PISTIL, a reactivity modulation device to probe the transfer function of the nuclear reactor CROCUS

Abstract

The present article summarizes the development and testing of a reactivity modulation device developed by the French Atomic and Alternative Energies Commission (CEA). It was installed in the CROCUS reactor of the Swiss Federal Institute of Technology in Lausanne (EPFL). Experimental tests were performed in the framework of a collaboration between CEA and EPFL. The so-called PISTIL device aims at measuring the nuclear reactor transfer function in the frequency range of interest between 1 mHz and 200 Hz, in order to probe the in-core kinetic behavior of prompt and delayed neutrons. The reactivity modulation is obtained from the rotation of cadmium foils. The design of the system was driven with the objective of installing PISTIL at the center of the CROCUS reactor. Neutronic simulations with TRIPOLI-4 Monte Carlo code were performed to select the suitable design parameters and meet the safety requirements of the reactor operation. The total reactivity worth of the device, as estimated by TRIPOLI-4 Monte Carlo calculation, was approximately 0.16 $ and the maximum amplitude of the reactivity modulation was about 0.013 $. In-core reactivity calibration was then performed and were consistent as compared to TRIPOLI-4 estimations.

Reactivity modulation experiments for nuclear data in CROCUS within a CEA-EPFL collaboration

“…Based on the assumption that the power distribution does not change with the rotation frequency, the modulation experiments at any frequency was analyzed and rescaled using the reactivity spectrum. The authors refer to [1] for more explanations on the data processing.…”

“…The objective was to measure the RTF of CROCUS, its power and phase, on a broad frequency range and with the same device in order to acquire information on the DN (range 1 mHz -1 Hz) but also on prompt neutrons (range 1 Hz -100 Hz). The study was conducted within a PhD work [1], which focused first on designing the modulation device, named PISTIL [2]. An experimental campaign was performed in CROCUS in June 2021.…”

Ten springs of experiments in CROCUS

“…In the case of APHRODITE, the goal is opposite to COLIBRI: a clean and well characterized perturbation allows providing feedback on the nuclear data for kinetic parameters. Carried out within a PhD thesis in collaboration between CEA and EPFL [40], a rotating oscillator -or oscillator of variable strengthcalled PISTIL was developed to induce a reactivity modulation over a large frequency range, from 1 mHz to 200 Hz [41]. Whereas branching noise allows only limited feedback at low frequencies, focusing on the integral 𝛽𝛽 eff parameter at most because of a lack of detection statistics, the gain observed in modulation noise allows overcoming this limitation.…”

“…Whereas branching noise allows only limited feedback at low frequencies, focusing on the integral 𝛽𝛽 eff parameter at most because of a lack of detection statistics, the gain observed in modulation noise allows overcoming this limitation. Hence the zero power transfer function (ZPTF) of CROCUS was determined in power and phase, with a focus on delayed neutrons, which is of most interest for nuclear data, especially for the determination of respective abundances of precursors (see Figure 3) [40], [42]. In addition to typical continuous rotations inducing unique frequency modulations, stepwise displacements allowed triggering a large range of frequencies at once, which was key in the resolution of low frequencies.…”