A phenomenological model for the explanation of a strongly space-dependent Decay Ratio

Paper i proceeding, 2003

It is commonly believed that the Decay Ratio (DR), a parameter characterizing the stability of Boiling Water Reactors (BWRs), is a space-independent parameter of the reactor, i.e. it is independent of which Local Power Range Monitor (LPRM) is used in the core to perform the evaluation. This paper shows that the presence of several simultaneous types or sources of instability with different stability properties and different space dependence renders the DR also space-dependent, and even strongly space dependent. Two cases were investigated: the case of a local instability (i.e. one induced by a local noise source) coexisting with a global instability (in-phase oscillations), and the case of two local instabilities (noise sources). The results of these calculations were compared to the Forsmark-1 channel instability event, where strongly space-dependent decay ratios had been found in the measurements. Good adequacy was found between the DR model applied to the Forsmark-1 event and the corresponding measured DR. The fact that one single noise source in the core does not allow explaining a non-homogeneous DR suggests that in the case of Forsmark-1, at least two types or sources of instability had to be present in the core at the same time. According to the results obtained in this paper, these could be either a local and a global instability, or two local ones.