Partial depletion of the primary circuit of a pressurized water reactor during a postulated small break loss of coolant accident can lead to interruption of one-phase flow natural circulation. In this case, the decay heat is removed from the core in the reflux-condenser mode. In this operation mode, slugs of lower borated water can accumulate in the cold legs. After refilling of the primary circuit, the natural circulation in the two loops not receiving emergency core cooling injection (ECC) re-establishes and the lower borated slugs are shifted towards the reactor pressure vessel (RPV). Entering the core, the lower borated water causes a reactivity insertion. Mixing inside the RPV is an important phenomenon limiting the reactivity insertion and preventing a re-criticality. The mixing of these lower borated slugs with the ambient coolant in the RPV was investigated at the 1:5 scaled coolant mixing test facility ROCOM. Wire mesh sensors based on electrical conductivity measurement are used in ROCOM to measure in detail the spreading of a tracer solution in the facility. The mixing in the downcomer was observed with a sensor which spans a measuring grid of 64 azimuthal and 32 positions over the height. The resulting distribution of the boron concentration at the core inlet was measured with a sensor integrated into the lower core support plate providing one measurement position at the entry into each fuel assembly. The boundary conditions for the mixing experiment were taken from an experiment at the thermal-hydraulic test facility PKL operated by FANP Germany. The slugs, which have a lower density, accumulate in the upper part of the downcomer after shifting into the RPV. The ECC-water injected into the RPV falls almost straight down through the lower borated water and accelerates. On the outer sides of the ECC-streak, lower borated coolant admixes and flows together with the ECC-water downwards. This is the only mechanism of transporting the lower borated water into the lower plenum. All these effects could be visualized and quantified by the downcomer sensor. On the way to the core, the lower borated water is effectively mixing with the ambient, high borated water. Therefore, in the core inlet plane, lower borated water is detected only in the outer part. The minimum boron concentration, measured at one fuel element inlet position at one certain time point, was 71% of the initial 2500 ppm. There is no change of the initial boron concentration in the inner part of the core inlet plane during the whole transient at all.

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