Lattice Contraction Behavior due to Atomic Ordering in Alloy 600

A PWSCC (primary water stress corrosion cracking) mechanism based on an ordering reaction has been proposed. However, there has been little investigation on the effects of ordering reaction on the lattice variation due to the ordering reaction in Alloy 600. To understand the lattice variation due to the ordering reaction in Alloy 600 is important, since this is unavoidable process in nuclear reactor environment. A lattice variation with ordering treatment at 400 .deg. C in Alloy 600 was systematically investigated using neutron diffraction. The magnitude of lattice contraction due to the ordering reaction is lager when the cooling rate is faster from the disordering temperature. The ordering treatment causes anisotropic lattice contraction according to the crystallographic planes. The ratio of minimum to maximum lattice contraction in WQ Alloy 600 is about 2. The maximum lattice contraction occurs in (111) and (200) planes are similarly about 0.04% in WQ Alloy 600. This saturates after 2,000H at 400 .deg. C. The effect of ordering reaction should be considered in the assessment of integrity of primary boundary materials made of Alloy 600, since the ordering reaction is an unavoidable phenomenon in nuclear reactor environment