant and y e t a c c e p t a b l e c h a n g e s due t o v a r y i n g t h e r m a l and o t h e r c o n d i t i o n s . O t h e r w i s e u n e x p l a i n e d c h a n g e s of s i z a b l e a m p l i t u d e s u s u a l l y p o i n t t o a more s e r i o u s c a u s e such as blade losses, cracks, etc ... Thus an i n t e l l i g e n t monitor must first follow a set of operational parameters best describing the above conditions. During a preliminary observation period, it then automatically establishes parameter-dependent confidence zones within which the vectors normally lie for a healthy machine. Using a c t i v e and r e a c t i v e powers as p a r a m e t e r s r e l a t e d to t h e t h e r m a l c o n d i t i o n s of t h e t u r b i n e and g e n e r a t o r in l a r g e t u r b o s e t s , chi-square statistics a r e b u i l t up by t h e s y s t e m t o produce a finite number of p a r a m e t e r - d e p e n d e n t confidence ellipses, e a c h a s s o c i a t e d w i t h an operational zone. Care is taken to incorporate the deterministic s p r e a d s c a u s e d by t h e p a r a meter variation within each zone. When e n o u g h s a m p l e s a r e c o l l e c t e d w i t h i n a z o n e , b o t h t h e statistics and c o r r e s p o n d i n g c o n f i d e n c e e l l i p s e s a r e f r o z e n and a s o - c a l l e d long-term vector m o n i t o r i n g i s a c t i v a t e d a s l o n g a s no m a j o r c h a n g e o f t h e m a c h i n e s u c h a s a r e - a l i g n m e n t o r a re-balancing is performed. A long-term alarm is issued as soon as the vectors leave their confidence ellipses, F i g . 1. A short-term chi-square statistics is constantly computed for all vectors. It is based on an e x p o n e n t i a l w e i g h t i n g o f t h e l a t e s t v e c t o r s and i s i n d e p e n d e n t o f t h e o p e r a t i o n a l parameters. If a vibration vector progressively moves, the corresponding short-term confidence ellipse follows the latest vibration v e c t o r a n d a l w a y s c o n t a i n s i t , F i g . 2. In case of a s u d d e n c h a n g e , t h i s i s no l o n g e r t h e c a s e . A short-term alarm is activated. It increments t h e number of v i b r a t i o n j u m p s , w h i c h i s t h e n m e m o r i z e d , F i g . 3. As i s c u r r e n t i n v i b r a t i o n pre-establtshed maximum v a l u e s

level monitoring, peak-peak amplitudes are also compared to o b t a i n e d from s u c h e s t a b l i s h e d n o r m s a s VDI 2056 o r 2 0 5 9 .

F o r e a c h s e n s o r , a summary o f a l a r m s i s a v a i l a b l e on t h e c o l o u r e d d i s p l a y s , as already described in the previous figure captions. It is organized as follows : a first row d i s p l a y s t h e s e n s o r by t h e i r i n v e r s e v i d e o s y m b o l s ( n u m b e r + c o l o u r c o d e s ) a s t h e y a p p e a r i n t h e multi-sensor vector plots ; entries i n a s e c o n d row i n d i c a t e f o r e a c h s e n s o r w h e t h e r i t i s o r has been in peak-peak alarm with a red or green P in inverse video ; entries i n a t h i r d row y i e l d t h e n u m b e r o f s h o r t - t e r m a l a r m s , i . e . how many t i m e s t h e v i b r a t i o n v e c t o r h a s Jumped for each sensor. Red n u m b e r s p o i n t t o t h e s e n s o r s w i t h t h e m o s t r e c e n t j u m p s . No e n t r y means no p a s t s h o r t - t e r m a l a r m . The l a s t row d e a l s w i t h l o n g - t e r m a l a r m s . Any e n t r y m e a n s that the corresponding vector lies outside the confidence ellipse t h a t was e s t a b l i s h e d the first t i m e t h e m a c h i n e was o p e r a t e d i n t h e same t h e r m a l c o n d i t i o n a s t h e c u r r e n t o n e . The user can then scan a special data file to retrieve w h a t h a s h a p p e n e d when t h e a l a r m was activated. Such f e a t u r e s g r e a t l y e n h a n c e t h e m a l f u n c t i o n d i a g n o s i s a n d t h e p r e d i c t i o n of incoming faults. For e x a m p l e , i t was p o s s i b l e t o d e t e c t c o u p l i n g s l i p o c c u r r i n g w i t h a 1000 ~ 1500 n u c l e a r rpm u n i t and N e w k i r k s e a l r u b w i t h a s m a l l e r c o a l - f i r e d turbine. In the future, such a m o n i t o r i n g w i l l be e x t e n d e d t o h i g h - m e r i t a u x i l i a r y machines such as f e e d and p r i m a r y pumps.

5.

CONCLUSIONS - FINAL REMARKS

The s i t u a t i o n in Belgium in the field of noise monitoring seems satisfactory for the moment. As g o o d r e l a t i o n s e x i s t between t h e o p e r a t o r s and t h e l a b o r a t o r y , frequent measurements are possible. The u t i l i t i e s u n d e r s t a n d t h a t t h e s e t e c h n i q u e s can h e l p them and t h i s i s t h e r e a s o n why t h e l a b o r a t o r y h a s t a k e n a p r a g m a t i c p o i n t o f v i e w t o w a r d s t h e s e t e c h n i q u e s . E m p h a s i s i s p u t on t r y i n g t o g e t p r a c t i c a l results r a t h e r t h a n make t h e o r e t i c a l research. The t r e n d s a r e to t h e d e v e l o p m e n t of a u t o m a t e d t e c h n i q u e s and t h e i n t r o d u c t i o n of artificial intelligence s y s t e m s , b u t i t w i l l be done i n p r u d e n t s t e p s .

Belgian experience in noise monitoring

17

In the field of loose parts monitoring systems, the effort is put on trying to make the existing systems better in order to get as low false alarms as possible. That means better understanding of the propagation of impacts, the sen~ors and the detection techniques. In the field of rotating machines vibration monitoring, a very powerful system has been developed and is being installed on a great number of turbosets. As a conclusion, the vibration monitoring techniques Belgian nuclear power plants and are used on a routinely warning systems.

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