In-reactor stress-relaxation tests were done on zirconium alloys with different compositions, grain sizes, heat-tratments, cold-work, and specimen stress directions at 568 ± 5 K in a fast neutron flux (E > 1 MeV) of 2.0 × 10¹⁷ neutrons (n)/m²s, for up to 8200 h with initial stresses up to 300 MPa. The relaxation behavior can be represented by σ/σ_o = D exp (-Kt) where σ/σ_o is the ratio of the unrelaxed stress at time t to the initial stress, D is a constant which approximates the stress ratio after a rapid initial drop, and K is a relaxation rate constant.

The effects of the metallurgical variables on the in-reactor stress relaxation of zirconium alloys is small. Variations in composition and heat-treatments change the relaxation rate constant by factors of 2 to 4, while variations due to cold-work and working direction change the constant by factors of up to 2. Stress-relieving (or aging) following cold-work decreases the initial stress drop.