Optimization of gap sizes for the high performance of annular nuclear fuels

Solid-type nuclear fuels have been used for nuclear reactors for a long time. Many countries are currently developing annular fuels to improve the efficiency of nuclear fuels. The thermoelastic-plastic-creep analyses of solid- and annular-type rods were conducted under the same conditions. The temperature and stress of the solid- and annular-type rods were compared on the basis of gap size. In this study, we examined the advantages and disadvantages of annular-type fuel regarding the temperature and stress of the pellet and cladding. The inner and outer gaps between the pellet and cladding play important roles in the temperature and stress distributions of fuel systems. Therefore, the optimization of gaps in fuel systems was conducted for a low temperature under certain stress conditions. Thermoelastic-plastic-creep analyses were conducted by using an in-house thermoelastic-plastic-creep finite element analysis program in Visual FORTRAN with the effective stress function algorithm. Nonlinear iterative stress analyses were conducted by nonlinear iterative temperature analyses; that is, a quasi-fully coupled algorithm was applied to this procedure. In this study, the thermoelastic-plastic-creep analysis of pressurized water reactor annular fuels was conducted to determine the contacting tendency of the inner-outer gaps between the annular fuel pellets and cladding, as well as to optimize the gap sizes by using the commercial package PIAnO for efficient heat transfer at certain stress levels. Most analyses were conducted until the gaps disappeared. However, certain analyses lasted for 1582 days, after which the fuels were replaced.