A procedure for solving transient nonlinear thermal problems of high burn-up nuclear fuel rods in a light water reactor

To ensure the safety of the high burn-up nuclear fuel rods (NFRs) in existing reactors, thermal behaviors of NFRs have been analyzed extensively. In this paper, an accurate and efficient procedure, called half-boundary method (HBM), is applied to solve the one-dimensional transient heat conduction problems of high burn-up NFRs with temperature-dependent material properties. HBM saves computation cost since it only needs to calculate the second-order matrix in the simulation. Compared with the low burn-up condition, the conductivity drops significantly in the rim zone of the high burn-up NFR. The huge temperature gradient in the radial direction render the nonlinear material properties affecting the temperature distribution greatly. In addition, the model contains a helium layer, so the radiation heat transfers are considered in this paper. The heating up process of a low burn-up nuclear fuel is first simulated, and these results are compared with those from reference to verify the accuracy of HBM in transient thermal problems of NFR. The high burn-up NFRs with and without central hole in heat-up process, reactivity initiated accidents (RIAs) and loss of coolant accidents (LOCAs) are then simulated. The results show that the NFR with the central hole (NFRCH) is safer than the NFR without the central hole (NFRWCH), and the temperature distribution of high burn-up NFR is higher than that of low burn-up NFR. The pellet, which is near the central axis, is going to melt when a critical RIA happens, while the inner surface of the cladding will melt first when serious LOCA happens.