Effect of additive (Zr) and dopants (Pd & Sb) on the depth of migration of lighter lanthanides (Nd & Ce) from U-bearing metal alloys into T91 steel

The metal alloy fuel, which is considered as the advanced nuclear fuel, has its own limitations; namely, lower liquidus temperature, higher swelling, and is highly prone to breach the cladding due to fuel-cladding chemical interaction (FCCI). The mobile and insoluble fission products (FPs), especially lanthanide FPs are mainly responsible for FCCI. In the present study, the influence of additive (Zr) and dopants (Pd and Sb) on the depth of migration of two lanthanide FPs, namely Nd and Ce, from different uranium-bearing metal alloys into T91 (ferritic-martensitic steel) cladding material is investigated. Seven metal alloys, namely, U-Nd, U-Nd-Pd, U-Ce, U-Ce-Pd, U-Zr-Nd, U-Zr-Nd-Pd, and U-Zr-Nd-Sb, were prepared using arc melting technique. The amount of lanthanide FPs taken in the above alloys corresponds to about 10 at.% burn-up of a typical fast breeder reactor (FBR) fuel. The pulsed laser deposition (PLD) method was used to obtain alloys in the form of thin film on T91 cladding, with simultaneous in-situ annealing. Subsequently, these films were subjected to elemental depth profiling using secondary ion mass spectrometry (SIMS). The depth of migration for Nd and Ce from the seven metal alloy systems into T91 cladding was determined based on SIMS data. The extent of the individual and simultaneous effect of additive and dopant on the depth of migration was determined. Both dopants (Pd and Sb) are effective individually in binding Nd and Ce within the fuel matrix (in the U matrix) by forming stable inter-metallic compounds. The effect of Zr on the migration of lanthanide FPs is negligible in the presence of dopants. Stable inter-metallic phases responsible for binding lanthanides FPs are identified. A comparative study on the efficiency of two dopants (Pd and Sb) is also discussed. For the first time, the depth of migration of Nd and Ce from U metal alloys into T91 cladding was measured in the presence of dopants (Sb/Pd) by using a novel technique of PLD followed by SIMS. These studies are very useful for understanding FCCI.