atw 2018-09v3


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


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


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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