STRENGTH AND METALLURGICAL PROPERTIES OF THE ZIRCALOY-2 PRESSURE TUBES FOR THE PRTR

The ultimate hoop strength of the annealed portion of the PRTR Zircaloy- 2 pressure tubes was determined by hydrostatic burst tests wherein the tube test specimens were internally pressurized to failure at temperatures from 425 to 750 deg F. In these tests, biaxial stresses simulating reactor operating conditions are imposed so that the circumferential stress is approximately twice the axial stress. The ultimate strength of the PRTR tubes thus tested is about 40 to 50% greater than the ultimate strength of rolled and annealed Zircaloy-2 strip determined from tensile tests. Extrapolation of long term (circa 5000 hours) tensile creep tests on longitudinal specimens cut from the annealed portion of the PRTR tubes showed that a stress of about 20000 psi will produce about 1% total deformation in ten years (the service limit used in the design of PRTR tubes) whereas for rolled strip Zircaloy-2 a stress of about 15000 psi would produce about 1% strain in ten years. These data further show that in tensite creep tests about 90% of the total creep deformation occurs during the primary creep stage. Very short term (circa 100 hours) pressure creep tests suggest that, under biaxial stress conditions (circumferential stress equal to twicemore » the axial stress) the primary creep deformation is much less than was anticipated from an analysis of the uniaxial tests. Using the annealed ultimate strength, determined from hydrostatic burst tests and the PRTR operating conditions (1050 psig at 550 deg F), the safety factor is nearly 3.5 compared to a safety factor of 2.5 based on the ultimate strength of annealed Zircaloy-2 strip; for PRTR scram type trip limits (1160 psig at 550 deg F) the safety factors are respectively about 3 and 2. (auth)« less