In the final objective of elaborating an optical sensor of dimensional change in a nuclear testing reactor, we present extreme high level irradiation tests, up to 1 3 1020 nfast cm2 and 16GGy, conducted on high purity silica and hollow core photonic bandgap (PBG) fibers. These tests are performed in the OSIRIS reactor at CEA Saclay.We measure continuously Radiation Induced Absorption (RIA), at 3 optical wavelengths: 980 nm, 1064 nm and 1310 nm. The evolution with the time of the spectral dependency over larger wavelength range is also reported. As well as reporting RIA up to higher dose than presented before, we demonstrate here the excellent superiority of hollow core PBG fibers over their conventional counterparts in term of radiation resistance up to over 1020 nfast cm2 and 10GGy. We also report some Optical Time Domain Reflectometry (OTDR) measurements that probe the irradiated parts of the fibers and showa back scattering peak which appears, widens spatially then duplicates. We provide an interpretation based on the change of silica when it turns to metamict phase.