Experimental and Numerical Investigation of Control Element Vibration During Abnormal Core Barrel Motion at a VVER-440 Type Reactor


Experimental and Numerical Investigation of Control Element Vibration During Abnormal Core Barrel Motion at a VVER-440 Type Reactor

Altstadt, E.; Weiß, F.-P.

In 1985 abnormal core barrel motion had occured at unit 2 of the Greifswald NPP indicated by extremly large neutron noise. Amplitudes of up to 4mm were estimated from the external neutron noise signals.
The coherence and phase relations between excore and incore neutron noise signals exhibited typical features from which it could be concluded that even the control elements were forced to vibrations by the moving core barrel.
To obtain principal understanding about these phenomena and to draw conclusions for the surveillance of the excore and incore neutron signals, experimental and numerical investigations were performed. An experimental set-up was constructed providing all necessary displacement signals via HALL probes. This physical model, being a double pendulum with an annular channel surrounding the lower pendulum, can be operated in a linear mode (small excitation amplitudes) as well as in a non-linear mode (larger amplitudes which cause impacts between channel and lower pendulum).
The numerical simulation algorithm is based on a mechanical system consisting of linear elements (inertia, dampers, springs) and non-linear elements (gaps). In this way impacts between control element and neighbouring fuel cassettes which are strongly non-linear events can be modeled. After simulating the time series, which is possible for any detector position at the control element or at the set-up respectively, transfer functions, coherences, phase relations etc. can be computed.
It could be shown that the typical linear phase shifts between excore and incore neutron noise signals are due to impacts between the control elements and the neighbouring fuel cassettes. The obtained results can be used to establish a sensitive detection procedure for control element vibrations induced by abnormal core barrel motion.

  • Lecture (Conference)
    International Simulators Conference (SIMULATORS X), Arlington / Viginia, 29th March - 1st April 1993

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Publ.-Id: 1851