However, it is demanding to interpret the structure and composition of complex fission products, especially those with elements that have overlapping X-ray lines, which is common because of the high fraction of fission-product yield with similar chemistries. Such a process accelerates the swelling, cracking, and mechanical degradation of the fuel and plenum. When the fission gas moves into the fuel cladding gap, the released fission gas decreases the heat transfer in the gap, leading to an increased central fuel pellet temperature with time. The fission gases migrate into the defect structures because of their low solubility and tend to coalesce in the grain interior as intragranular bubbles or at the grain boundaries and triple junctions as intergranular bubbles.
Since the temperature decreases from the pellet center to the periphery, the generated fission products distribute heterogeneously in the UO 2 matrix. These features can be represented by noble gases (e.g., Kr and Xe) and metallic fission products (e.g., Mo, Tc, Ru, Rh, and Pd), whereas other fission products form oxides such as Cs, Ba, Zr, or lanthanides and transuranic elements 1, 7, 8, 9. The burned nuclear fuel contains multiple phases, including fission-product phases with complex chemistry. The current fleet of light-water reactors (LWRs) utilizes oxide fuel such as UO 2. Understanding the evolution of the nuclear fuel in service is critical to ensuring a reliable environment for geological storage sites 1, improving the fuel efficiency and safety as the operation is pushed to higher burnups 2, 3, and providing a realistic low carbon emission energy solution 4, 5, 6.
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