Irradiation Tests of Optical Fibers and Cables Devoted to Corium Monitoring in Case of a Severe Accident in a Nuclear Power Plant

Abstract

The DIstributed Sensing for COrium Monitoring and Safety (DISCOMS) project considers the use of optical fiber sensing cables embedded into the concrete floor under the reactor vessel for remote monitoring of a severe nuclear accident. This article focuses on the selection and testing of single-mode optical fibers with limited radiation-induced attenuation (RIA). To simulate the normal operation period of the reactor, followed by a severe accident, several gamma and mixed (neutron–gamma) irradiations were performed, thanks to the Atomic Energy Commission (CEA) Saclay facilities, POSEIDON irradiator and ISIS reactor, up to a gamma cumulative dose of about 2 MGy and fast neutrons fluence ( ${E} \,\, >1$ MeV) of $6\times 10^{15}$ n/cm2. In comparison with the first gamma test on standalone fibers, a significant increase of the RIA at 1550 nm appeared during the second test on fibers encapsulated in sensing cables. Molecular hydrogen generated by the cable compound radiolysis induced an increase of the hydroxyl (OH) attenuation broad spectral peak centered at 1380 nm. The radiation-induced OH growth mechanism is confirmed by the comparison of the extended absorption spectral responses of the standalone and encapsulated fibers. A third gamma irradiation run permitted to measure the radiolytic hydrogen production yield of some compounds of a dedicated temperature cable sample. The efficiency of a carbon-coating layer over the silica cladding, acting as a barrier against hydrogen diffusion into the optical fiber core, was successfully tested under gamma irradiation as well as in a final test under neutron irradiation.