Irradiation effects in low T$_{c}$ superconductors

The effects of neutron irradiation on the superconducting parameters Tc, Bc2 and Jc of Nb$_{3}$Sn are reviewed in view of the determination of the radiation limit in the LHC upgrade magnets. The variation of Jc in binary as well as in Ti and Ta alloyed Nb$_{3}$Sn wires is presented. The coexisting defect mechanisms in irradiated Nb$_{3}$Sn type compounds are briefly presented and a model is discussed explaining the site exchange mechanism which leads to a decrease of atomic ordering after irradiation. Based on calculations of F. Cerutti and coworkers (CERN), the neutron fluence at the inner winding of the quadrupole Q2a is estimated to values below 1018 neutrons /cm2 for a life time of 10 years, which is within the safety margin with respect to the critical current density and Bc2.