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During the last years one focus of German PWR safety analyses was boron dilution events with the potential of reactivity transients. Coolant with a low boron concentration could be collected in localized areas of the reactor coolant system e. g. by separation of borated reactor coolant into highly concentrated and diluted fractions (inherent dilution) which can occur during reflux-condenser heat transfer after a small break loss of coolant accident with a limited availability of the emergency core cooling systems. The TüV NORD SysTec appraises in the framework of follower core assessments safety analyses of boron dilution events presented by the utilities. These analyses are based on the simulation of boron dilution and transport processes in conjunction with a number of dedicated experiments. The analyses shall demonstrate that boron dilution events cannot lead to recriticality of the core. Hence the boron concentration at the core inlet has to be determined. TüV NORD Sys Tec applies the CFD code FLUENT for the investigation of boron dilution events in pressurized water reactors. To affirm the FLUENT abilities for the simulation of boron dilution events a validation against the ROCOM experiment T6655_21 with density driven coolant mixing was performed. This validation proves that FLUENT is able to determine the boron concentration in the reactor core with sufficient accuracy and to solve the safety issue, whether the core gets critical or not.

Validation of the CFD code FLUENT by post test calculation of a ROCOM experiment with density driven coolant mixing