Procedures for Multiaxial Creep-Fatigue Verifications of Nuclear Components

Nuclear high heat flux components (HHFCs) experience large thermal gradients and high heat flux variations, which induce severe thermal cyclic loadings. The most critical design issue for these components is their endurance strength under the required number of thermal cycles.

The aim of this work is to provide procedures to perform the multiaxial creep-fatigue life assessment of HHFCs. Since the existing design codes present limitations due to simplifying assumptions concerning procedures for the multiaxial fatigue verification considering interactive effects of creep-fatigue and local stress and temperature conditions, better accurate verification methods and rules are developed starting from the available scientific literature and experimental data. The new verification methods identify the shape of the most damaging hysteresis loop considering plasticity and creep strains in both tensile and compressive conditions.

The developed procedures are used to post-process the thermomechanical results of finite element (FE) analyses. They foresee the calculation of the creep-fatigue damage in each node and for each cyclic loading of the analyzed FE model by using the fatigue curve corresponding to the shape of the local hysteresis loop. Furthermore, the most fatigued elements are bounded and the causes of damage are identified to improve the local design. The fatigue damage is evaluated considering the effects of local conditions: temperature, multiaxial stress-strain state, strain intensity range, effect of local mean stresses, material shakedown, accumulated damage for multiple cyclic loads, combined effect of creep-fatigue, hold periods, and neutron flux.

The developed procedures are successfully verified by comparing the results with experimental data for different levels of mean stress.

This paper presents a description of the procedures and design rules focusing on the innovative aspects. The new procedures have been developed in the framework of the activities for the design, manufacturing, and procurement of the ITER neutral beam injector, and they are applied for creep-fatigue verifications of the in-vessel HHFCs.