Partial depletion of the primary circuit during a hypothetical small break loss of coolant accident can lead to the interruption of one-phase flow natural circulation. In this case, the decay heat is removed from the core in the reflux-condenser mode. For the scenario of a hot leg side leak and hot leg safety injection thermal hydraulics analyses using the system code ATHLET showed, that weakly borated condensate can accumulate in particular in the pump loop seal of those two loops, which do not receive safety injection. According to these ATHLET-calculations, one-phase flow is maintained in the remaining two loops at high residual heat conditions because of the entrainment of safety injection coolant into the steam generators. After refilling of the primary circuit, natural circulation in the two stagnant loops simultaneously re-establishes and the de-borated slugs are shifted towards the reactor pressure vessel (RPV). Mixing in the downcomer and the lower plenum is an important phenomenon mitigating the reactivity insertion into the core in this postulated scenario. Therefore, mixing of the de-borated slugs with the ambient coolant in the RPV was investigated at the four loop 1:5 scaled ROCOM mixing test facility. Based on the ATHLET-calculations, a volume flow rate of 5% of the nominal rate was set in the loops running in one-phase flow. The volume flow rate in the two restarting loops increases from zero to 6%. In these two loops, de-borated slugs of 7.2 m3 were assumed corresponding to the volume of the whole loop seal. An experimental parameter study was carried out with different duration of the flow ramp and variation of the density difference between de-borated slug and ambient coolant due to differences in boron concentration and temperature. The variation of the density difference significantly changes the mixing behavior. With no density difference, the weakly borated coolant almost perpendicularly flows down in the downcomer and a maximum of 64% of the initial perturbation is detected in the core entry section below the loops where the slugs were formed. Increasing the density difference, a stratification is observed in the downcomer. The less dense slugs flow around the core barrel at the top of the downcomer. At the opposite side the lower borated coolant is entrained by the colder safety injection water and transported to the core. This entrainment effect leads to the admixture of boron from the safety injection to the under-borated slugs. Consequently, the maximum under-boration at the core entry is lower. For the maximum investigated density difference of 2%, a value of 31% only of the initial under-boration was measured at the core entrance.

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