New creep correction methods of the finite length-tube in cladding creep collapse

Generally, plane strain assumption, i.e., infinite-length unsupported tube assumption is applied in the specialty cladding creep collapse analysis code. Actually the unsupported cladding tube length is usually less than 20 mm, and this real condition will result in lower creep rate than the one with infinite-length unsupported. In order to make the prediction better consistent with reality, the finite tube-length correction on creep rate, which is calculated based on infinite-length unsupported tube assumption, should be done. Two methods are introduced to calculate the finite-length tube correction factor based on 3D fuel rod creep collapse code ABCREEP. One method is called stiffness method, and it is developed based on that the stiffness difference between finitelength tube and infinite-length tube results in the creep rate difference. Other method is called lifetime method, and it is developed based on that the tube length difference between finite-length tube and infinite-length tube results in the creep collapse difference. The calculation investigation shows that, for stiffness method, the correction factor is mainly dependent on the ratio of unsupported cladding tube length and cladding mid-wall diameter. And for lifetime method, the correction factor is dependent on cladding material and the ratio of unsupported cladding tube length and cladding mid-wall diameter. When unsupported cladding tube length and cladding mid-wall diameter are both same, the correction factor of stiffness method is greater than lifetime method’s, i.e., stiffness method is more conservative than lifetime method in cladding creep collapse analysis.