PASTELS aims to significantly increase the knowledge within Europe ofinnovative passive systems, namely SACOs and CWCs, and the ability of severalEuropean system and CFD computational codes to be able to accurately model keyphenomena such as natural circulation loops and condensation. This is verychallenging due to their very specific properties, i.e. small driving forcesworking against high resistive forces which are specific to the concept ofthese technologies. Given the growing use of the SACO and CWC technologies innon-European NPPs, it is essential, especially with the foreseen future use ofSmall Medium Reactors (SMR) that the European nuclear community is able toadapt its current numerical tools to this promising technology. Extensive experimentaltesting (SET, CET and integral experiments) with representative operatingconditions on semi-industrial full scale test facilities (PKL facility [DE] andPASI facility [FI]) will provide essential data to support the improvement ofthe numerical activities. Existing data from PERSEO and HERO-2 facilities willalso be used. The numerical and experimental activities will be conducted in anintegrated step-by-step approach. PASTELS will investigate improvements tomodels, novel methodologies for the coupling of system and CFD codes working atdifferent scales. Additionally, important knowledge on the behaviour of theSACO and CWC will be captured through the observation of their behavior duringthe test campaigns. Different and similar computational codes will be used bythe partners in order to be able to benchmark and compare the different resultsobtained, understand the causes and propose strategies to improve them. Allproject results will feed into extensive methodology guidelines and a roadmapto achieving licensing and implementation of these innovative passive systemtechnologies in future European NPPs.