The Effect of Different Perturbations on the Stability Analysis of Light Water Reactors
Neutron noise analysis techniques are studied and developed, with primary use
of determining the stability of Boiling Water Reactors (BWRs). In particular, the role of
a specific perturbation prevailing in Light Water Reactors, the propagating density
perturbation, in the stability of BWRs and on the noise field of LWRs in general, is
investigated by considering three topics.
In the first topics, we investigate how the neutronic response of the reactor, usually
described as a second order system driven by a white noise driving force, is affected by
a non-white driving force. This latter arises from the reactivity effect of the propagating
density perturbations. The investigation is performed by using spectral and correlation
analysis. Propagating perturbations with different velocities are analyzed. We investigate
how the accuracy of the determination of the so-called decay ratio (DR) of the system,
based on the assumption of white noise driving force, deteriorates
with deviations from the white noise character of the driving force.
In the second topics, the space dependence of the neutron noise, induced by
propagating density perturbations, represented through the perturbation of
the absorption, is determined and discussed. A full analytical solution was obtained
by the use of the Green's function technique. The solution was analyzed for different
frequencies and different system sizes. An interesting new interference effect
between the point-kinetic and space-dependent components of the induced noise was
discovered and interpreted in physical terms.
In the last topics, a non-linear stability analysis of a BWR is performed,
using so called Reduced Order Model (ROM) techniques. A ROM is usually constructed
by reducing the full set of 3D space-time dependent neutron-kinetics,
thermal-hydraulics and heat transfer equations to time-dependent ones, by
considering space dependence in a lumped parameter model (one or two discrete channels).
The main novelty of our work is to treat the
space dependence by four heated channels. This extension makes it
possible to account for the effect of three neutronic modes: fundamental, first
and second azimuthal ones. The Forsmark-1 instability event in 1996/1997
was chosen to be investigated by the ROM developed in this work. The reactor response was determined
for various operational points to identify the stable/unstable reactor
behavior. The suitability of using the DR as the stability parameter in case of non-linear oscillations
is also being investigated.
BWR and PWR stability
propagating density perturbation
density wave oscillations
non-white driving force
damped harmonic oscillator
reduced order models
Green's function technique