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The vortex lattice (VL) in UPt₃ was studied using small-angle neutron scattering, and found to undergo a gradual disordering due to neutron beam induced fission of ²³⁵U. The fission events temporarily heat regions of the sample above the critical temperature, in which the vortices remain in a disordered state (most likely a quenched vortex glass) upon re-cooling. The disordering rate is proportional to the magnetic field, indicating that it is governed by the collective elastic properties of the vortex matter. An ordered VL can be re-formed by applying a small field oscillation, indicating that the fission does not cause significant radiation damage to UPt₃ on the time scale of the neutron scattering experiments. The ability to introduce localized quenched disorder, which at the same time is reversible, presents a novel avenue for vortex matter studies.