Second transit time in a BWR power plant: Experimental evidence

Abstract

An important feature of a BWR is the axial propagation of the coolant density perturbation. A transit time can be associated to such phenomenon and its estimation is generally performed via neutron noise crosscorrelation techniques. Transit time measurements have been performed in Caorso Power Plant (860 MWE, BWR-4) at different radial positions and different power levels by using the local power range monitors (LPRM) and the traversing in core probe (TIP). A remarkable result is the observation of two transit times in the upper part of the core. The effect is clearly detectable at high power conditions and only above the level of the LPRM C (≅ 228 cm. over the core bottom). The problem of the double time delay is examined at first. Secondly a review of the analysis method is presented with reference to the multiple delay c a se. Experimental results are finally referred to validate the hypothesis of a second transit effect.