Coupling of a Lumped Parameter and a Finite Difference Model for Estimation of a Reactivity-Induced Transient in a PWR with Annular Fuel Rods

This paper develops a finite difference and a semianalytical model to evaluate the thermal behavior of fuel rods during a hypothetical reactivity-induced transient (absorber rod ejection) in a pressurized water reactor (PWR). The calculations are carried out for two different reactor core designs, namely, for a typical PWR core with typical fuel rods and for a modified PWR core containing annular fuel rods. The overall dimensions of the core internals and fuel assemblies are unchanged, and the total number of fuel assemblies is the same in both designs. The finite difference code was verified on the results provided by the semianalytical model. In the calculations, two point models were used (neutron kinetics to compute the fission power of the reactor; power balance of coolant in the active core region). The point models were coupled to the one-dimensional, finite difference heat conductivity model to calculate the radial temperature profile in the solid and the annular cylindrical pellets (UO₂). The latter are the constituent part of annular fuel rods cooled both on their external and internal surfaces. The fuel, cladding, and fluid temperatures were evaluated for the annular and the solid fuel design in the hot spot, where the maximum allowable power rating of rods is 2.5 times higher than the average one. It was assumed that before the transient, (a) the reactor with annular fuel runs continuously at higher nominal power (150%) and at higher nominal coolant flow rate (150%) at the core inlet than the reference reactor with solid fuel (100%) and (b) the modified and the reference reactors have the same coolant temperature at the core inlet and the same temperature rise of coolant along their core. These conditions correspond to a 150% power uprate of the reference reactor. The coupled models require limited computational resources only. The calculated results showed that during the transient, in the annular fuel pellet, the temperatures peaked at considerably lower values, even at 150% power, than in the solid pellets at 100% power. These evaluations show that in the case of the reactivity-induced transient analyzed, the annular fuel cooled from both sides has a better safety performance than the solid fuel cooled only on its external surface.