Development of a qualified plant nodalization for steady state and transient analysis of table top facility

MYRRHA is a flexible, fast spectrum, pool type research reactor cooled by lead bismuth eutectic (LBE), under development at SCK CEN. The Primary Heat eXchangers (PHX) of MYRRHA are designed to operate with pressurized water as secondary coolant and are located in the reactor pool. The PHX design is based on a double-walled bayonet tube bundle, with intermediate Helium leak detection system. Three experimental facilities have been designed and constructed at SCK CEN with the aim to assess the performance of this innovative component: The Leak Detection System facility has been designed, constructed and operated during the period 2017 - 2019 with the goal to perform a standalone investigation of the PHX Helium leak detection system. The HEXACOM facility has been designed and constructed during the period 2018-2023 with the aim to investigate the performance of a full scale single PHX tube including the use of LBE from the primary coolant loop of the COMPLOT facility, which is already operational, as heat source. The Table Top facility has been designed and constructed during the period 2019-2021 and has been operational since 2022. This latter facility is discussed further in the current paper. Table Top is a 2.5 kW heated water/steam pressurized loop in open configuration that works with natural convective flow in the pressure range of [8-16] bara. The objective of the facility is to obtain a first estimation of the thermal resistance of the double walled PHX tube as well as provide experimental data for thermal hydraulic system code assessment and model development. The facility represents a single scaled down (in length) PHX tube and does not represent the primary LBE coolant, as heating is provided by electric heating elements. Three large experimental campaigns have been carried out including more than twenty-nine experiments. A procedure to develop a qualified thermal hydraulic system code nodalization for steady state and transient analyses is introduced in the paper. The procedure is applied to two experimental tests on Table Top to develop and refine a "immutable and qualified" nodalization of the facility. The activity investigates the capabilities of the thermal hydraulic system code RELAP 5 mod 3.3 of representing the main phenomena which occur during a typical transient as well as a typical steady state operational mode of Table Top. Emphasis will be given to the need of representing the plant layout to within an acceptable range of accuracy as defined in the procedure. This is necessary to capture the physical behavior of the system numerically.