Remark on the role of the driving force in BWR instability

• Simple models of BWR instability, used e.g. in understanding the role of the various oscillation modes inthe overall stability of the plant, assume that each oscillation mode can be described by a second ordersystem (a damped harmonic oscillator) driven by a white noise driving force. Change of the decay ratio(DR) of the observed signal is, as a rule, associated with the changing of the parameters of the dampedoscillator, mainly its damping coefficient, and is interpreted in terms of the change of the stability ofthe system. However, conceptually, one cannot exclude cases when the change of the response of a drivendamped oscillator is due to the change of the properties of the driving force. In this work we investigatethe effect of a non-white driving force on the behaviour of the system. A question of interest is howchanges of the spectrum of the driving force influence the observed autocorrelation function (ACF) ofthe resulting signal. Hence we calculate the response of a damped harmonic oscillator driven by anon-white driving force, corresponding to the reactivity effect of propagating density fluctuations intwo-phase flow. It is shown how in some special cases such a driving force, when interpreting the neutronnoise as if induced by a white noise driving source, can lead to an erroneous conclusion regarding thestability of the system. It is also concluded that in the practically interesting cases the effect of the coloureddriving force, arising from propagating density fluctuations, is negligible.

Subject headings

NATURVETENSKAP  -- Fysik -- Subatomär fysik (hsv//swe)

NATURAL SCIENCES  -- Physical Sciences -- Subatomic Physics (hsv//eng)

TEKNIK OCH TEKNOLOGIER  -- Annan teknik -- Övrig annan teknik (hsv//swe)

ENGINEERING AND TECHNOLOGY  -- Other Engineering and Technologies -- Other Engineering and Technologies not elsewhere specified (hsv//eng)

NATURVETENSKAP  -- Fysik -- Annan fysik (hsv//swe)

NATURAL SCIENCES  -- Physical Sciences -- Other Physics Topics (hsv//eng)

Keyword

autocorrelation

neutron noise

spectral density

BWR stability

decay ratio

Publication and Content Type

art (subject category)

ref (subject category)