atw 2018-09v3

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atw Vol. 63 (2018) | Issue 8/9 ı August/September OPERATION AND NEW BUILD 454 tail distribution. High but short peaks and low prob ability are mostly responsible for the activation of the power limitation function of the digital I&C system. Probability density function (e.g. Generalized Extreme Value GEV a) [6]) and fits of measured parameters were calculated in an attempt to predict maximal values and frequency occurrences of the measured neutron flux [3], which are of relevance for operational core control. Although ex-core raw signals from the ionisation chambers are electronically filtered in the signal processing, high amplitude noise are nevertheless registered with a certain low residual probability of triggering b) | | Fig. 4. Radial cross-correlations of the four ex-core detectors (a) and axial cross-correlations of in-core detector G02 (b) measured on 18.12.2014. a) | | Fig. 5. Probability Density Functions (PDF) a) and probability distribution b) of in-core detectors at position J14 and position G02 axial level 5. The signals are fitted with the Generalized Extreme Value (GEV) and Gauss functions (b). GEV fit is well suited for asymmetric distributions as observed at certain core positions in KKG. b) a) | | Fig. 6. Signal sample from vertical movement detector A1 located on the reactor pressure lid of the oscillation surveillance system (SÜS) recorded at MOC (a) and probability distribution of two absolute position sensors at MOC (b). b) 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
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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