Jl. Munozcobo et al., NON-LINEAR ANALYSIS OF OUT-OF-PHASE OSCILLATIONS IN BOILING WATER-REACTORS, Annals of nuclear energy, 23(16), 1996, pp. 1301-1335
Out of phase oscillations have been observed recently in many boiling
water reactors during stability tests and also in start up conditions.
Many authors have attempted to explain these regional oscillations, b
ut the explanations given are not complete. In this paper, we develop
a non linear phenomenological model that can explain, both in phase an
d out of phase oscillations. The neutronic loop has been described on
the basis of an expansion in terms of lambda-modes. Furthermore, for a
semiquantitative representation of the dynamics, reduced order model
have been obtained reducing the number of regions, modes and energy gr
oups considered in the problem. In this line, we propose a model that
qualitatively explains the dynamic behavior of these oscillations veri
fying that in phase oscillations only appear when the azimuthal mode h
as not enough thermal-hydraulic feedback to overcome the eigenvalue se
paration and also, that it is possible that self-sustained out of phas
e oscillations arise due to the different thermal-hydraulic properties
of the two reactor core lobes, if the modal reactivities have appropr
iate feedback gains. Copyright (C) 1996 Elsevier Science Ltd