The paper is focused on the detailed thermo-hydraulic (TH) modelling of the Reactor Pressure Vessel (RPV) only with the help of a TH system code including the full scope of Nuclear Power Plant (NPP) modelling (primary and secondary sides, RPV, active core and all operational and safety systems). The idea of the pseudo-3D TH methodology of the GRS system code ATHLET  is based on the application of a set of parallel TH (PTH) channels with cross flow connections between them. The aim of the activities described in the paper is to validate the recent developments of the methodology based on a fine TH nodalization of ATHLET on different experiments and thus to meet the challenge of detailed modelling of neutron-kinetics (NK) in the active core and the TH processes in the whole RPV with a good accuracy applying only a system TH code. This methodology enables to boost the performance of coupled code safety analysis for production purposes and it also allows the efficient application of uncertainty and sensitivity (U&S) analyses. Primary concern of the validation is related to the methodology itself that includes the correct modelling of the mixing flow phenomena mainly in the coolant legs, downcomer, bottom and upper plenum of RPV, and active core. Another aim of the study is to find the limits of application and accuracy of the system code ATHLET by the simulation and prediction of local (core) safety related parameters with the enhanced nodalization and PTH channel application.
The paper describes and discusses the comparisons done with the ATHLET system code using the new enhanced nodalization scheme with 3 types of measured data: data from two experiments (C1 and C3) on fluid mixing at Upper Plenum Test Facility (UPTF)  and a set of detailed neutron-physical/thermal-hydraulic measurements performed at Kalinin-3 NPP during an asymmetric transient.
The performed studies are of great interest because they have shown acceptable accuracy and also some advantages by performing coupled code production pseudo-3D analysis of NPPs applying the PTH channel methodology within a 1-D TH system code.