The present article summarizes the development and testing of a reactivity modulation device developed by the French Atomic Energy 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 reactivity modulator (Periodic reactivity Injection System Transients Induced Locally) aims at measuring the nuclear reactor transfer function in the frequency range of interest, between 1 mHz and 200 Hz. The device is made up of several aluminum tubes (outer diameter 10 mm), two inner ones holding Cadmium foils. The center tube can be rotated by means of a brushless motor, while the outer one remains static. During the rotation, the radiative capture reaction rate of Cadmium can be varied. This gives rise to a controlled reactivity modulation when the device is within or close to a nuclear reactor core, as the in-core neutron population is varied. The height of the ensemble, as well as the relative position between rotary and static parts are adjustable, offering flexibility on reactivity worth and reactivity amplitude.
Thanks to its compactness, PISTIL can be inserted at the center of the fuel lattice of CROCUS. In the experimental setup, the maximum reactivity worth, as estimated using TRIPOLI-4 Monte Carlo code, was less than 0.15 $\$$ and the maximum amplitude of the reactivity modulation was about 0.02 $\$$.
Out-of-pile tests inside a dummy mock-up of CROCUS were conducted for the mechanical characterization of PISTIL, while the device was driven periodically to rotate up to 100 Hz. Recorded motion profiles and induced vibrations were analyzed for performance and safety studies. In-core reactivity calibration was then performed for several reference configurations of PISTIL. The measured reactivity worth and modulation amplitude were consistent as compared to TRIPOLI-4 estimations.