atw 2018-09v3

atw Vol. 63 (2018) | Issue 8/9 ı August/September
OPERATION AND NEW BUILD 452
Revised version of a
paper presented at
the Annual Meeting
of Nuclear Technology
(AMNT 2017), Berlin.
Detailed Measurements and Analyses
of the Neutron Flux Oscillation
Phenomenology at Kernkraftwerk Gösgen
G. Girardin, R. Meier, L. Meyer, A. Ålander and F. Jatuff
1 Introduction This paper summarises recent investigations [1], [2], [3] on measured neutron flux noise at
the Kernkraftwerk Gösgen-Däniken AG, who is operating since 1979 a German KWU pre-KONVOI, 3-Loop PWR with a
thermal power of 3,002 MWth (1,060 MWe). In a period of approx. 7 cycles from 2010 to 2016, an increase of the
­measured neutron noise amplitudes in the in- and out-core neutron detectors has been observed, although no ­significant
variations have being detected in global core, thermo-hydraulic circuits or instrumentation parameters. Verifications of
the instrumentation were performed and it was confirmed that the neutron flux instabilities increased from cycle to
cycle in this period. In the last two years, the level of neutron flux noise remains high but seems to have achieved a
saturation state.
In a power reactor, neutron noise is
the result of random fluctuations of
many parameters, primarily neutronic
ones such as the number of neutrons
emitted per fission, thermal-hydraulic
parameters such as the fluctuations of
the primary water inlet temperature,
and mechanical parameters as for
example main circulation pump vibrations
or core internal vibrations. In
a KWU-PWR as KKG, the significant
neutron noise is observed at a frequency
in the range of 0.1 Hz to about
10 Hz, with a peak close to 1 Hz. Each
component has a typical spectral
response in the frequency domain,
and such a spectrum analysis can be
used as a diagnostic tool for surveillance
[4]. A significant variation of
the measured spectrum during a cycle
can be potentially interpreted as of
relevance for the plant performance
or safety. For that reason the Reactor
Pressure Vessel (RPV) and main
| | Fig. 1.
Schematic representation of the 3002 MW 3-Loop KKG core and the radial
positions of the in-core (left white on the map) and ex-core neutron flux
detectors. The colour map shows the relative power map (Fq) at the
assembly level. The inner axial flux distribution is monitored via six axially
and uniformly distributed in-core Self-Powered Neutron Detectors, while
the four radial ex-core channels contain two compensated ionisation
chambers, i.e. for the upper and lower core regions.
cir culation pumps at KKG are
equipped with acceleration and absolute
position sensors.
To deepen the understanding of
this behaviour, neutron flux signals at
different core locations and burnup
have been newly measured at a
sampling rate up to 100 Hz in order to
analyse possible spatial correlations
between the measured signals. The
measurements corresponded to
Middle- of-Cycle (MOC) and End-of-
Cycle (EOC) conditions, for two
successive cycles aiming at analysing
noise evolution, additionally to the
known linear increase during the
cycle. During the cycle itself, the noise
amplitude increase is linearly correlated
to the decrease of the negative
moderator temperature reactivity
­coefficient (Γ T ), which is caused by
the decrease of the boron con centration
in the primary circuit; this
behaviour is well known and predictable.
The phenomena to be
investigated here is the variation from
cycle-to-cycle, which was unexpected.
Auto- and cross-correlations between
neutron signals in the time and
frequency domain were investigated
by means of signal analysis tools. In
this respect several hypotheses behind
the increase of neutron noise – e.g.
core loading pattern, fuel structure
design, variations of the core inlet
temperature, core asymmetry, etc. –
were identified and checked on
the measured high-frequency data.
Globally it was observed that the
highest neutron noise amplitudes
were to be found in one single core
quadrant, located between Loop 1 and
Loop 3 of the core. Radial correlations
were also identified between core
quadrants, but no measurable time
delays were found axially between
measurements from top and bottom
neutron signals.
Additional measurements of various
plant parameters were also performed,
in a second phase, to extend
the analysis not only to neutron flux
signals, but also temperature, pressure
or component vibrations. Correlations
between vibration signals and
neutron flux signals were analysed as
well.
A brief description of the KKG core
is provided in Section 2. The performed
measurements, neutron noise analysis
performed at KKG [3], along with the
results are described in Section 3.
Section 4 presents a summary of the
performed analysis and the current
model explaining its origin.
2 KKG Core design
The reactor is a Pressurized Water
Reactor (PWR) pre-KONVOI 3-Loop,
manufactured by KWU-Siemens with
a thermal power of 3002 MWth
(1060 MWe). The core contains 177
fuel assemblies with a 15 x 15 fuel
assembly layout and an active core
height of 352 cm.
Since 2014 (Cycle 36) the core is
for the first time fully loaded with
HTP fuel assemblies manufactured
by AREVA GmbH, whose fuel design
features Zircaloy/Duplex cladding
material, modern spacer grid geometries
and UO 2 fuel with 4.95%-wt
enrichment equivalent. The reactor is
typically operated at full power for
12-month cycles and has five different
radial burnup regions. The moderator
temperature coefficient of reactivity
Γ T is in the range of 30 pcm/K at BOC
to 70 pcm/K at EOC. The boron
concentration is typically 950 ppm at
BOC and is continuously decreasing
at a rate of ~ 3 ppm/day. The core
is operated at a maximal Linear
Heat Generation Rate (LHGR) of
525 W/cm, with an average power
density q’’’ of about 105 W/cm 3 [5].
Operation and New Build
Detailed Measurements and Analyses of the Neutron Flux Oscillation Phenomenology at Kernkraftwerk Gösgen ı G. Girardin, R. Meier, L. Meyer, A. Ålander and F. Jatuff