State of the art regarding the safety analysis of boron dilution events in Germany

In the German practice of considering boron dilution transients (BDT) in safety analysis reports (SAR), a strongly conservative approach is applied, although it is not explicitly requested in German rules and guidelines for SAR. The approach is based on recommendations of the German Gesellschaft fuer Anlagen- und Reaktorsicherheit (GRS) and the technical expert organizations (TUV) and accepted by the Reactor Safety Commission (RSK) and is currently followed by the industries and the facilities . No final recommendation and guidelines exist, because the item is subject of comprehensive discussions and research in Germany currently. This conservative approach is based on a combination of analytical and experimental steps. First of all, in a series of thermal hydraulic system code calculations, the bounding scenarios are determined which lead to maximum realistic (under the conditions assumed for nuclear safety verification calculations) volumes of the lower-borated coolant which can be transported to the reactor core by re-establishing circulation in the primary circuit. These are mainly small break loss of coolant accidents (SBLOCA), during which the decay heat is removed from the core by the reflux-condenser regime. The position and maximum size of lower-borated slugs are identified, and the circulation re-start conditions are determined. The main SBLOCA scenarios are verified on experiments at large scale test facilities (e.g. PKL in Germany). While in the integral tests, the formation and transport of the slugs in the loops is assessed, the mixing is investigated in detail in dedicated mixing test facilities (ROCOM) using the boundary conditions either from the integral tests or from the system code calculations. In this way, the minimum boron concentration at the core inlet, which is reached during the transient, is estimated. Additionally, the minimum boron concentration is calculated by using computational fluid dynamics (CFD) codes. The critical boron concentration of a reactor core should be lower than the minimum boron concentration at the core inlet, determined in the mixing experiments or obtained from reliable calculations. This approach contains a high amount of conservatism, because it is assumed that the boron concentration is uniform in the whole reactor core. The spatial and temporal distribution of the boron concentration in the reactor core, available from the detailed analyses, is not taken into account. A best-estimate approach has been developed at FZR and was applied to generic studies of BDT scenarios connected with start-up of the first MCP for German KONVOI type reactors. This approach starts with the definition of a scenario which covers all possible BDT scenarios with respect to reactivity consequences (maximum reactivity insertion). This is followed by a reactivity initiated accident (RIA) analysis using appropriate best-estimate tools (coupled 3D neutron kinetics/thermal hydraulic codes). Best-estimate boundary conditions for the boron concentration at the core inlet are applied. These are time-dependent boron concentration at the core inlet determined from experiments or CFD calculations. The final goal of the analysis is to show the integrity of the fuel rods. Usual acceptance limits with respect to maximum fuel temperature, maximum cladding temperature, radially averaged enthalpy deposited in the fuel and maximum cladding oxide layer thickness for RIA are applied. The use of this best estimate approach, which should be accompanied by an uncertainty analysis, can help to make the design of future reactor cores more flexible and economically not decreasing the safety level of the nuclear installations (Authors)