atw Vol. 63 (2018) | Issue 8/9 ı August/September
an alarm. If two channels out of
four are simultaneously measuring a
reactor power PPKG.2.Max._Signal >
103 %, an alarm will be activated in
the control room and RCCA insertion
will be activated in order to reduce the
neutron flux. For this reason, probability
density functions of the in-core
and ex-core detectors were specifically
analysed (Figure 5).
Additional to physical measurements
of neutron flux and vibration
signals (Figure 6), special care was
given to the signal analysis of digitallybuilt
signals, for example the corrected
reactor thermal power, used into the
digital I&C system. Useful insights,
among other the evolution of the
positions with high neutron noise,
were obtained by comparing statistical
distributions at MOC and EOC of
those signals.
The neutron noise evolutions of
the in-core and ex-core detectors are
presented in Figure 7. The withincycle
evolutions of the neutron noise
amplitude are to be seen mostly as
linear; local trends are observed
and well coincide with the average
neutron flux trend within the cycle,
whose distribution is a result of boron
acid concentration, burnup of hot
spots in the core, decrease of the
radial peaking factors and RCCA
positions.
The signal correlations given in
Figure 4 revealed that the noise
signals at two opposite sides of the
core had strong negative correlations;
detectors of instrumentation channels
1 and 3 are strongly correlated. This
means that the measured flux increase
in one quadrant is at the same time
compensated by a flux reduction in
the opposite core quadrant. The
analysis has also shown, as illustrated
in Figure 7, that the largest noise
| | Fig. 7.
In-cycle evolution of neutron noise (1-σ standard deviation) measured during Cycle 36: ex-core ionization chambers (S1 – S4) and
in-core SPNDs at axial position 5 (close to fuel assembly inlet). The peak observed at ~20 EFPD is the result of a conducted power
level change.
amplitudes are located primarily in
one quadrant of the core centred on
core position J14 between Loop 1 and
3. The reason for the high neutron
noise in this region was analysed.
It is to note here that the core fuel
loading is 90° symmetric whereas the
RPV with the three loops is 120°
symmetric, implying that there is no
simple core symmetry; in addition,
the individual symmetries show
deviations from theory. To illustrate
this assumption, it can be mentioned
that the thermal loops have different
thermal powers, and their layout is no
perfectly 120° from one another.
Further thermo-hydraulic investigations
would be required to check the
impact of these asymmetries on the
neutron noise amplitudes. It can also
be mentioned that the 48 RCCA are
not positioned with a 90° symmetry in
the core.
Finally, the within-cycle evolution
of neutron noise was compared, at a
macroscopic level, to plant-specific
parameters such as the reactor power,
calibrated ex-core and in-core LHGRs,
and the calculated core flowrate
deduced from the pressure sensors
in the three loops. For illustration
purposes, the neutron flux measured
by two different channels (Middle
range and SPNDs) and the primary
water temperature span are shown in
Figure 8.
4 Summary
The phenomena leading to an increase
of the neutron flux noise from
cycle to cycle since about 2010 have
been studied in detail through
detailed measurements performed in
the timeframe 2014 to 2015 over two
cycle at MOC and EOC states. The
results show that this increase can
OPERATION AND NEW BUILD 455
a) b)
| | Fig. 8.
Cycle Evolution during Cycle 36 at KKG of a) Measured neutron flux and b) Average core temperature difference (ΔT = T oulet – T inlet ).
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
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