ELSEVIER Nuclear Engineering and Design 158 (1995) 149-156 Nudem Technical Engineering Note Positron affinity for precipitates in reactor pressure vessel steels * Gerhard Brauer ~, Martti J. Puska b, Mojmir Sob c, Timo Korhonen b a Positron Group of Technical University Dresden, c/o Research Centre Rossendorf Inc., PO Box 510119. D-OI314 Dresden. Germany b Laboratory of Physics, Helsinki University of Teehnology. SF-O21.qOEspoo, Finland c Institute of Physics of Matertals, Academy of .Sciences of the Czech Republic, ZizkotvJ 22, CZ-616 62 Brno. Czech Republic Received November 1993 Abstract The sensitivity of positron annihi!ation spectroscopy to irradiation-induced precipitates in reactor pressure vessel steels is discussed in the light of recent positron affinity and lifetime calcu'Jations.Carbide and nitride precipitates are found to tr~p positrons only if they contain metal vacancies. Copper precipitates are also attractive to positrons but they are probably detected through annihilation at the precipitate-matrix interface. These findings are related to available experimental data. 1. Introduction Neutron embrittlement of reactor pressure ves- sel (RPV) steels is a limiting factor in the lifetime of vessels of today's nuclear power plant (NPP) reactors. Until now, the underlying mechanisms of irradiation damage have not been completely understood (Eyre, 1993, Phythian, 1993). The presence of Cu impurities most strongly influences the irradiation behaviour of RPV steels, and irra- diation-induced precipitates play a dominant role. However, pure Cu precipitates are definitely not formed (Beaven, 1989; Buswell, 1986; Phythian, 1993; Solt, 1989). Therefore, a lot of effort has * Presented at the Fifth Meetingof the International Group on RadiationDamageMechanisms in PressureVesselSteels (IG-RDM-5), May 2-6, 1994,Santa Barbara, CA, USA. been undertaken worldwide to identify the prop- erties and chemical composition of such irradia- tion-induced precipitates, which could finally lead to a more general understanding of neutron em- brittlement. Several microscopic experimental methods have been employed to investigate neutron embrittle- ment processes, mostly small angle neutron scat- tering (SANS), atom probe field io~ m~ro~opy (APFIM) and transmission electron microscopy (TEM) -- for reviews see Phythian and English (1993), Solt et al. (1992). Positron annihilation spectroscopy (PAS) is a powerful tool for investigating defects with a less than average electron density, particularly vacan- cies and vacancy clusters (voids), and can also give useful information about precipitates (Brandt, 1983; Hautojfirvi, i979). PAS has been 0029-5493/951509.50 © 1995 Elsevier Science S.A. All rights reserved SSDi 0029-5493(95)01022-X