Pile-oscillation results for stainless steel with local and 3D global measurements

The BLOOM pile-oscillation program on stainless steel and minor alloying elements was successfully conducted in the CROCUS reactor at EPFL between fall 2024 and spring 2025. For this purpose, a pile-oscillator and a new instrumented channel capable of measuring thermal neutrons as close as 2 mm from the sample were developed in-house, thanks to the previous development in miniature neutron scintillators. Furthermore, the SAFFRON detector array, consisting of 160 such miniature detectors, was used to measure the neutron flux across the core and within the instrumented channel. Using SAFFRON, it was verified that the chosen oscillation location was suitable and point kinetics could be applied, as the induced perturbation did not cause globalized flux redistributions. Local perturbations were measurable up to 15 cm from the sample, setting the minimum distance beyond which the global perturbation was measurable. A total of 45 pile-oscillations were performed, including measurements of 40 different samples and five repetitions of a gold sample at different points of the campaign. For each experiment, the amplitudes of the local and global perturbations and the corresponding reactivity worth were successfully measured, with relative uncertainties as low as 0.07% for the heaviest samples local perturbation, and uncertainty on reactivity worths of 0.04 pcm Future developments will include the addition of new samples and references such as the D 2 O. Further developments are also planned to be able to perform pile-oscillations at higher power.