The dependence of the upper critical field (Hc2) and the critical current density ( jc) on fluence has been measured up to 1.2×1019cm−2(E≳1 MeV) for V3Si bulk material irradiated with neutrons at ambient reactor temperatures. Dislocation loops are identified by TEM as the part of the radiation induced defects responsible forjc. Using these results it is shown that the increase injcof irradiatedA15 superconductors with grain boundaries is not due to the raisedHc2but corresponds to the radiation induced dislocation loops. The volume pinning force of these defects has the same temperature dependence but quite a different field dependence than the grain boundary pinning. By superposing the dislocation loop pinning on the grain boundary pinning we obtain an enhancement ofjcwhich increases with increasing field in excellent agreement with the observed field dependent increase ofjcin neutron irradiatedA15 superconductors. Deviations from the experimental values for high‐jcmaterials and consequences for the high field mechanism of Kramer’s model are discussed.