atw Vol.

atw Vol. 63 (2018) | Issue 8/9 ı August/September

OPERATION AND NEW BUILD 450

There are three possibilities for an

excitation in general:

• Stochastic fluid forces from turbulent

flow would lead to oscillations

of components at their natural

frequencies. The lowest natural

frequency of the fuel assemblies is

reported around 2.6 to 4 Hz [6, 18],

which would not explain the coherence

maximum at 1 Hz. Calculations

with simplified finite element

models show that depending on

design and operational behavior,

i.e. lateral stiffness decrease due to

radiation induces spring relaxation

in the spacers, the lowest natural

frequency can be shifted significantly

towards lower values. In [6,

16] an additional mode of the fuel

assemblies around 1 Hz in form of

synchronously moving cantilevered

beams (fixed at the bottom) is supposed.

Nevertheless, regarding the

fixture of the fuel assemblies in the

grid plate, the manifestation of this

mode is questionable. A further

explanation is the excitation of the

coupled system of core barrel, grid

plate and fuel assemblies, which

might have additional natural

system frequencies below the natural

frequencies of the single fuel

assemblies.

• A second possibility would be the

existence of an excitation force,

which is oscillating at around 1 Hz

and evokes a subsequent transient

deflection of the fuel assemblies.

Pressure fluctuations from residual

imbalances of the coolant pump,

standing waves, cavity resonances

in the pressurizer or vibrations of

other components of the loop are

known to induce core barrel motions

which could propagate to the

fuel assemblies. Fluid mechanical

oscillating forces with direct effect

on the fuel assemblies, e.g. pressure

differences, are also possible.

• A third possibility would be a selfexcitation

of fuel assemblies in a

constant axial flow. Research on

fuel assembly bow gives hints that

in fluid-structure-interaction (FSI)

simulations local forces can arise

leading to instability of the zero

position of the fuel assembly [19].

To investigate and prove the mentioned

hypotheses, a coupled FSI

model of core components and the

surrounding fluid is essential.

Simulations of reflector

influence

Further, the reflector influence has

been studied by means of a simplified

2D core model, in which the reflector

Case description

Maximum (relative)

increase on the left side

cross-sections are manipulated in

order to simulate the effect of varying

water gap between core barrel and

reactor pressure vessel, which corresponds

to the reflector region. These

variations could be caused by mechanical

motions, e.g. of core barrel or

fuel assemblies at the core periphery,

and their effect increases with decreasing

boron concentration. In this

model the TH parameters are homogeneous

and representative of the

hot full power state at zero burnup.

Further assumptions are: fuel temperature

= 900 K, moderator density =

702 kg/m 3 and boron concentration

= 1,300 ppm.

Table 1 summarizes the results

obtained for different variations of

the thermal absorption and fast-tothermal

scattering crosssection. The

reflector is modified only in one half of

the core (the left side) to reproduce

the spatial oscillations observed in the

PWR. The results show that the effects

of thermal absorption and scattering

are additive. The amplitude of the

power variation can reach the same

order of magnitude as observed in the

PWR.

Additional study is necessary to

determine if actual mechanical motions

can cause such changes leading

to increase/decrease of the moderator

volume (coolant water) in the reflector

zone and in that way changing

the homogenized assembly crosssections.

In addition, time-dependent

simulations are needed to check if

the frequency observed in the PWR

can be reproduced. Nevertheless, this

preliminary result shows that this

hypothesis is very promising. The

recently published study [20] showed,

that a variation of the gap size

between fuel elements of about one

centimeter can result in changes

of the neutron flux amplitudes at

the ex-core detectors of up to the

order of magnitude of 10 %. Therefore,

the influence of mechanical

motions of the fuel elements relative

to each other and as an ensemble

relative to the reflector cannot be

ruled out as explanation of the observed

neutron flux oscillations.

Summary and outlook

Several models based on single

physical effects (TH fluctuations at

the core inlet, movement of a point

source, coupled oscillations of core

internals, changes in the reflector

coefficients) are used to simulate the

neutron flux. Each of these simple

models can reproduce some of the

characteristics of the observed neutron

flux fluctuations but does not

encompass all features observed in a

real reactor. This suggests that further

work on the combination of models

is needed. Thereby, the biggest challenges

will lie in FSI simulations of

fuel assemblies including further core

internals, neutron physics simulations

using time-dependent geometries,

and possibly the coupling of all three

physical models.

Acknowledgment

This work has been performed in the

framework of the German Reactor

Safety Research and was funded by

the German Federal Ministry for

Economic Affairs and Energy (BMWi,

project no. RS1533). The authors

would like to thank the operators of

one German Vorkonvoi PWR and one

Konvoi PWR for providing data of

neutron flux measurements.

References

Maximum (relative)

decrease on the right side

-10 % thermal absorption 4 % -3 %

-10 % scattering 7 % -5 %

-10 % thermal absorption

-10 % scattering

10 % -8 %

-20 % thermal absorption 11 % -7 %

-20 % scattering 14 % -11 %

| | Tab. 1.

Summary of the reflector study results.

1. M. Seidl et al., Review of the historic

neutron noise behavior in German

GWU built PWRs, Progress in Nuclear

Energy 85, pp 668-675, 2015.

2. Reaktor-Sicherheitskommission,

Stellungnahme DWR-Neutronenflussschwankungen,

457. Sitzung vom

11.04.2013.

3. Bundesamt für Strahlenschutz,

Kurzbeschreibung und Bewertung der

meldepflichtigen Ereignisse in Kernkraftwerken

und Forschungsreaktoren

der Bundesrepublik Deutschland im

Zeitraum Januar 2011, Stand

14.12.2012.

Operation and New Build

Analyses of Possible Explanations for the Neutron Flux Fluctuations in German PWR ı Joachim Herb, Christoph Bläsius, Yann Perin, Jürgen Sievers and Kiril Velkov

atw Vol. 63 (2018) | Issue 8/9 ı August/September OPERATION AND NEW BUILD 450 There are three possibilities for an excitation in general: • Stochastic fluid forces from turbulent flow would lead to oscillations of components at their natural frequencies. The lowest natural frequency of the fuel assemblies is reported around 2.6 to 4 Hz [6, 18], which would not explain the coherence maximum at 1 Hz. Calculations with simplified finite element models show that depending on design and operational behavior, i.e. lateral stiffness decrease due to radiation induces spring relaxation in the spacers, the lowest natural frequency can be shifted significantly towards lower values. In [6, 16] an additional mode of the fuel assemblies around 1 Hz in form of synchronously moving cantilevered beams (fixed at the bottom) is supposed. Nevertheless, regarding the fixture of the fuel assemblies in the grid plate, the manifestation of this mode is questionable. A further explanation is the excitation of the coupled system of core barrel, grid plate and fuel assemblies, which might have additional natural system frequencies below the natural frequencies of the single fuel assemblies. • A second possibility would be the existence of an excitation force, which is oscillating at around 1 Hz and evokes a subsequent transient deflection of the fuel assemblies. Pressure fluctuations from residual imbalances of the coolant pump, standing waves, cavity resonances in the pressurizer or vibrations of other components of the loop are known to induce core barrel motions which could propagate to the fuel assemblies. Fluid mechanical oscillating forces with direct effect on the fuel assemblies, e.g. pressure differences, are also possible. • A third possibility would be a selfexcitation of fuel assemblies in a constant axial flow. Research on fuel assembly bow gives hints that in fluid-structure-interaction (FSI) simulations local forces can arise leading to instability of the zero position of the fuel assembly [19]. To investigate and prove the mentioned hypotheses, a coupled FSI model of core components and the surrounding fluid is essential. Simulations of reflector influence Further, the reflector influence has been studied by means of a simplified 2D core model, in which the reflector Case description Maximum (relative) increase on the left side cross-sections are manipulated in order to simulate the effect of varying water gap between core barrel and reactor pressure vessel, which corresponds to the reflector region. These variations could be caused by mechanical motions, e.g. of core barrel or fuel assemblies at the core periphery, and their effect increases with decreasing boron concentration. In this model the TH parameters are homogeneous and representative of the hot full power state at zero burnup. Further assumptions are: fuel temperature = 900 K, moderator density = 702 kg/m 3 and boron concentration = 1,300 ppm. Table 1 summarizes the results obtained for different variations of the thermal absorption and fast-tothermal scattering crosssection. The reflector is modified only in one half of the core (the left side) to reproduce the spatial oscillations observed in the PWR. The results show that the effects of thermal absorption and scattering are additive. The amplitude of the power variation can reach the same order of magnitude as observed in the PWR. Additional study is necessary to determine if actual mechanical motions can cause such changes leading to increase/decrease of the moderator volume (coolant water) in the reflector zone and in that way changing the homogenized assembly crosssections. In addition, time-dependent simulations are needed to check if the frequency observed in the PWR can be reproduced. Nevertheless, this preliminary result shows that this hypothesis is very promising. The recently published study [20] showed, that a variation of the gap size between fuel elements of about one centimeter can result in changes of the neutron flux amplitudes at the ex-core detectors of up to the order of magnitude of 10 %. Therefore, the influence of mechanical motions of the fuel elements relative to each other and as an ensemble relative to the reflector cannot be ruled out as explanation of the observed neutron flux oscillations. Summary and outlook Several models based on single physical effects (TH fluctuations at the core inlet, movement of a point source, coupled oscillations of core internals, changes in the reflector coefficients) are used to simulate the neutron flux. Each of these simple models can reproduce some of the characteristics of the observed neutron flux fluctuations but does not encompass all features observed in a real reactor. This suggests that further work on the combination of models is needed. Thereby, the biggest challenges will lie in FSI simulations of fuel assemblies including further core internals, neutron physics simulations using time-dependent geometries, and possibly the coupling of all three physical models. Acknowledgment This work has been performed in the framework of the German Reactor Safety Research and was funded by the German Federal Ministry for Economic Affairs and Energy (BMWi, project no. RS1533). The authors would like to thank the operators of one German Vorkonvoi PWR and one Konvoi PWR for providing data of neutron flux measurements. References Maximum (relative) decrease on the right side -10 % thermal absorption 4 % -3 % -10 % scattering 7 % -5 % -10 % thermal absorption -10 % scattering 10 % -8 % -20 % thermal absorption 11 % -7 % -20 % scattering 14 % -11 % | | Tab. 1. Summary of the reflector study results. 1. M. Seidl et al., Review of the historic neutron noise behavior in German GWU built PWRs, Progress in Nuclear Energy 85, pp 668-675, 2015. 2. Reaktor-Sicherheitskommission, Stellungnahme DWR-Neutronenflussschwankungen, 457. Sitzung vom 11.04.2013. 3. Bundesamt für Strahlenschutz, Kurzbeschreibung und Bewertung der meldepflichtigen Ereignisse in Kernkraftwerken und Forschungsreaktoren der Bundesrepublik Deutschland im Zeitraum Januar 2011, Stand 14.12.2012. Operation and New Build Analyses of Possible Explanations for the Neutron Flux Fluctuations in German PWR ı Joachim Herb, Christoph Bläsius, Yann Perin, Jürgen Sievers and Kiril Velkov

atw Vol. 63 (2018) | Issue 8/9 ı August/September 4. C. Bläsius et al., Untersuchungen der Ursachen für Neutronenflussschwankungen, GRS-408, Gesellschaft für Anlagen- und Reaktorsicherheit (GRS) gGmbH, 2016. 5. G. Kaiser et al., Reaktorinstrumentierung. Prozeßtechnik und Leistungsregelung im Kernkraftwerk, VDE Verlag, 1983 6. J. Runkel, Rauschanalyse in Druckwasserreaktoren, 1987. 7. L. J. Kostić, J. Runkel, D. Stegemann, Thermohydraulics Surveillance of Pressurized Water Reactors by Experimental and Theoretical Investigations of the Low Frequency Noise Field, Progress in Nuclear Energy 21, pp. 421-430, 1988. 8. J. Fiedler, Schwingungsüberwachung von Primärkreiskomponenten in Kernkraftwerken, 2002. 9. G. Kosaly, M. M. R. Williams, Point theory of the neutron noise induced by in-let temperature fluctuations and random mechanical vibrations, Atomkernenergie 18(3) p. 203-208, 1971. 10. G. Kosaly., L. Mesko, Remarks on the transfer function relating inlet temperature fluctuations to neutron noise, Atomkernenergie 20(1), pp. 33-36, 1972. 11. A. Abarca et al., Analysis of Thermalhydraulic Fluctuations in Trillo NPP with CTF/PARCSv2.7 Coupled Code, 23 nd International Conference Nuclear Energy for New Europe, Portoroz, Slovenia, 2014. 12. G. Verdú et al., Study of the Noise Propagation in PWR with Coupled Codes, International Conference on Mathematics and Computational Methods Applied to Nuclear Science and Engineering (M&C 2011), Rio de Janeiro, Brazil, 2011. 13. S. Langenbuch, K. Velkov, Overview on the Development and Application of the Coupled Code System ATHLET- QUABOX/CUBBOX, Proceedings of Mathematics and Computation, Supercomputing, Reactor Physics and Nuclear and Biological Applications, Avignon, France, 2005. 14. J. O. Hinze, Turbulence, McGraw-Hill, 1975. 15. G. C. Van Uitert, H. Van Dam, Analysis of Pool-Type Reactor Noise, Progress in Nuclear Energy 1, pp. 73-84, 1977. 16. E. Türkcan, Review of Borssele PWR noise experiments, analysis and instrumentation, Progress in Nuclear Energy 9, pp. 437–452, 1982. 17. Hashemian et al., Sensor Response Time Monitoring Using Noise Analysis, Progress in Nuclear Energy 21, pp. 583-592, 1988. 18. R. Sunder, Sammlung von Signalmustern zur DWR-Schwingungs überwachung – Informationsgehalt der Neutronenflussrauschsignale, GRS-A-1074, Gesellschaft für Reaktorsicherheit (GRS) mbH, 1985. 19. A. J. Petrarca, Y. Aleshin, Y. Xu, R. Corpa Masa, J.M. Gómez Palomino, Effect of lateral hydraulic forces on fuel assembly bow, Proceedings of the TopFuel Conference in Zurich, Switzerland, 2015. 20. J. Konheiser et al., Investigation of the effects of a variation of fuel assembly position on the ex-core neutron flux detection in a PWR, Journal of Nuclear Science and Technology 54(2), pp. 188-195, 2017. Authors Joachim Herb Christoph Bläsius Yann Perin Jürgen Sievers Kiril Velkov Gesellschaft für Anlagen- und Reaktorsicherheit (GRS) gGmbH Boltzmannstr. 14 85748 Garching, Germany OPERATION AND NEW BUILD 451 Advertisement WEITERBILDUNGSKURS NUKLEARFORUM SCHWEIZ IT-SICHERHEIT IM ALLTAG – PRAXISWISSEN FÜR MITARBEITER IN DER NUKLEARTECHNIK Donnerstag, 22. November 2018, Trafo, Baden Der eintägige Weiterbildungskurs gibt einen Einblick in die Gesamt problematik der Cybersecurity und die vielfältigen Formen der Be drohung. Das an der betrieblichen Praxis orientierte Programm hilft, potenzielle Gefahrenstellen im Alltag zu erkennen und den Hinter grund betrieblicher Massnahmen zur IT-Sicherheit zu verstehen. Der Kurs richtet sich an Mitarbeitende aller Funktionsstufen im Bereich der Nuklear technik, die in ihrem Büroalltag computerbasierte Informations- und Kommunikations systeme benutzen. Besondere IT-Kenntnisse sind nicht erforderlich. Programm und Anmeldung unter www.nuklearforum.ch SwissNuklear_Weiterbildung_Inserat_166x115mm_20180730.indd 1 30.07.18 14:30 Operation and New Build Analyses of Possible Explanations for the Neutron Flux Fluctuations in German PWR ı Joachim Herb, Christoph Bläsius, Yann Perin, Jürgen Sievers and Kiril Velkov