A Critical Heat Generation for Safe Nuclear Fuels after a LOCA

Abstract

This study applies a thermo-elasto-plastic-creep finite element procedure to the analysis of an accidental behavior of nuclear fuel as well as normal behavior. The result will be used as basic data for the robust design of nuclear power plant and fuels. We extended the range of mechanical strain from small or medium to large adopting the Hencky logarithmic strain measure in addition to the Green-Lagrange strain and Almansi strain measures, for the possible large strain situation in accidental environments. We found that there is a critical heat generation after LOCA without ECCS (event category 5), under which the cladding of fuel sustains the internal pressure and temperature for the time being for the rescue of the power plant. With the heat generation above the critical value caused by malfunctioning of the control rods, the stiffness of cladding becomes zero due to the softening by high temperature. The weak position of cladding along the length continuously bulges radially to burst and to discharge radioactive substances. This kind of cases should be avoid by any means.

Nonlinear dynamic analysis of nuclear fuels considering thermo–elastic–plastic–creep phenomena

“…The deviatoric stress is computed from the deviatoric strain while regarding the plastic and creep strain 1,4 adopting the same symbols in Kojic and Bathe 1 as…”

“…Plasticity analysis 4 was performed using the present program and commercial package ADINA. The boundary conditions are shown in Figure 3.…”

“…Creep analysis was performed using the present program and commercial package ADINA. 4 The boundary conditions are shown in Figure 5. The von Mises, radial, and hoop stresses of the present program and ADINA are compared each other for creep problems as shown in Figure 6.…”

“…After a LOCA during the behavior of nuclear fuels, an unstable situation develops where the cladding stiffness becomes zero because of a local uncontrollable deformation. 4 In this case, an analysis using the post-buckling method applied to quasi-static problems is not a proper method for analyzing this transient problem because properties of the materials, which composed the nuclear fuel rods, are not constant. The material properties are considered functions of time and temperature.…”

“…2,3 Thermo-elastic-plastic-creep analysis considering large strain plasticity has been applied to nuclear fuel in loss of coolant accident (LOCA) situation. 4 However, the quasi-static analysis could not 1 handle the bulging phenomenon at the final stage of deformation.…”