Parametric analyses of partial sub-assembly blockages of the 50 MWth gas cooled experimental technology and demonstrator reactor (ETDR)

This paper presents a parametric study of thermal hydraulic and structural mechanic analyses of accidental blockages of the hottest sub-assembly of the 50 MWth gas-cooled fast reactor, ETDR. The blockage ratios were 60% and 100% of the sub-assembly flow cross-section located at the first row of grid spacers. Temperature profiles in the fuel and cladding were calculated as a function of time using computational fluid dynamics. The results were incorporated into finite elements analyses to evaluate thermal and mechanical stresses and strains in the cladding and fuel. 2D simulations with generalized plane strains were used in the structural analyses applying the maximum power density in the pellet as calculated by CFD. The thermal analyses showed that a 60% partial blockage increases the maximum cladding temperature by 130 °C within a time period of 50 s, whereas a full blockage will lead to clad melting (1320 °C) in about 8 s after accident initiation. In the finite element analyses several, mainly conservative, assumptions have been made to incorporate the phenomena occurring in the pellet–cladding interaction. The results of the finite element analyses represent a first study of the pellet–cladding mechanical interface under given transients, exploring the development of a methodology to be used for future analyses. The model has been verified to predict realistic contact stresses in line with analytical solutions during partial or full blockages. However, more elaborate 2D and 3D contact models, with creep and irradiation creep material models for both fuel and clad, together with parametric studies on friction coefficients and number of fuel fragments are foreseen for future work on failure criteria.