Cross correlation of neutronic and acoustic noise signals from local boiling

Abstract

A boiling simulator was constructed to model the void and pressure effects of local boiling in an LMFBR. By forcing superheated steam into subcooled water the boiling simulator produced a series of single bubbles separated by waiting times. The bubbles ranged from 1 to 4 cm in diameter and had lifetimes of the order of 60 msec, and waiting times of 90 msec. The boiling simulator was placed in the core of a zero power reactor (GfK, ARK), and the neutron and pressure signals were recorded and cross correlated. The neutron-pressure cross correlation exhibited an almost perfect correlation at the bubble repetition frequency (7 Hz). If (as expected) the normal pressure and reactivity background noises in a power reactor are uncorrelated, then this strong correlation will improve the signal-to-noise ratio of a boiling detection scheme. Other advantages of the cross correlation technique are that only simple linear transfer functions are necessary to relate the neutronic noise to the pressure noise and that the zone of the reactor (positive or negative void coefficient zone) can easily be determined. Because of these advantages, the neutron-pressure cross correlation technique is considered a serious alternative for the detection of local boiling in sodium cooled reactors.