Critical Review of Through-Wall Attenuation of Mechanical Properties in RPV Steels

This paper reviews the current state of knowledge on attenuation of damage parameters in reactor pressure vessels (RPVs). There are two methods for evaluating attenuation of properties through a reactor pressure vessel wall. The first is by direct measurement of the change in mechanical properties from decommissioned RPV sections or from simulated RPV wall experiments. It is shown that, although this approach is appealing, issues associated with knowledge of the start-of-life properties throughout the vessel wall sample and material property data scatter have made past measurements non-definitive in establishing attenuation changes. There is a need for further data on the direct measurement of attenuation, and an experiment is described, that is planned in 2002 under IAEA sponsorship. An alternative method for evaluating embrittlement is the use of a neutron damage exposure parameter and attenuation model coupled with an embrittlement correlation developed from surveillance capsule testing. The significant change in neutron flux spectrum when neutrons are attenuated through the RPV wall defines the need for a suitable neutron exposure damage parameter. The best available neutron exposure damage parameter is dpa. It is shown that plant-specific calculation of dpa through the RPV wall is the best method to be used for the neutron exposure. However, it can lead to slightly less attenuated values for damage at 1/4-T and 3/4-T for the vessel, as compared to using the simple exponential model quoted in Regulatory Guide 1.99, Rev. 2. Finally it is concluded that when using a surveillance correlation model to predict the attenuation of mechanical properties through the RPV wall, the use of a mechanistically guided model appears to be more appropriate than the embrittlement correlation provided in Regulatory Guide 1.99, Revision 2.