Implementation of an Elastoplastic UMAT Model Applied to the Irradiation Behavior of the Atucha I Moderator Tank

Abstract

The irradiation behavior of Atucha I’s Moderator Tank (MT) is modeled using finite elements. Under neutron irradiation, the MT experiences changes in mechanical properties such as ductility and fracture toughness. Irradiation creep adds to the existing thermoelastic strain. Assuming DIN 1.4550 steel as the sole material, an irradiation constitutive law is implemented as a UMAT in Code-Aster. An additive strain decomposition is considered, including elastic, plastic, irradiation creep strains, and swelling. The algorithm and its numerical implementation are detailed, including the calculation of the consistent Jacobian matrix via Newton-Raphson. Finally, the model is applied to study dimensional changes of the CNAI MT.