atw 2018-09v3

inforum

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

OPERATION AND NEW BUILD 448

| | Fig. 4.

Coherence and phase angles between different ex-core detectors (top),

relative neutron flux measurements at six different elevations of the C04

in-core measurement rod (bottom); all measurements in a Vorkonvoi PWR.

progressions of the curves are identical

for all six elevations. It has to be

emphasized that no time lag can be

identified between measurements at

the bottom of the reactor core

compared with measurements at

the top. The same signal pattern can

be observed for all eight in-core

measurement positions.

All these observations are consistent

with different measurements

and analyses done during the last

decades [6, 7, 8]. Fiedler [8] compared

neutron flux fluctuation levels

in different plant types. He found that

the prominence of the 180° phase

difference between opposing detectors

(referred to as “beam mode”) is

special to KWU type PWRs.

Possible explanation based on

thermo-hydraulics effects

Already at the beginning of the

1970s, a model was published [9, 10]

coupling a point-kinetics neutron

physics model with a one-dimensional

TH model. It allows predicting neutron

flux fluctuation levels based

on coolant temperature or density

oscillations. Based on this model

it is already possible to understand

essential characteristics of the neutron

| | Fig. 5.

Simulated temperature fluctuations in frequency (top, left) and time (top, right) domain; layout of the coupled ATHLET-QUABOX/

CUBBOX model for a mini-core (bottom, left) and the resulting neutron flux fluctuations spectrum (bottom, right).

noise spectrum qualitatively, e. g. the

dependency of the neutron flux fluctuation

amplitude on the value of the

moderator temperature coefficient.

Following this approach and based

on some new simulations with the

CTF/PARCS codes [11, 12] a model of

the reactor core has been developed

using a coupled version of ATHLET

and QUABOX/CUBBOX [13]. In [12]

temperature fluctuations at the core

inlet were applied based on different

spectral properties. Temperature

oscil lations based on a white noise

spectrum resulted in much smaller

power/neutron flux oscillations than

temperature oscillations based on a

low-pass-filtered spectrum. A possible

explanation for that observation

might be alias-effects due to the limited

spatial and temporal resolution of

the coupled system. To avoid such

problems with the coupled system of

ATHLET and QUABOX/CUBBOX, a

Kolmogorov type spectrum [14] has

been applied for the temperature

­fluctuations at the inlet of the reactor

core. Figure 5 (top row, left) shows

the power spectral density of the

temperature oscillations over the

frequency. Such spectra were observed

in different reactors [15, 16,

17].

Based on the assumption that the

temperature fluctuations follow such

a Kolmogorov type spectrum the time

dependent temperature fluctuations

are calculated (Figure 5, top right).

The temperature fluctuations have the

same variance as a sine-wave with an

amplitude corresponding to 1 K.

The TH model layout is shown in

Figure 5 (bottom, left). It consists of

nine interconnected core channels

with common inlet and outlet thermofluid

elements. The mini core has a

typical neutron-physics characteristic

of an end of fuel cycle (EOC).

Figure 5 (bottom, right) shows the

power spectral density of the resulting

fluctuations in the reactor power

production, which is proportional to

the neutron flux amplitude. For frequencies

smaller than 3 Hz the calculated

power spectral density fits the

measured ex-core detector signals of a

Vorkonvoi PWR quite well over several

orders of magnitude. This suggests

that temperature fluctuations at the

inlets of the core channels are part of

the explanation. This model can also

explain the correlation between the

amplitude of the fluctuations and the

moderator temperature coefficient.

However, it is not possible to explain,

why no phase differences could be

observed between measurements of

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

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