The 3 GeV synchrotron accelerator for JAERI-KEK joint project is aimed to supply a high-intensity proton beam to both the 50 GeV synchrotron ring and the neutron production target. For this purpose, the 3 GeV synchrotron, having a circumference of 313.5 m, is designed to accelerate a high-power beam of 1MW, by setting 25 Hz as a repetition rate and 8.3 x 1013 as a proton number for each acceleration. A rapid-cycling magnetic field is necessary to operate a synchrotron with such a high repetition rate. Since an eddy-current effect in a metal duct would produce an unacceptable perturbation of the magnetic field and unnecessarily large ohmic losses, an alumina ceramic is a candidate material for vacuum ducts in the dipole and quadrupole magnets. The required beam apertures, being a circular cross-section, are 187 and 247 mm in diameter for the vacuum ducts to be installed in the dipole and quadrupole magnets, respectively. The duct lengths for each magnet are 3.5 and 1 m. Since such a long duct is difficult to make with accuracy in the radial dimension, when sintered at once, it can only be manufactured by jointing duct segments of about 0.5 m long. The mechanical strength and deterioration under high level of radiation were examined for various kinds of alumina ceramic and for the joint. Almost all the ceramics to be tested have a sufficiently high flexural strength of 300 MPa, or more, before irradiating a 2.5 GeV-electron beam, and they did not show any significant deterioration in the flexural strength as far as an absorption dose is up to 1000 MGy.