The Life of a Fuel Assembly
A nuclear fuel assembly enters the reactor as a precisely engineered array of ceramic UO₂ pellets sealed in zirconium alloy tubes. Over 3–5 years, it undergoes a profound transformation: U-235 atoms split, releasing energy and fission products; U-238 atoms transmute into plutonium; and the crystal structure of the ceramic degrades under relentless neutron and fission fragment bombardment. Tracking this evolution is essential for reactor operation, safety analysis, and spent fuel management.
U-235 Depletion and Pu-239 Breeding
Fresh 4% enriched fuel contains about 40 kg of U-235 per tonne. As the reactor operates, U-235 is consumed by fission at a rate proportional to neutron flux. Simultaneously, the abundant U-238 captures neutrons and transmutes through neptunium-239 into fissile plutonium-239. By mid-cycle, Pu-239 fission contributes a significant fraction of reactor power — the fuel effectively breeds its own replacement fissile material, extending the useful cycle length.
Fission Product Buildup
Each fission event creates two highly radioactive fission product nuclei from a library of over 800 possible isotopes. Some, like Xe-135, have enormous neutron absorption cross-sections that poison the chain reaction. Others, like Cs-137 and Sr-90, define the long-term radioactivity of spent fuel. The accumulation of fission products is the primary reason fuel must eventually be replaced — not because fissile material is exhausted, but because the neutron economy can no longer overcome the poison load.
Burnup and Cycle Optimization
This simulation tracks U-235 depletion, Pu-239 buildup, and integrated burnup over a fuel cycle. Adjust enrichment, power density, cycle length, and capacity factor to explore the tradeoffs between fuel utilization, cycle economics, and reactivity management. Higher enrichment enables longer cycles and deeper burnup, but increases fuel fabrication costs and the reactivity swing that must be managed with burnable absorbers or control rods.