DYNAMIC BEHAVIOUR OF WWER 1000/320 REACTOR ......The 6-th International Conference “Safety Assurance of NPP with WWER” EDO “GIDROPRESS”, Podolsk, Russia 26-29 May, 2009 1 DYNAMIC

The 6-th International Conference “Safety Assurance of NPP with WWER” EDO “GIDROPRESS”, Podolsk, Russia

26-29 May, 2009

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DYNAMIC BEHAVIOUR OF WWER 1000/320 REACTOR FUEL ASSEMBLIES

AND INFLUENCE OF MAIN CIRCULATING PUMP PRESSURE PULSATIONS

P.Stulík

Nuclear Research Institute, Rez, Czech Republic

Annotation: The evaluating of changes in fuel assemblies dynamic behaviour is particularly needed

and required. Self power neutron detectors installed in reactor fuel assemblies can hold important

information about vibration in spite of complex surrounding influencing signal output by many

parameters. The paper deals with the analysis of available set of self power neutron detectors to show

influence of main circulating pump excitement forces.

Key words: Reactor vibration, pressure pulsations, fuel assembly, fuel rod, self power neutron

detector, power spectral density, transfer function, time domain, frequency domain, joint time

frequency domain, beat frequencies.

1. Introduction In the past many start-up and operational measurements of pressure vessel and internals

vibrations of both VVER 1000/320 NPP Temelin reactor units have been performed. These

measurements included pressure sensors (TP), accelerometers (ACC), ex-core ionisation chambers

(XNN) and self-powered neutron detectors (INN). The measurement data sets were originated

from several diagnostic systems – operational diagnostic system RVMS (the part of in-plant

diagnostic system TDMS delivered by Westinghouse, USA ), start-up special system ANALOG

(implemented by ŠJS start-up supplier– [1], [2]) and system DMTS (developed by NRI Rez for the

distributed acquisition of large volumes of data, their relevant processing and evaluation in

the different domains primarily by means of noise diagnostics ([3]).

In [4] the accelerometers installed on the flange of reactor pressure vessel have been used

for measurement, the evaluated significant frequencies were compared with the exciting ones and

the good agreement was indicated.

The influence of the NPP Temelin reactor operational vibrations on the core barrel stability was

investigated in [5] by means of the developed 3D mathematical reactor model and evaluated

measurement results under the condition of full MCPs operation with slightly different revolving

frequencies.

Some following works were further conducted ([6] - [7]) in which the dynamic responses of NPP

Temelin reactor in full and less loop main circulating pump combinations were examined on reactor

models and experimentally verified.

With the Gidropress experience in the area of reactor internals and fuel vibration research

published in ([9], [12]) on mind, the NPP Temelin new fuel elements project ([10]) has been

initiating the solution of further investigations ([13], [14]).


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2. NPP Temelin diagnostic measurements In-plant system RVMS delivered by Westinghouse and described in [1] or [2] has being used not

only during both units commissioning tests (2000, 2002) but then primarily in the normal plant

operation. It features 12 bit amplitude resolution in six frequency ranges up to 200Hz with 0.5 Hz

frequency resolution and 1 kHz sampling frequency enabling thus measurements of 4 accelerometers

ACC on the upper reactor flange, 6 accelerometers ACC on the steam generator (Unit 1), 8 ionization

chambers XNN in upper and lower positions, 5 pressure fluctuation sensors PFT (only Unit 2) placed

on cold loops, 12 reactor output thermocouples, 256 self powered neutron detectors INN measured

in 16 groups just as it is shown in Table 1.

The Škoda system ANALOG was used during both units commissioning tests on set of specially

arranged sensors. Measurement was accomplished with 12 bit amplitude resolution, with variable

frequency domain resolution 0,1 – 0,5 Hz and 1 kHz sampling frequency.

The NRI Řež system DMTS has been applied in various stages of both units operation

by acquiring and processing extensive time records with 24 bit amplitude and selected 0,122 Hz

frequency resolution with 1kHz sampling frequency.

It is worthwhile to note in this context that there are important issues for diagnostic

investigations especially for the case of actual or anticipated operational events with known

implications but with unknown causalities

it is necessary to have a well arranged frequency domain results (APSD, CPSD, COH, PHASE etc.) with pertaining operation parameters enabling comparison of similar reactor units

the existence of sufficient time records is necessary for more detailed processing and evaluation.

Table 1

NPP Temelín RVMS sensors

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3. Reactor vibrations There are three main types of reactor internals exciting forces – pressure pulsations generated

by main circulation pumps (MCP), pressure pulsations in turbulent boundary coolant flow layer

in the core barrel - reactor pressure vessel gap and the acoustic pressure sources in the primary circuit

coolant.

The analysis of reactor pressure vessel vibration was fulfilled in [4] and [5]. Four piezoelectric

accelerometers (0.5 – 300 Hz) installed on reactor cover had been measured at nominal 100% power

with enhanced amplitude resolution by system DMTS. The acquired signals were processed

in frequency range of 0.5 – 100 Hz. The used 3D mathematical reactor model with 137 degree of

freedom was tuned in by means of measured operational frequency 18.738 Hz.

The analysis summary is as follows

authors classified the set of frequencies 6.714, 13.57, 33.05, 53.386 Hz as the operational acoustic frequencies with the assumption that the corresponding pressure fluctuations cause

forced reactor vibrations

the frequency 9.278 Hz of reactor vertical movement depends on the coolant temperature and there is authors´ opinion that this reactor vibration is generated by tube – cavity resonance well

explained by the Helmholtz resonator theory

the frequency 18.738 Hz represent the horizontal pendulum motion of the system reactor pressure vessel – core barrel driven by turbulent pressure fluctuations appearing in the near boundary

layers of coolant flow

there are operational frequencies of 16,602, 33,204, 49,926, 66,528, 83,130, c Hz of forced reactor vibrations induced by corresponding MCP revolution frequencies.

Power spectral densities of all accelerometers A511-A514 are shown in Fig. 1 with marked MCP revolution frequencies up to 6

th harmonic one.

Fig. 1 NPP Temelín Unit 1 reactor head accelerometers A511-A514 power spectral densities


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The peak amplitudes of reactor vessel vibration induced by pressure pulsations generated by

main circulation pumps MCP revolution are given in Table 2. Their values are well below under

acceptable level of 0,1 g in frequency domain [9] but we can mark the 3 order difference

at operational 99.730 Hz which corresponds 6th

MCP harmonic. When converting values of Table 2

into Fig. 2 we can see slight curve resemblance of A511 with A512 and A513 with A514

accelerometer couples, which could indicate the similar vibration behaviour of reactor vessel

on these induced frequencies.

In the past the revolutions of main circulation pumps rotor were supposed to be constant.

The authors of [6] reported that the power spectral densities of reactor pressure vessel are not stable

in time. The operational JTFS spectrograms of A511 head reactor accelerometr presented in Fig. 3

(NPP Temelin Unit No2, 3

rd fuel cycle, Nnom=100%) demonstrate clear indication of reactor beat

vibration in 0 – 150 Hz frequency range. The different beat forms of particular MCP harmonics

from the same measurement are shown in Fig. 4 for the time interval 0 – 1200 s. Tab 3 gives the

amplitude statistics for these beat harmonics where namely frequencies of 49.926, 83.130 and 99.730

Hz are dominating by their average, standard, maximal and range values.

Table 2

NPP Temelín reactor head accelerometers PSD : frequency peak values of

vibration forced by MCP revolutions

ACC Apeak [10

-5 g]

1 2 3 4 5 6

A511 29,7 33,0 20,7 17,6 5,4 1797,4

A512 5,1 24,1 19,3 9,1 2,6 2788,9

A513 7,3 18,6 25,9 8,1 2,8 2811,6

A514 9,8 26,7 27,8 13,3 2,2 4618,6

Frequency peak values of vibration forced by MCP revolutions

1,0

10,0

100,0

1000,0

10000,0

100000,0

16,601 33,203 49,926 66,528 83,129 99,731

Operational frequency [Hz]

Ap

eak [

10

-5g

]

A514

A513

A512

A511

Fig. 2


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Fig. 4 MCP harmonics (16,6 – 99,7 Hz) with beat character from JTFS spectrogram of A511 accelerometer

0 – 150 Hz 0 – 50 Hz

Fig. 3 Operational JTFS spectrograms of A511 head reactor accelerometer (NPP Temelin Unit No2)

Table 3

A511 accelerometer statistics of operational beat harmonics forced by MCP

revolutions

MCP Beat

Harmonics

[Hz]

[dB]

Average StandDev Min Max Range

16,602 40,8 5,2 20,3 48,5 28,2

33,204 43,5 5,1 13,1 49,6 32,3

49,926 46,6 5,2 22,2 54,4 36,5

66,528 53,0 2,6 43,9 57,6 13,7

83,130 41,9 5,7 16,2 49,7 33,4

99,730 69,7 4,8 56,3 78,2 21,9


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As proved in [7] the similar beat character is observed also for exciting pressure pulsations

measured at reactor input and output. The two couples of operational JTFS pressure pulsation

spectrograms shown in Fig. 5 (NPP Temelin Unit No2, 5

th fuel cycle, Nnom=0%) demonstrate

however another feature. The noticeable change of spectrogram couples measured in two hours span

is due to changed reactor operation conditions when less loop main circulating pump combinations

were examined between the pressure pulsation measurements. It is necessary to mark that these

measurements themselves were realized in steady state within time interval 0 – 1000 s.

The common beat display of exciting pressure pulsations and reactor vibration at MCP

harmonics can be demonstrated when calculating the transfer function between signals of these

quantities. In Fig. 6 the close relationship between reactor input/output pressure pulsations TP1/TP5

and reactor vibration A511 is presented when good coherence is achieved also for the MCP

harmonics regions.

3.7.2008, 22:38 3.7.2008, 00:38

Fig. 5 JTFS spectrograms of reactor input/output pressure pulsations TP1/TP5 (NPP Temelin 2th

Unit, 5th

cycle, Nnom=0%)

REACTOR INPUT

REACTOR OUTPUT


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The developed model of forced reactor vibrations excited by pressure pulsations generated

by main circulation pumps was described in paper [8]. The vibration analysis based on this new

generalised reactor model with spatial localization of fuel assemblies, protection tubes and linear

stepper control rod drives has confirmed that the slightly different pump revolutions are sources

of the beating effects. These effects cause an vibration amplification and increase a possibility

of the contact loss in internal core barrel linkages.

4. Dynamic fuel assemblies behaviour NPP Temelín units were connected to grid in June 2002 or in April 2003 with initial VV6 fuel

load. Then there were successive changes to VV6 Phase 0, VV6 Phase 1X fuel assemblies

from WEC during 4 – 6th

fuel cycles of both units. Nowadays they have decided to change fuel

supplier to TVEL with TVSA-T fuel assemblies to be loaded in May 2010 in U1C9 cycle.

Both fuel assemblies are shown in Fig.7 ([10]). Fuel assembly mechanical, hydraulic and neutron-

physical compatibility is expected having been supported by many preparatory works including

vibration research ones.

From the operation view of point it is necessary to evaluate continually changes in fuel

assemblies dynamic behaviour. The knowledge of fuel assemblies and fuel rods natural frequencies

is then an essential step for the utilization of self power neutron detectors SPND signals. These are

composed by many source contributions including e.g. collective fuel assemblies vibration but their

signal structure is not yet fully identified for the time being. Nevertheless, the qualified information

about fuel rods fretting and fuel assembly bowing is required

As described in [11] the natural frequencies of selected reactor components were computed

by program “WWER 1000 reactor modal analysis”. This program is the part of mathematical reactor

model with 137 degrees of freedom incorporating 8 subsystems of main reactor components

(pressure vessel, core barrel, active zone with 163 VV6 fuel assemblies, etc.). The first six bending

modes of VV6 fuel assembly natural frequencies are listed in Table 4 together with

the corresponding values for UTVS and TVS-2M assemblies, which were get from the modal

experiments with hammer and shaker excitement ([12]).

Fig. 6 Transfer function of reactor input/output pressure pulsations TP1/TP5 [kPa] and A511 acceleration [g]


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The analysis of one possible reason of VV6 fuel rod integrity infringement was done for Temelin

U2C3 cycle [13]. The starting assumption was the loss of contact between rod and grid in the area

of spacer grids 2, 3, 4 and computed hydrodynamic forces acting on fuel rods were compared with

natural bending frequencies of fuel rods. The evaluation of exciting force frequency was done for the

velocities of coolant with the characteristic values for a flow around fuel rods (outer diameter of fuel

rod as characteristic dimension, Strouhal, Reynolds numbers from velocity models based on Temelin

U2C3 operation data). The pulsation of hydrodynamic forces has a frequency in the range 1 – 9 Hz

and acts primarily on the level of 1 – 4, 8, 9 spacer grid. Even if the fuel rod natural frequencies

estimation was not based on exact VV6 dimension and material data, the overall result (Tab.5)

is considered to be sufficient for the early comparison with the fuel rod exciting forces. It shows

relatively big fuel rod infringement probability in the frequency range 1 – 11 Hz.

Table 5

Bending natural frequencies of unreleased/released fuel rods

#

Spacer grid

Mode

#

Spacer grid

Mode

OK … spacer grid in contact with fuel rod, x … released fuel rod in spacer grid,

red marked numbers … 1st and 2nd modes of fuel rod natural frequencies up to 11 Hz

Kodl, Macák, 2006, [13]

Table 4

Bending natural frequencies of fuel assemblies

Mode [Hz]

VV6 1 UTVS

2 TVS-2M

2

1 3,00 4,7 5

2 6,35 10,5 10,5

3 10,82 17,7 16,5

4 16,2 25,3 23

5 21,5 34 28,5

6 31,7 - 35,5

1 Zeman, WWER1000/320 Mathematical model, 2008, [11]

2 Makarov, TVS modal analysis, 2007, [12]


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Data from 256 SPND signals are normally acquired by operational diagnostic system RVMS

at 12 bit resolution with 1 kHz sampling frequency and reduced into frequency domain results

with 0,5 Hz resolution. The more detailed measurements by system DMTS (NRI Řež, 24 bit

resolution in time domain, 0,122Hz in frequency domain) were made at Nnom=100% with all working

MCPs during Temelin U1C3 a U2C3 cycles. The acquired data of 15 min unified length were

centred, normalized to maximum value and processed in time and frequency domain.

The frequency interval 0 – 70 Hz for processing was determined with regard to above mentioned

works [11], [13]. 52 SPND from altogether 272 measured ones were chosen for further evaluation in

spectral maps. These maps are constructed as 3D graphs in XY view with arbitrary variables

(frequency and spectra serial number) and power spectral density PSD value as a dependent variable.

This layout yields overall and quick overview namely in the case when comparison between units is

required what was an original aim of the work done in [14]. There are spectral maps for both Temelin

units presented in Fig.8 and Fig.9 together with the above mentioned fuel assembly and fuel rod

natural frequencies, which are divided in two regions according of released/unreleased, state rod to

grid.

When interpreting these spectral maps the following conclusions can be made:

the peak frequencies and amplitudes occurrence of both units with the same VV6 basic design fuel in the same fuel cycle is distinctively different

the 2nd unit amplitudes are almost ten times higher with the significant occurrence number also in the region of 1 – 3

rd mode released fuel rods natural frequencies

there are however frequency individual values, smaller or larger regions where the both units behave in similar manner

the neighbourhood of 9 and 13 Hz

the revolution frequency 16,6 Hz and its harmonic 33,3 Hz

the neighbourhood of 18 Hz

the bands 25 – 30, 30 – 32, 41 – 47 and 49 – 50 Hz. It is necessary however to be aware of the fact that these conclusions are valid only for the one

of both units cycles.

In order to cover qualified detection of fuel rods infringements, more systematic approach is

required in the future.


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Fig. 8 NPP Temelin unit #1 (3th cycle) SPND power spectral density map with fuel rod and assembly natural frequencies

1-3th mode of released fuel rods

All modes of unreleased fuel rods

Kodl, Macák [13]

FR

3 6,35 10,82 16,2 21,5 31,7 Hz VV6

FA

Zeman [11]

4,7 10,5 17,7 25,3 34 Hz UTVS

FA

Makarov [12]

5 10,5 16,5 23 28,5 35,5 Hz TVSA-2M

FA

Makarov [12]

Fig. 9 NPP Temelin unit #2 (3th cycle) SPND power spectral density map with fuel rod and assembly natural frequencies

Kodl, Macák [13]

3 6,35 10,82 16,2 21,5 31,7 Hz VV6

FR

FA

Zeman [11]

4,7 10,5 17,7 25,3 34 Hz UTVS

FA

Makarov [12]

5 10,5 16,5 23 28,5 35,5 Hz TVSA-2M

FA

Makarov [12]

1-3th mode of released fuel rods

All modes of unreleased fuel rods


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5. Conclusions The paper has introduced NPP Temelin diagnostic measurements in conjunction with

the description of reactor vibration. The phenomena of vibration beats namely on main circulation

pump revolution harmonics was referred to pressure pulsations generated by main circulation pumps

with slightly differed revolutions. The amplitude statistics shows that particularly at 4th

– 6th

harmonic frequencies the beat character is dominating by average, standard, maximal and range

values. These beat phenomena represent inconsiderable component in overall reactor vibration,

which can cause contact loss of internal parts during long-term operation.

The operational data from selected self power neutron detectors of both NPP Temelin units were

processed together with modal parameters of VV6 and TVSA assemblies to show probable region

of possible fuel rod infringement.

In order to be able to detect fuel incorrect behaviour during the reactor operation it is advisable

to extend synchronous measurement and processing to other diagnostic sensors inclusive main

circulating pumps parameters.


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References

[1] Tanzer M., Mašek V., Smolík J., Macák P. : „The signal measurement evaluation and RVMS system setting-up of NPP Temelin Unit 1“, Škoda JS plc report N

o Ae 967/Dok, Plzeň,

September 2002, in Czech

[2] Smolík J.,Tanzer M., Mašek V. : „The signal measurement evaluation and RVMS system setting-up of NPP Temelin Unit 2“, Škoda JS plc report N

o Ae 11 112/Dok, Plzeň, June 2003

[3] Stulík P. : “Calibration and On-line Monitoring Methods on Czech NPP Operational Diagnostic Systems”, Technical IAEA meeting on Increasing instrument calibration interval

through on-line monitoring technologies, OECD, Halden Reactor Project Halden, Norway,

September 2004

[4] Pečínka L., Stulík P., Šípek B. : “Operating shapes vibration analysis of NPP Temelin WWER 1000/320 reactor”, Computational Mechanics 2006, 22nd Conference with International

Participation, Nečtiny, November 2006, in Czech

[5] Pečínka L., Stulík P., Zeman V. : “Influence of the operational vibrations on the WWER 1000/320 reactor of NPP Temelin core barrel stability”, 5

th International Conference “Safety

Assurance of NPP with WWER”., May 2007, Podolsk, in Russian

[6] Pečínka L., Stulík P. : “Experimental verification of WWER 1000/320 reactor dynamic response to pressure pulsations generated by the main circulating pumps”, Colloquium

Dynamics of Machines, Prague, February 2008, in Czech

[7] Stulík P., Šípek B. : “The operational dynamic responses of NPP Temelin WWER 1000/320 reactor in less loop main circulating pump combinations ”, Nuclear Research Institute Rez plc,

Report No Z 2125, February 2008, in Czech

[8] Zeman V., Hlaváč Z. : “Dynamic response of WWER 1000 type reactor excited by the main circulating pump pressure pulsations“, Colloquium Dynamics of Machines, Prague, February

2008, in Czech

[9] Dragunov J.G., Dranchenko B.N, Abramov V.V., Chairetdinov V.U. : ”Vibration studies for WWER designs”,5

th International Conference “Safety Assurance of NPP with WWER”.,

May 2007, Podolsk, in Russian

[10] Mečíř V. : ”Temelin NPP Fuel experience”, 7th International Conference on WWER Fuel Performance, May 2007, Albena

[11] Zeman V., Hlaváč Z., Pečínka L. : “Dynamic response of WWER 1000 type reactor excited by pressure pulsations generated by main circulating pumps“, West Bohemian University report

No 52 120 – 01 - 08, Plzeň, May 2008, in Czech

[12] Makarov V.V., Afanasiev A.V., Matvienko I.V. : ”Modal analysis of the WWER fuel assembly dummies during the force and kinematic vibration excitation”,5

th International

Conference “Safety Assurance of NPP with WWER”., May 2007, Podolsk, in Russian

[13] Kodl P., Macák P. : “The possible reason evaluation of fuel assemblies damage”, ŠKODA JS plc report N

o Jad-Výp/E26/06, Plzeň, October 2006, in Czech

[14] Stulík P. : “NPP Temelin in-core detectors analysis in the frequency range important from the fuel behaviour view”, Nuclear Research Institute Rez plc, Report N

o 13 010, June 2008, in

Czech
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