The results of surveillance tests on the High Flux Isotope Reactor (HFIR) pressure vessel at the Oak Ridge National Laboratory (ORNL) revealed that a greater than expected embrittlement had taken place after about 17.5 effective full power years of operation, and an operational assessment program was undertaken to fully evaluate the vessel condition and recommend conditions under which operation could be resumed. A research program was undertaken that included irradiating specimens in the Oak Ridge research reactor. Specimens of the A212 grade B vessel shell material were included, along with specimens from a nozzle qualification weld and a submerged arc weld fabricated at ORNL to reproduce the vessel seam weld. The results of the surveillance program and the materials research program performed in support of the evaluation of the HFIR pressure vessel are presented and show the welds to be more radiation resistant than the A212B. An important result that may have implications for power reactors was a higher than expected Charpy V-notch shift for the surveillance materials at relatively low neutron fluences. Moreover, to obtain embrittlement (as measured by either Charpy shift or increase in yield strength) equal to that from the low flux HFIR surveillance program, the neutron fluence (>1 MeV) required in the high flux Oak Ridge Research Reactor is about ten times that experienced by the HFIR surveillance specimens. Results of irradiated tensile and annealing experiments are described as well as a discussion of mechanisms which may be responsible for enhanced hardening at low damage rates.