atw 2018-09v3

inforum

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

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