Coolant Mixing Studies for the Analysis of Hypothetical Boron Dilution Transients in a PWR

PWR transients caused by a perturbation of boron concentration or coolant temperature at the inlet nozzles depend on the mixing inside the reactor pressure vessel (RPV). Initial steep gradients are partially reduced by turbulent mixing with the ambient coolant in the RPV. However, the assumption of an ideal mixing in the downcomer and the lower plenum of the reactor leads to unrealistically small reactivity inserts. Moreover, the reactivity differences between ideal mixing and total absence of mixing are too large to be acceptable for safety analyses. In reality, a partial mixing takes place. For realistic predictions it is necessary to study the mixing within the three-dimensional flow field in the complicated geometry of a PWR. For this purpose, a 1:5 scaled model (ROCOM) of the German PWR KONVOI was built. The emphasis was put on extensive measuring instrumentation and on maximum flexibility of the facility to cover different test scenarios. The use of special electrode-mesh sensors together with a salt tracer technique allows to measure concentration fields within the downcomer and at the core entrance with a high resolution in space and time. Especially the instrumentation in the downcomer provides detailed information about the mixing phenomena. The obtained data was used to support code development for mixing modeling and validation.
Scenarios investigated are: (1) Steady-state flow in several coolant loops with a temperature or boron concentration perturbation in one of them. (2) Transient flow situations with flow rates changing in time in one or more loops, such as pump start-up scenarios with deborated slugs in one of the loops or onset of natural circulation after boiling-condenser-mode. (3) Gravity driven flow caused by large density gradients, e.g. mixing of cold ECC water with the warmer inventory in the RPV. In all cases, the experimental results show an incomplete mixing with typical concentration and temperature distributions at the core inlet which strongly depend on the conditions of the considered scenario. CFD calculations were found to be in good agreement with the experiments but take long calculation times.
Therefore, an efficient semi-analytical model (Semi-Analytical Perturbation Reconstruction) has been developed allowing the description of the coolant mixing inside the RPV by the superposition of response functions at the core entrance on Dirac-shaped perturbations in the cold leg. The validation of the model against experimental data from the ROCOM-facility is presented.
SAPR provides realistic time-dependent boron concentration fields at the core inlet that can be used for the analysis of a hypothetical boron dilution transient after start-up of the first main coolant pump in a generic four-loop PWR. Core calculations were performed with the 3D reactor dynamics code DYN3D. By varying the initial slug volume it was found, that for the given core loading pattern slugs of less than 20 m3 do not lead to re-criticality of the shut-off reactor. Calculations with the bounding slug volume of 36 m3 show, that the corresponding reactivity insertion does not result in core damage.