Disclosed are a superconductor having improved critical current density when exposed to high-energy neutron radiation and high magnetic fields, such as found in a compact nuclear fusion reactor, and a method of making the same. The method includes, prior to deployment in the exposure environment, irradiating a polycrystalline (e.g. cuprate) superconductor with ionic matter or neutrons at a cryogenic temperature to create “weak” magnetic flux pinning sites, such as point defects or small defect clusters. Irradiation temperature is chosen, for example as a function of the superconducting material, so that irradiation creates the beneficial flux pinning sites while avoiding detrimental widening of the boundaries of the crystalline grains caused by diffusion of the displaced atoms. Such a superconductor in a coated-conductor tape is expected to be beneficial when used, for example, as a toroidal field coil in a fusion reactor when cooled well below its critical temperature.