Reevaluation of HFIR [High-Flux Isotope Reactor] pressure vessel integrity

The design of the High-Flux Isotope Reactor (HFIR) pressure vessel included consideration of radiation embrittlement, which enhances the chances of vessel failure as a result of propagation of crack-like defects (flaws). The intent was to surround the core and beryllium reflector with sufficient water and to use sufficiently tough beam-tube nozzle material so that a vessel lifetime of at least 20 effective full-power years (EFPY) could be achieved. The criterion by which vessel integrity was to be judged was that the operating temperature of the vessel should always be greater than the nil-ductility temperature (NDT) by 33 degree C. Specimens withdrawn in 1983 and 1986 indicated that in 1983, NDT + 33 degree C was as much as 25 degree C above the minimum permissible temperature for pressurization (20 degree C), and that by 1986, NDT + 33 degree C exceeded the vessel normal operating temperature (49 degree C) by 8 degree C. Indications were that the radiation damage rate was greater than expected. Thus, HFIR was shut down and a reevaluation of vessel integrity commenced. The reevaluation effort is described