ISABELLE 1 loop for ramp testing in the OSIRIS reactor. Upgrade and quality assurance issues

The ISABELLE 1 loop, installed in the OSIRIS reactor at Saclay, has been equipped since 1992 with sophisticated instrumentation for power determinations by heat balance. An inter-comparison programme including nuclear measurements (gamma spectrometry, dosimetry, radiochemical analysis) has been set up to qualify these thermal measurements. A significant reduction of the uncertainties involved in fuel rod linear heat rate determination was obtained. Thermohydraulic and neutron modelling of the ISABELLE 1 loop substantiated the various corrections to be made to the heat balance. The reproducibility of the tests was established by using a neutron signal to slave the position of the loop according to the ramp sequence and by a special quality assurance program, needed to ensure that all technical requirements were actually fulfilled. For this reason a comprehensive set of requirements has been scheduled by the program manager of each party involved, including nominal specifications and actions in anomalous situations before or during each ramp test. In addition a specific quality assurance plan dedicated to the ISABELLE 1 loop was carried out by the OSIRIS project engineers, covering mainly the calibration of all sensors signals involved in the thermal balance determination as well as the software package validation for each ramp test. Ramp tests performed in the ISABELLE 1 loop on similar preirradiated fuel rods have proven the reproducibility of the results, in particular clad failures due to pellet-clad interaction