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.
The ISP COAL involves international participants comparing simulation results run with a variety of codes to the ASNR COAL reflooding experiments which are based on four 7x7 full-length electrically heated rod bundles. The fuel rods correspond to a 17x17 pressurised water reactor design.
The task group held its meeting on 26-27 June 2025 at the NEA headquarters in Paris. The event gathered nearly 40 experts from 10 countries to discuss the progress in the activities. The task group is jointly carried out by the Working Group on Analysis and Management of Accidents (WGAMA) and Working Group on Fuel Safety (WGFS).
ASNR, the French Nuclear Safety and Radiation Protection Authority, reported on the “open” phase of the ISP, which involved comparing the international participants’ simulations of the two COAL experiments performed on bundles with undeformed rods identified as B0.
Then, participants presented the results of their calculations for the second, “blind” phase of the ISP, which involved the simulation of the two COAL experiments performed on a bundle containing deformed, ballooned rods, identified as B2. Presentations were made without knowledge of the experimental outcomes of the blind tests.
Finally, the blind experimental results were presented by ASNR at the end of the meeting.
The third phase of the ISP will start at the end of July 2025. It will involve an open benchmark of simulations of the two deformed COAL experiments investigated in the blind phase, and incorporating the provided experimental results.
An uncertainty propagation exercise will be conducted by the participants for each of the three phases.
