The NEA task group focused on the international standard problem 52 (ISP-52) held its meeting on 8-10 July 2025 in Bologna, Italy, to discuss progress in its activities and the importance of exercises to assess thermal-hydraulic code capabilities.
International standard problems (ISP) are comparative exercises in which the predictions of different computer codes for a given physical problem are compared with each other or with the results of a carefully controlled experimental study. The main goal of an ISP is to increase confidence in the validity and accuracy of tools which are used in assessing the safety of nuclear installations, following dedicated procedures.
ISP-52 focuses on the PKL III J5.1 Run2 experiment, which simulates a multiple steam generator U-tube rupture (MSGTR) scenario under design extension conditions without significant fuel degradation (DEC-A). It is expected to contribute to improving thermal-hydraulic modeling practices and enhancing the robustness of safety assessments for operating nuclear power plants. The ISP-52 benchmark is carried out by the NEA’s Working Group on Analysis and Management of Accidents (WGAMA) and is based on data from the PKL test facility that was operated by Framatome GmbH in Erlangen, Germany, and is now dismantled. This facility replicated the thermal-hydraulic behavior of a 1 300 MWe 4-loop pressurised water reactor, scaled to preserve hydrostatic head and natural circulation characteristics.
The task group’s meeting was hosted by ENEA (the Italian National Agency for New Technologies, Energy and Sustainable Economic Development) and gathered nearly 30 experts from 13 countries.
ENEA reported on the “blind” phase of the ISP, which involved comparing the participants’ simulations of the PKL experiments. In this phase participants were provided only with the boundary and initial conditions of the PKL III J5.1 Run2 test for the execution of the calculations.
Then, participants presented the results of their calculations for the second, “open” phase of the ISP, after getting access to the experimental results. This allowed them to improve the calculation results and better understand the reasons for the discrepancies. Finally, a session was dedicated to plant calculations, where computational tools are used to apply the experiment results to nuclear power plants.
The meeting provided an occasion to discuss the importance of such analytical exercises, not only to assess and better understand thermal-hydraulic code capabilities in predicting complex scenarios, but also to help users evaluate the limits of their simulation processes, improve their skills and address the user effect.
On the second day of the meeting, participants had the opportunity to visit the ENEA Brasimone Research Centre, an important nuclear research and innovation centre in Italy. The visit included tours of several major experimental facilities such as the Newcleo infrastructures supporting advanced reactor development, the PEC (Prova Elementi di Combustibile) reactor facility, the ESPRESSO loop for thermal-hydraulic testing, and the RSA (refrigerated storage area) used for component and material studies. These visits provided participants with insight into the experimental capabilities that support safety analysis and code validation efforts relevant to ISP-52 and beyond.
