Optical fiber semiconductor absorption temperature sensor for temperature monitoring in a gas-cooled nuclear reactor

Optical fiber sensor technology has not yet penetrated the relatively conservative world of nuclear instrumentation. Although the main effect of ionizing radiation on optical fibers is well known to be an increase of the optical absorption, little is known about the response of optical fiber sensors exposed to radiation fields. Our experiments therefore aimed at gaining insight about the behavior of standard commercially available optical fiber sensors in a representative nuclear environment. As experimental target, we have chosen semiconductor absorption optical fiber temperature sensors for monitoring temperature excursions in-situ, within SCK·CEN's BR1 gas-cooled graphite-moderated nuclear reactor. The sensors are located in the reactor's graphite, where they are exposed to thermal neutron fluxes on the order of 10¹¹ n.cm^-2.s^-1. Temperature excursions were measured for several reactor cycles, i.e. from reactor start to reactor shut-down, and compared to readings obtained from iron-constantan thermocouples. The sensor performed well until severe embrittlement of the fiber protective jacket caused a premature mechanical failure, after only a few days of reactor operation.