Speaker
Description
The French Alternative Energies and Atomic Energy Commission (CEA) is developing innovative neutron detectors to support Generation IV reactors. For this purpose, the CEA is working on optical fission chambers, a technology based on noble gas scintillation. An optical fission chamber is a non-polarised fission chamber that uses de-excitation photons emitted by the gas filling the chamber, which is excited by a fission fragment. The signal is transmitted from the core via an optical fibre to a single-photon avalanche photodiode, which provides a count rate proportional to the number of photons detected.
One of the main challenges of this technology is related to signal transport in a nuclear environment. Indeed, irradiated fibers suffer from a phenomenon known as radiation-induced attenuation, which restricts the use of optical fission chambers, as the measured optical signal is no longer proportional to reactor power. Moreover, radiation-induced emission, particularly the Cherenkov effect in fibers, is also a limitation for optical fission chambers, as it is superimposed on the neutron radiation from the gas and introduces optical noise into the measurement due to gamma interactions.
For this reason, an experimental campaign was conducted on JSI's TRIGA reactor in June 2024 to characterize the radiation-induced properties (emission and attenuation) of various optical material irradiated in a mixed neutron/gamma field, in order to determine which optical elements are most suitable. These experiments demonstrated that the OH content in the core of an optical fiber strongly affects the fiber's radiation-induced attenuation and highlighted preferential spectral ranges that may be useful for optical fission chambers. Phenomena related to the reactor environment, such as neutron activation of silica, were also observed. Lastly, measurements taken on sapphire glass, known for its resistance to radiation and high temperatures, showed that its use in reactors would be unwise due to its radiation-induced emission, which is much more intense than that of silica.
