Creep of Tubes under External Pressure

29 tubes made of an E110 alloy for VVER and PWR reactors are subjected to creep tests under external pressure (temperature of 350 and 380 degrees C, circumferential stress of 70-130 MPa, holding time up to 4250 h). Creep is found to be accelerated after deformation up to 0.9% and ovality up to b/a = 0.86 and is ended in the collapse of a tube. The equations of tube equilibrium and creep under pressure are generalized for tubes of any ovality (ellipses of any eccentricity). The solutions to these equations predict the presence of a general curve in the contour length-eccentricity coordinates and give tube instability conditions under pressure.