Deformation features of fuel rods with mixed uranium–plutonium nitride fuel

Fuel rods containing a mixed uranium–plutonium nitride (MNUP) fuel were subjected to irradiation in BOR-60 and BN-600 reactors to assess their performance and potential use in BREST-OD-300 and BN-1200M reactors. Post-irradiation examinations revealed that their deformation behavior differs from that of fuel rods with oxide fuel. Nitride fuel rods demonstrated increased axial elongation and cladding ovalization under identical irradiation conditions within the same assembly. Elongation and ovalization studies were carried out for 12 fuel rods with cold-worked ChS68-ID cladding, 38 rods with cold-worked EK164-ID cladding, and 69 rods with EP823-Sh cladding. The fuel rods were irradiated in 17 experimental fuel assemblies, covering a range of maximum fuel burnup from 3.1–9.1% heavy atoms and displacement damage from 26–108 displacements per atom (dpa). The experimental data suggest that axial elongation is predominantly attributable to axial forces that emerge from thermomechanical interactions between the fuel and cladding. These interactions are attributed to the random displacement of pellets and/or their fragments from an axisymmetric position.