In operating light water reactor (LWR) commercial power plants, neutron radiation induces embrittlement of the pressure vessel (PV) and its support structures. As a consequence, LWR-PV integrity is a primary safety consideration. LWR-PV integrity is a significant economic consideration, since the PV and its support structures are nonreplaceable power plant components and embrittlement of these components can, therefore, limit the effective operating lifetime of the plant.

To define the effects of neutron radiation damage on LWR pressure-temperature operating limits and to assess fracture toughness of power reactor PV, trend curves for the prediction of PV embrittlement have been developed. These trend curves are very general PV embrittlement curves that are used to evaluate current PV status as well as to predict the future state of the PV. In such trend curves, the two main measures of radiation damage are the adjusted reference nil-ductility temperature ART_NDT(RT_NDT initial + ΔRT_NDT) and the decrease in upper-shelf energy level determined from Charpy V notch impact tests. Current measures of neutron exposure most commonly used in trend curve analyses are fluence >1 MeV and displacements per atom (dpa) in iron.

Since trend curves play such a crucial role in the assessment of PV embrittlement of operating commercial LWR power plants, a critical appraisal of trend curve analysis is essential. To this end, current limitations in trend curve analysis for the prediction of reactor PV embrittlement are examined. It is concluded that a number of systematic effects can arise because environmental differences exist between test reactors, surveillance capsule locations, and the actual irradiation conditions that accure within the PV of an operating LWR commercial power plant. An irradiation test program is advanced to investigate these systematic effects and to produce the requisite data needed to correct for such systematic biases in trend curve analysis.